C/C++ Interview Questiions

 

Q: Is it possible to have Virtual Constructor? If yes, how? If not, Why not possible?

A: There is nothing like Virtual Constructor. The Constructor can’t be virtual as the constructor is a code which is responsible for creating an instance of a class and it can’t be delegated to any other object by virtual keyword means.

Q: What is constructor or ctor?

A: Constructor creates an object and initializes it. It also creates vtable for virtual functions. It is different from other methods in a class.

Q: What about Virtual Destructor?

A: Yes there is a Virtual Destructor. A destructor can be virtual as it is possible as at runtime depending on the type of object caller is calling to, proper destructor will be called.

Q: What is the difference between a copy constructor and an overloaded assignment operator?

A: A copy constructor constructs a new object by using the content of the argument object. An overloaded assignment operator assigns the contents of an existing object to another existing object of the same class.

Q: Can a constructor throws an exception? How to handle the error when the constructor fails?

A:The constructor never throws an error.

Q: What is default constructor?

A: Constructor with no arguments or all the arguments has default values.

Q: What is copy constructor?

A: Constructor which initializes the it’s object member variables ( by

shallow copying) with another object of the same class. If you don’t implement one in your class then compiler implements one for you. for example:

  • Boo Obj1(10); // calling Boo constructor
  • Boo Obj2(Obj1); // calling boo copy constructor
  • Boo Obj2 = Obj1;// calling boo copy constructor

Q: When are copy constructors called?

A: Copy constructors are called in following cases:

  • when a function returns an object of that class by value
  • when the object of that class is passed by value as an argument to a function
  • when you construct an object based on another object of the same class
  • When compiler generates a temporary object

Q: Can a copy constructor accept an object of the same class as parameter, instead of reference of the object?

A: No. It is specified in the definition of the copy constructor itself. It should generate an error if a programmer specifies a copy constructor with a first argument that is an object and not a reference.

Q: What is conversion constructor?

A: constructor with a single argument makes that constructor as conversion ctor and it can be used for type conversion.

for example: class Boo

{

public: Boo( int i );

};

Boo BooObject = 10 ; // assigning int 10 Boo object

Q:What is conversion operator??

A:class can have a public method for specific data type conversions. for example:

class Boo

{

double value; public: Boo(int i )

operator double()

{

return value;

}

};

Boo BooObject;

double i = BooObject; // assigning object to variable i of type double. now conversion operator gets called to assign the value.

Q: How can I handle a constructor that fails?

A: throw an exception. Constructors don’t have a return type, so it’s not possible to use return codes. The best way to signal constructor failure is therefore to throw an exception.

Q: How can I handle a destructor that fails?

A: Write a message to a log-_le. But do not throw an exception. The C++ rule is that you must never throw an exception from a destructor that is being called during the “stack unwinding” process of another exception. For example, if someone says throw Foo(), the stack will be unwound so all the stack frames between the throw Foo() and the } catch (Foo e) { will get popped. This is called stack unwinding. During stack unwinding, all the local objects in all

those stack frames are destructed. If one of those destructors throws an exception (say it throws a Bar object), the C++ runtime system is in a no-win situation: should it ignore the Bar and end up in the } catch (Foo e) { where it was originally headed? Should it ignore the Foo and look for a } catch (Bare) { handler? There is no good answer:either choice

loses information. So the C++ language guarantees that it will call terminate() at this point, and terminate() kills the process. Bang you’re dead.

Q: What is Virtual Destructor?

A: Using virtual destructors, you can destroy objects without knowing their type – the correct destructor for the object is invoked using the virtual function mechanism. Note that destructors can also be declared as pure virtual functions for abstract classes. if someone will derive from your class, and if someone will say “new Derived”, where “Derived” is derived from your class, and if someone will say delete p, where the actual object’s type is “Derived” but the pointer p’s type is your class.

Q: Can a copy constructor accept an object of the same class as parameter, instead of reference of the object?

A: No. It is specified in the definition of the copy constructor itself. It should generate an error if a programmer specifies a copy constructor with a first argument that is an object and not a reference.

Q: What’s the order that local objects are destructed?

A: In reverse order of construction: First constructed, last destructed.

In the following example, b’s destructor will be executed first, then a’s destructor:

void userCode()

{

Fred a; Fred b;

}

Q: What’s the order that objects in an array are destructed?

A: In reverse order of construction: First constructed, last destructed.

In the following example, the order for destructors will be a[9], a[8], …, a[1], a[0]:

void userCode()

{

Fred a[10];

}

Q: Can I overload the destructor for my class?

A: No.

You can have only one destructor for a class Fred. It’s always called Fred::~Fred(). It never takes any parameters, and it never returns anything.

You can’t pass parameters to the destructor anyway, since you never explicitly call a destructor (well, almost never).

Q: Should I explicitly call a destructor on a local variable?

A: No!

The destructor will get called again at the close } of the block in which the local was created. This is a guarantee of the language; it happens automagically; there’s no way to stop it from happening. But you can get really bad results from calling a destructor on the same object a second time! Bang! You’re dead!

Q: What if I want a local to “die” before the close } of the scope in which it was created? Can I call a destructor on a local if I really want to?

A: No! [For context, please read the previous FAQ].

Suppose the (desirable) side effect of destructing a local File object is to close the File. Now suppose you have an object f of a class File and you want File f to be closed before the end of the scope (i.e., the }) of the scope of object f:

void someCode()

{

File f;

…insert code that should execute when f is still open… We want the side-effect of f’s destructor here!

…insert code that should execute after f is closed…

}

There is a simple solution to this problem. But in the mean time, remember: Do not explicitly call the destructor!

Q: OK, OK already; I won’t explicitly call the destructor of a local; but how do I handle the above situation?

A: Simply wrap the extent of the lifetime of the local in an artificial block {…}:

void someCode()

{

{

File f;

…insert code that should execute when f is still open…

} f’s destructor will automagically be called here!

…insert code here that should execute after f is closed…}

Q: What if I can’t wrap the local in an artificial block?

A: Most of the time, you can limit the lifetime of a local by wrapping the local in an artificial block ({…}). But if for some reason you can’t do that, add a member function that has a similar effect as the destructor. But do not call the destructor itself!

For example, in the case of class File, you might add a close() method. Typically the destructor will simply call this close() method. Note that the close() method will need to mark the File object so a subsequent call won’t re-close an already-closed File. E.g., it might set the fileHandle_ data member to some nonsensical value such as -1, and it might check at the beginning to see if the fileHandle_ is already equal to -1:

class File { public:

void close();

~File();

private:

int fileHandle_; // fileHandle_ >= 0 if/only-if it’s open

};

File::~File()

{

close();

}

void File::close()

{

if (fileHandle_ >= 0) {

…insert code to call the OS to close the file… fileHandle_ = -1;

}

}

Note that the other File methods may also need to check if the fileHandle_ is -1 (i.e., check if the File is closed).

Note also that any constructors that don’t actually open a file should set fileHandle_ to -1.

Q: But can I explicitly call a destructor if I’ve allocated my object with new?

A: Probably not.

Unless you used placement new, you should simply delete the object rather than explicitly calling the destructor. For example, suppose you allocated the object via a typical new expression:

Fred* p = new Fred();

Then the destructor Fred::~Fred() will automagically get called when you delete it via:

delete p; // Automagically calls p->~Fred()

You should not explicitly call the destructor, since doing so won’t release the memory that was allocated for the Fred object itself. Remember: delete p does two things: it calls the destructor and it deallocates the memory.

Q: What is “placement new” and why would I use it?

A: There are many uses of placement new. The simplest use is to place an object at a particular location in memory. This is done by supplying the place as a pointer parameter to the new part of a new expression:

#include // Must #include this to use “placement new”

#include “Fred.h” // Declaration of class Fred

void someCode()

{

char memory[sizeof(Fred)]; // Line #1 void* place = memory; // Line #2

Fred* f = new(place) Fred(); // Line #3 (see “DANGER” below)

// The pointers f and place will be equal

}

Line #1 creates an array of sizeof(Fred) bytes of memory, which is big enough to hold a Fred object. Line #2 creates a pointer place that points to the first byte of this memory (experienced C programmers will note that this step was unnecessary; it’s there only to make the code more obvious). Line #3 essentially just calls the constructor Fred::Fred(). The this pointer in the Fred constructor will be equal to place. The returned pointer f will therefore be equal to place.

ADVICE: Don’t use this “placement new” syntax unless you have to. Use it only when you really care that an object is placed at a particular location in memory. For example, when your hardware has a memory-mapped I/O timer device, and you want to place a Clock object at that memory location.

DANGER: You are taking sole responsibility that the pointer you pass to the “placement new”

operator points to a region of memory that is big enough and is properly aligned for the object type that you’re creating. Neither the compiler nor the run-time system make any attempt to check whether you did this right. If your Fred class needs to be aligned on a 4 byte boundary but you supplied a location that isn’t properly aligned, you can have a serious disaster on your hands (if you don’t know what “alignment” means, please don’t use the placement new syntax). You have been warned.

You are also solely responsible for destructing the placed object. This is done by explicitly calling the destructor:

void someCode()

{

char memory[sizeof(Fred)]; void* p = memory;

Fred* f = new(p) Fred();

f->~Fred(); // Explicitly call the destructor for the placed object

}

This is about the only time you ever explicitly call a destructor.

Note: there is a much cleaner but more sophisticated way of handling the destruction / deletion situation.

Q: When I write a destructor, do I need to explicitly call the destructors for my member objects?

A: No. You never need to explicitly call a destructor (except with placement new).

A class’s destructor (whether or not you explicitly define one) automagically invokes the destructors for member objects. They are destroyed in the reverse order they appear within the declaration for the class.

class Member { public:

~Member();

};

class Fred { public:

~Fred();

private: Member x_; Member y_; Member z_;

};

Fred::~Fred()

{

// Compiler automagically calls z_.~Member()

// Compiler automagically calls y_.~Member()

// Compiler automagically calls x_.~Member()

}

Q: When I write a derived class’s destructor, do I need to explicitly call the destructor for my base class?

A: No. You never need to explicitly call a destructor (except with placement new).

A derived class’s destructor (whether or not you explicitly define one) automagically invokes the destructors for base class subobjects. Base classes are destructed after member objects. In the event of multiple inheritance, direct base classes are destructed in the reverse order of their appearance in the inheritance list.

class Member { public:

~Member();

};

class Base { public:

virtual ~Base(); // A virtual destructor

};

class Derived : public Base { public:

~Derived();

private: Member x_;

};

Derived::~Derived()

{

// Compiler automagically calls x_.~Member()

// Compiler automagically calls Base::~Base()

}

Note: Order dependencies with virtual inheritance are trickier. If you are relying on order dependencies in a virtual inheritance hierarchy, you’ll need a lot more information than is in this

FAQ.

Q: Is there any difference between List x; and List x();?

A: A big difference!

Suppose that List is the name of some class. Then function f() declares a local List object called x:

void f()

{

List x; // Local object named x (of class List)

}

But function g() declares a function called x() that returns a List:

void g()

{

List x(); // Function named x (that returns a List)

}

Q: Can one constructor of a class call another constructor of the same class to initialize the this object?

A: Nope.

Let’s work an example. Suppose you want your constructor Foo::Foo(char) to call another constructor of the same class, say Foo::Foo(char,int), in order that Foo::Foo(char,int) would help initialize the this object. Unfortunately there’s no way to do this in C++.

Some people do it anyway. Unfortunately it doesn’t do what they want. For example, the line Foo(x, 0); does not call Foo::Foo(char,int) on the this object. Instead it calls Foo::Foo(char,int) to initialize a temporary, local object (not this), then it immediately destructs that temporary when control flows over the ;.

class Foo { public: Foo(char x);

Foo(char x, int y);

};

Foo::Foo(char x)

{

Foo(x, 0); // this line does NOT help initialize the this object!!

}

You can sometimes combine two constructors via a default parameter:

class Foo { public:

Foo(char x, int y=0); // this line combines the two constructors

};

If that doesn’t work, e.g., if there isn’t an appropriate default parameter that combines the two constructors, sometimes you can share their common code in a private init() member function:

class Foo { public: Foo(char x);

Foo(char x, int y);

private:

void init(char x, int y);

};

Foo::Foo(char x)

{

init(x, int(x) + 7);

}

Foo::Foo(char x, int y)

{

init(x, y);

}

void Foo::init(char x, int y)

{

}

BTW do NOT try to achieve this via placement new. Some people think they can say new(this) Foo(x, int(x)+7) within the body of Foo::Foo(char). However that is bad, bad, bad. Please don’t write me and tell me that it seems to work on your particular version of your particular compiler; it’s bad. Constructors do a bunch of little magical things behind the scenes, but that bad technique steps on those partially constructed bits. Just say no.

Q: Is the default constructor for Fred always Fred::Fred()?

A: No. A “default constructor” is a constructor that can be called with no arguments. One example of this is a constructor that takes no parameters:

class Fred { public:

Fred(); // Default constructor: can be called with no args

};

Another example of a “default constructor” is one that can take arguments, provided they are given default values:

class Fred { public:

Fred(int i=3, int j=5); // Default constructor: can be called with no args

};

Q: Which constructor gets called when I create an array of Fred objects?

A: Fred’s default constructor (except as discussed below).

class Fred { public: Fred();

};

int main()

{

Fred a[10];  calls the default constructor 10 times

Fred* p = new Fred[10]; calls the default constructor 10 times

}

If your class doesn’t have a default constructor, you’ll get a compile-time error when you attempt to create an array using the above simple syntax:

class Fred { public:

Fred(int i, int j); assume there is no default constructor

};

int main()

{

Fred a[10]; ERROR: Fred doesn’t have a default constructor

Fred* p = new Fred[10]; ERROR: Fred doesn’t have a default constructor

}

However, even if your class already has a default constructor, you should try to use std::vector rather than an array (arrays are evil). std::vector lets you decide to use any constructor, not just the default constructor:

#include int main()

{

std::vector a(10, Fred(5,7));  the 10 Fred objects in std::vector a will be initialized with Fred(5,7)

}

Even though you ought to use a std::vector rather than an array, there are times when an array might be the right thing to do, and for those, you might need the “explicit initialization of arrays” syntax. Here’s how:

class Fred { public:

Fred(int i, int j);  assume there is no default constructor

};

int main()

{

Fred a[10] = {

Fred(5,7), Fred(5,7), Fred(5,7), Fred(5,7), Fred(5,7), // The 10 Fred objects are Fred(5,7), Fred(5,7), Fred(5,7), Fred(5,7), Fred(5,7) // initialized using Fred(5,7)

};

}

Of course you don’t have to do Fred(5,7) for every entry you can put in any numbers you want, even parameters or other variables.

Finally, you can use placement-new to manually initialize the elements of the array. Warning: it’s ugly: the raw array can’t be of type Fred, so you’ll need a bunch of pointer-casts to do things like compute array index operations. Warning: it’s compiler- and hardware-dependent: you’ll need to make sure the storage is aligned with an alignment that is at least as strict as is required for objects of class Fred. Warning: it’s tedious to make it exception-safe: you’ll need to manually

destruct the elements, including in the case when an exception is thrown part-way through the loop that calls the constructors. But if you really want to do it anyway, read up on placement- new. (BTW placement-new is the magic that is used inside of std::vector. The complexity of getting everything right is yet another reason to use std::vector.)

By the way, did I ever mention that arrays are evil? Or did I mention that you ought to use a std::vector unless there is a compelling reason to use an array?

Q: Should my constructors use “initialization lists” or “assignment”?

A: Initialization lists. In fact, constructors should initialize as a rule all member objects in the initialization list. One exception is discussed further down.

Consider the following constructor that initializes member object x_ using an initialization list: Fred::Fred() : x_(whatever) { }. The most common benefit of doing this is improved performance. For example, if the expression whatever is the same type as member variable x_, the result of the whatever expression is constructed directly inside x_ the compiler does not make a separate copy of the object. Even if the types are not the same, the compiler is usually able to do a better job with initialization lists than with assignments.

The other (inefficient) way to build constructors is via assignment, such as: Fred::Fred() { x_ = whatever; }. In this case the expression whatever causes a separate, temporary object to be created, and this temporary object is passed into the x_ object’s assignment operator. Then that temporary object is destructed at the ;. That’s inefficient.

As if that wasn’t bad enough, there’s another source of inefficiency when using assignment in a constructor: the member object will get fully constructed by its default constructor, and this might, for example, allocate some default amount of memory or open some default file. All this work could be for naught if the whatever expression and/or assignment operator causes the object to close that file and/or release that memory (e.g., if the default constructor didn’t allocate a large enough pool of memory or if it opened the wrong file).

Conclusion: All other things being equal, your code will run faster if you use initialization lists rather than assignment.

Note: There is no performance difference if the type of x_ is some built-in/intrinsic type, such as int or char* or float. But even in these cases, my personal preference is to set those data members in the initialization list rather than via assignment for consistency. Another symmetry argument in favor of using initialization lists even for built-in/intrinsic types: non-static const and non- static reference data members can’t be assigned a value in the constructor, so for symmetry it makes sense to initialize everything in the initialization list.

Now for the exceptions. Every rule has exceptions (hmmm; does “every rule has exceptions” have exceptions? reminds me of Gdel’s Incompleteness Theorems), and there are a couple of exceptions to the “use initialization lists” rule. Bottom line is to use common sense: if it’s cheaper, better, faster, etc. to not use them, then by all means, don’t use them. This might happen when your class has two constructors that need to initialize the this object’s data members in

different orders. Or it might happen when two data members are self-referential. Or when a data- member needs a reference to the this object, and you want to avoid a compiler warning about using the this keyword prior to the { that begins the constructor’s body (when your particular compiler happens to issue that particular warning). Or when you need to do an if/throw test on a variable (parameter, global, etc.) prior to using that variable to initialize one of your this members. This list is not exhaustive; please don’t write me asking me to add another “Or when…”. The point is simply this: use common sense.

Q: Should you use the this pointer in the constructor?

A: Some people feel you should not use the this pointer in a constructor because the object is not fully formed yet. However you can use this in the constructor (in the {body} and even in the initialization list) if you are careful.

Here is something that always works: the {body} of a constructor (or a function called from the constructor) can reliably access the data members declared in a base class and/or the data members declared in the constructor’s own class. This is because all those data members are guaranteed to have been fully constructed by the time the constructor’s {body} starts executing.

Here is something that never works: the {body} of a constructor (or a function called from the constructor) cannot get down to a derived class by calling a virtual member function that is overridden in the derived class. If your goal was to get to the overridden function in the derived class, you won’t get what you want. Note that you won’t get to the override in the derived class independent of how you call the virtual member function: explicitly using the this pointer (e.g., this->method()), implicitly using the this pointer (e.g., method()), or even calling some other function that calls the virtual member function on your this object. The bottom line is this: even if the caller is constructing an object of a derived class, during the constructor of the base class, your object is not yet of that derived class. You have been warned.

Here is something that sometimes works: if you pass any of the data members in this object to another data member’s initializer, you must make sure that the other data member has already been initialized. The good news is that you can determine whether the other data member has (or has not) been initialized using some straightforward language rules that are independent of the particular compiler you’re using. The bad news it that you have to know those language rules (e.g., base class sub-objects are initialized first (look up the order if you have multiple and/or virtual inheritance!), then data members defined in the class are initialized in the order in which they appear in the class declaration). If you don’t know these rules, then don’t pass any data member from the this object (regardless of whether or not you explicitly use the this keyword) to any other data member’s initializer! And if you do know the rules, please be careful.

Q: What is the “Named Constructor Idiom”?

A: A technique that provides more intuitive and/or safer construction operations for users of your class.

The problem is that constructors always have the same name as the class. Therefore the only way to differentiate between the various constructors of a class is by the parameter list. But if there are lots of constructors, the differences between them become somewhat subtle and error prone.

With the Named Constructor Idiom, you declare all the class’s constructors in the private or protected sections, and you provide public static methods that return an object. These static methods are the so-called “Named Constructors.” In general there is one such static method for each different way to construct an object.

For example, suppose we are building a Point class that represents a position on the X-Y plane. Turns out there are two common ways to specify a 2-space coordinate: rectangular coordinates (X+Y), polar coordinates (Radius+Angle). (Don’t worry if you can’t remember these; the point isn’t the particulars of coordinate systems; the point is that there are several ways to create a Point object.) Unfortunately the parameters for these two coordinate systems are the same: two floats. This would create an ambiguity error in the overloaded constructors:

class Point { public:

Point(float x, float y); // Rectangular coordinates

Point(float r, float a); // Polar coordinates (radius and angle)

// ERROR: Overload is Ambiguous: Point::Point(float,float)

};

int main()

{

Point p = Point(5.7, 1.2); // Ambiguous: Which coordinate system?

}

One way to solve this ambiguity is to use the Named Constructor Idiom:

#include // To get sin() and cos() class Point {

public:

static Point rectangular(float x, float y); // Rectangular coord’s static Point polar(float radius, float angle); // Polar coordinates

// These static methods are the so-called “named constructors”

private:

Point(float x, float y); // Rectangular coordinates float x_, y_;

};

inline Point::Point(float x, float y)

: x_(x), y_(y) { }

inline Point Point::rectangular(float x, float y)

{ return Point(x, y); }

inline Point Point::polar(float radius, float angle)

{ return Point(radius*cos(angle), radius*sin(angle)); }

Now the users of Point have a clear and unambiguous syntax for creating Points in either coordinate system:

int main()

{

Point p1 = Point::rectangular(5.7, 1.2); // Obviously rectangular Point p2 = Point::polar(5.7, 1.2); // Obviously polar

}

Make sure your constructors are in the protected section if you expect Point to have derived classes.

The Named Constructor Idiom can also be used to make sure your objects are always created via new.

Note that the Named Constructor Idiom, at least as implemented above, is just as fast as directly calling a constructor modern compilers will not make any extra copies of your object.

Q: What is virtual function?

A: When derived class overrides the base class method by redefining the same function, then if client wants to access redefined the method from derived class through a pointer from base class object, then you must define this function in base class as virtual function.

class parent

{

void Show()

{

cout << “i’m parent” << endl;

}

};

class child: public parent

{

void Show()

{

cout << “i’m child” << endl;

}

};

parent * parent_object_ptr = new child; parent_object_ptr->show() // calls parent->show() now we goto virtual world…

class parent

{

virtual void Show()

{

cout << “i’m parent” << endl;

}

};

class child: public parent

{

void Show()

{

cout << “i’m child” << endl;

}

};

parent * parent_object_ptr = new child; parent_object_ptr->show() // calls child->show()

Q: What is a “pure virtual” member function?

A: The abstract class whose pure virtual method has to be implemented by all the classes which derive on these. Otherwise it would result in a compilation error. This construct should be used when one wants to ensure that all the derived classes implement the method defined as pure virtual in base class.

Q: How virtual functions are implemented C++?

A: Virtual functions are implemented using a table of function pointers, called the vtable. There is one entry in the table per virtual function in the class. This table is created by the constructor of the class. When a derived class is constructed, its base class is constructed _rst which creates the vtable. If the derived class overrides any of the base classes virtual functions, those entries in the vtable are overwritten by the derived class constructor. This is why you should never call virtual functions from a constructor: because the vtable entries for the object may not have

been set up by the derived class constructor yet, so you might end up calling base class implementations of those virtual functions

Q: What is pure virtual function? or what is abstract class?

A: When you de_ne only function prototype in a base class without implementation and do the complete implementation in derived class. This base class is called abstract class and client won’t able to instantiate an object using this base class. You can make a pure virtual function or abstract class this way..

class Boo

{

void foo() = 0;

}

Boo MyBoo; // compilation error

Q: What is Pure Virtual Function? Why and when it is used?

A: The abstract class whose pure virtual method has to be implemented by all the classes which derive on these. Otherwise it would result in a compilation error. This construct should be used when one wants to ensure that all the derived classes implement the method defined  as pure virtual in base class.

Q: How Virtual functions call up is maintained?

A: Through Look up tables added by the compile to every class image. This also leads to performance penalty.

Q: What is a virtual destructor?

A: The simple answer is that a virtual destructor is one that is declared with the virtual attribute. The behavior of a virtual destructor is what is important. If you destroy an object through a caller or reference to a base class, and the base-class destructor is not virtual, the derived-class destructors are not executed, and the destruction might not be complete.

Q: What is inheritance?

A: Inheritance allows one class to reuse the state and behavior of another class. The derived class inherits the properties and method implementations of the base class and extends it by overriding methods and adding additional properties and methods.

Q: When should you use multiple inheritance?

A:There  are  three  acceptable  answers:-  “Never,””Rarely,”  and  “When  the  problem  domain cannot be accurately modeled any other way.” Consider an Asset class, Building class, Vehicle class, and CompanyCar class. All company cars are vehicles. Some company cars are assets because the organizations own them. Others might be leased. Not all assets are vehicles. Money accounts are assets. Real estate holdings are assets. Some real estate holdings are buildings. Not all  buildings  are  assets.  Ad  infinitum.  When  you  diagram  these  relationships,  it  becomes apparent that multiple inheritance is a likely and intuitive way to model this common problem domain. The applicant should understand, however, that multiple inheritance, like a chainsaw, is a useful tool that has its perils, needs respect, and is best avoided except when nothing else will do.

Q: Explain the ISA and HASA class relationships. How would you implement each in a class design?

A: A specialized class “is” a specialization of another class and, therefore, has the ISA relationship with the other class. This relationship is best implemented by embedding an object of the Salary class in the Employee class.

Q: When is a template a better solution than a base class?

A: When you are designing a generic class to contain or otherwise manage objects of other types, when the format and behavior of those other types are unimportant to their containment or management, and particularly when those other types are unknown (thus, the generality) to the designer of the container or manager class.

Q: What is multiple inheritance(virtual inheritance)? What are its advantages and disadvantages? A: Multiple Inheritance is the process whereby a child can be derived from more than one parent class. The advantage of multiple inheritance is that it allows a class to inherit the functionality of more than one base class thus allowing for modeling of complex relationships.

The disadvantage of multiple inheritance is that it can lead to a lot of confusion(ambiguity) when two base classes implement a method with the same name.

Q: What a derived class inherits or doesn’t inherit?

A: Inherits:

Every data member defined in the parent class (although such members may not always be accessible in the derived class!)

Every ordinary member function of the parent class (although such members may not always be accessible in the derived class!)

The same initial data layout as the base class. Doesn’t Inherit :

The base class’s constructors and destructor. The base class’s assignment operator.

The base class’s friends

Q: What is Polymorphism??

A: Polymorphism allows a client to treat di_erent objects in the same way even if they were created from di_erent classes and exhibit di_erent behaviors. You can use implementation inheritance to achieve polymorphism in languages such as C++ and Java. Base class object’s pointer can invoke methods in derived class objects. You can also achieve polymorphism in C++ by function overloading and operator overloading.

Q: What is problem with Runtime type identification?

A: The run time type identification comes at a cost of performance penalty. Compiler maintains the class.

Q: What is Polymorphism?

A: Polymorphism allows a client to treat different objects in the same way even if they were created from different classes and exhibit different behaviors. You can use implementation inheritance to achieve polymorphism in languages such as C++ and Java. Base class object’s pointer can invoke methods in derived class objects. You can also achieve polymorphism in C++ by function overloading and operator overloading.

Q: What is a class?

A: A class is an expanded concept of a data structure: instead of holding only data, it can hold both data and functions.

Q: What are the differences between a C++ struct and C++ class?

A: The default member and base class access specifies are different. This is one of the commonly misunderstood aspects of C++. Believe it or not, many programmers think that a C++ struct is just like a C struct, while a C++ class has inheritance, access specifes, member functions, overloaded operators, and so on. Actually, the C++ struct has all the features of the class. The only differences are that a struct defaults to public member access and public base class inheritance, and a class defaults to the private access specified and private base-class inheritance.

Q: How do you know that your class needs a virtual destructor?

A: If your class has at least one virtual function, you should make a destructor for this class virtual. This will allow you to delete a dynamic object through a caller to a base class object. If the destructor is non-virtual, then wrong destructor will be invoked during deletion of the dynamic object.

Q: What is encapsulation?

A: Containing and hiding Information about an object, such as internal data structures and code. Encapsulation isolates the internal complexity of an object’s operation from the rest of the application. For example, a client component asking for net revenue from a business object need not know the data’s origin.

Q: What is “this” pointer?

A: The this pointer is a pointer accessible only within the member functions of a class, struct, or union type. It points to the object for which the member function is called. Static member functions do not have a this pointer. When a nonstatic member function is called for an

object, the address of the object is passed as a hidden argument to the function. For example, the following function call

myDate.setMonth( 3 );

can be interpreted this way: setMonth( &myDate, 3 );

The object’s address is available from within the member function as the this pointer. It is legal, though unnecessary, to use the this pointer when referring to members of the class.

Q: What happens when you make call “delete this;”?

A: The code has two built-in pitfalls. First, if it executes in a member function for an extern, static, or automatic object, the program will probably crash as soon as the delete statement executes. There is no portable way for an object to tell that it was instantiated on the heap, so the class cannot assert that its object is properly instantiated. Second, when an object commits suicide this way, the using program might not know about its demise. As far as the instantiating program is concerned, the object remains in scope and continues to exist even though the object

did itself in. Subsequent dereferencing of the pointer can and usually does lead to disaster.

You should never do this. Since compiler does not know whether the object was allocated on the stack or on the heap, “delete this” could cause a disaster.

Q: What is assignment operator?

A: Default assignment operator handles assigning one object to another of the same class. Member to member copy (shallow copy)

Q: What are all the implicit member functions of the class? Or what are all the functions which compiler implements for us if we don’t define one?

A:

  • default ctor
  • copy ctor
  • assignment operator
  • default destructor
  • address operator

Q: What is a container class? What are the types of container classes?

A: A container class is a class that is used to hold objects in memory or external storage. A container class acts as a generic holder. A container class has a predefined behavior and a well- known interface. A container class is a supporting class whose purpose is to hide the topology used for maintaining the list of objects in memory. When a container class contains a group of mixed objects, the container is called a heterogeneous container; when the container is holding a group of objects that are all the same, the container is called a homogeneous container.

Q: What is Overriding?

A: To override a method, a subclass of the class that originally declared the method must declare a method with the same name, return type (or a subclass of that return type), and same parameter list.

The definition of the method overriding is:

  • Must have same method
  • Must have same data
  • Must have same argument

Overriding a method means that replacing a method functionality in child class. To imply overriding functionality we need parent and child classes. In the child class you define the same method signature as one defined in the parent class.

Q: How do you access the static member of a class?

A: ::

Q: What is a nested class? Why can it be useful?

A: A nested class is a class enclosed within the scope of another class. For example:

// Example 1: Nested class

//

class OuterClass

{

class NestedClass

{

// …

};

// …

};

Nested classes are useful for organizing code and controlling access and dependencies. Nested classes obey access rules just like other parts of a class do; so, in Example 1, if NestedClass is public then any code can name it as OuterClass::NestedClass. Often nested classes contain private implementation details, and are therefore made private; in Example 1, if NestedClass is private, then only OuterClass’s members and friends can use NestedClass. When you instantiate as outer class, it won’t instantiate inside class.

Q: What is a local class? Why can it be useful?

A:  Local class is a class defined within the scope of a function _ any function, whether a member function or a free function. For example:

// Example 2: Local class

//

int f()

{

class LocalClass

{

// …

};

// …

};

Like nested classes, local classes can be a useful tool for managing code dependencies.

Q: What a derived class can add?

A: New data members New member functions

New constructors and destructor New friends

Q: What happens when a derived-class object is created and destroyed?

A: Space is allocated (on the stack or the heap) for the full object (that is, enough space to store the data members inherited from the base class plus the data members defined in the derived class itself)

The base class’s constructor is called to initialize the data members inherited from the base class

The derived class’s constructor is then called to initialize the data members added in the derived class

The derived-class object is then usable

When the object is destroyed (goes out of scope or is deleted) the derived class’s destructor is called on the object first

Then the base class’s destructor is called on the object Finally the allocated space for the full object is reclaimed Q: How do I create a subscript operator for a Matrix class?

A: Use operator() rather than operator[].

When you have multiple subscripts, the cleanest way to do it is with operator() rather than with operator[]. The reason is that operator[] always takes exactly one parameter, but operator() can take any number of parameters (in the case of a rectangular matrix, two parameters are needed).

For example:

class Matrix { public:

Matrix(unsigned rows, unsigned cols);

double& operator() (unsigned row, unsigned col);  subscript operators often come in pairs double operator() (unsigned row, unsigned col) const;  subscript operators often come in pairs

~Matrix(); // Destructor

Matrix(const Matrix& m); // Copy constructor

Matrix& operator= (const Matrix& m); // Assignment operator

private:

unsigned rows_, cols_; double* data_;

};

inline

Matrix::Matrix(unsigned rows, unsigned cols)

: rows_ (rows)

, cols_ (cols)

//data_ <–initialized below (after the ‘if/throw’ statement)

{

if (rows == 0 || cols == 0)

throw BadIndex(“Matrix constructor has 0 size”); data_ = new double[rows * cols];

}

inline Matrix::~Matrix()

{

delete[] data_;

}

inline

double& Matrix::operator() (unsigned row, unsigned col)

{

if (row >= rows_ || col >= cols_)

throw BadIndex(“Matrix subscript out of bounds”); return data_[cols_*row + col];

}

inline

double Matrix::operator() (unsigned row, unsigned col) const

{

if (row >= rows_ || col >= cols_)

throw BadIndex(“const Matrix subscript out of bounds”); return data_[cols_*row + col];

}

Then you can access an element of Matrix m using m(i,j) rather than m[i][j]:

int main()

{

Matrix m(10,10); m(5,8) = 106.15;

std::cout << m(5,8);

}

Q: Why shouldn’t my Matrix class’s interface look like an array-of-array?

A: Here’s what this FAQ is really all about: Some people build a Matrix class that has an operator[] that returns a reference to an Array object (or perhaps to a raw array, shudder), and that Array object has an operator[] that returns an element of the Matrix (e.g., a reference to a double). Thus they access elements of the matrix using syntax like m[i][j] rather than syntax like m(i,j).

The array-of-array solution obviously works, but it is less flexible than the operator() approach. Specifically, there are easy performance tuning tricks that can be done with the operator() approach that are more difficult in the [][] approach, and therefore the [][] approach is more likely to lead to bad performance, at least in some cases.

For example, the easiest way to implement the [][] approach is to use a physical layout of the matrix as a dense matrix that is stored in row-major form (or is it column-major; I can’t ever

remember). In contrast, the operator() approach totally hides the physical layout of the matrix, and that can lead to better performance in some cases.

Put it this way: the operator() approach is never worse than, and sometimes better than, the [][] approach.

The operator() approach is never worse because it is easy to implement the dense, row-major physical layout using the operator() approach, so when that configuration happens to be the optimal layout from a performance standpoint, the operator() approach is just as easy as the [][] approach (perhaps the operator() approach is a tiny bit easier, but I won’t quibble over minor nits).

The operator() approach is sometimes better because whenever the optimal layout for a given application happens to be something other than dense, row-major, the implementation is often significantly easier using the operator() approach compared to the [][] approach.

As an example of when a physical layout makes a significant difference, a recent project happened to access the matrix elements in columns (that is, the algorithm accesses all the elements in one column, then the elements in another, etc.), and if the physical layout is row- major, the accesses can “stride the cache”. For example, if the rows happen to be almost as big as the processor’s cache size, the machine can end up with a “cache miss” for almost every element access. In this particular project, we got a 20% improvement in performance by changing the mapping from the logical layout (row,column) to the physical layout (column,row).

Of course there are many examples of this sort of thing from numerical methods, and sparse matrices are a whole other dimension on this issue. Since it is, in general, easier to implement a sparse matrix or swap row/column ordering using the operator() approach, the operator() approach loses nothing and may gain something it has no down-side and a potential up-side.

Use the operator() approach.

Q: Should I design my classes from the outside (interfaces first) or from the inside (data first)?

A: From the outside!

A good interface provides a simplified view that is expressed in the vocabulary of a user. In the case of OO software, the interface is normally the set of public methods of either a single class or a tight group of classes.

First think about what the object logically represents, not how you intend to physically build it. For example, suppose you have a Stack class that will be built by containing a LinkedList:

class Stack { public:

private: LinkedList list_;

};

Should the Stack have a get() method that returns the LinkedList? Or a set() method that takes a LinkedList? Or a constructor that takes a LinkedList? Obviously the answer is No, since you should design your interfaces from the outside-in. I.e., users of Stack objects don’t care about LinkedLists; they care about pushing and popping.

Now for another example that is a bit more subtle. Suppose class LinkedList is built using a linked list of Node objects, where each Node object has a pointer to the next Node:

class Node { /*…*/ };

class LinkedList { public:

private: Node* first_;

};

Should the LinkedList class have a get() method that will let users access the first Node? Should the Node object have a get() method that will let users follow that Node to the next Node in the chain? In other words, what should a LinkedList look like from the outside? Is a LinkedList really a chain of Node objects? Or is that just an implementation detail? And if it is just an implementation detail, how will the LinkedList let users access each of the elements in the LinkedList one at a time?

The key insight is the realization that a LinkedList is not a chain of Nodes. That may be how it is built, but that is not what it is. What it is is a sequence of elements. Therefore the LinkedList abstraction should provide a LinkedListIterator class as well, and that LinkedListIterator might have an operator++ to go to the next element, and it might have a get()/set() pair to access its value stored in the Node (the value in the Node element is solely the responsibility of the LinkedList user, which is why there is a get()/set() pair that allows the user to freely manipulate that value).

Starting from the user’s perspective, we might want our LinkedList class to support operations that look similar to accessing an array using pointer arithmetic:

void userCode(LinkedList& a)

{

for (LinkedListIterator p = a.begin(); p != a.end(); ++p) std::cout << *p << ‘\n’;

}

To implement this interface, LinkedList will need a begin() method and an end() method. These return a LinkedListIterator object. The LinkedListIterator will need a method to go forward, ++p; a method to access the current element, *p; and a comparison operator, p != a.end().

The code follows. The important thing to notice is that LinkedList does not have any methods

that let users access Nodes. Nodes are an implementation technique that is completely buried. This makes the LinkedList class safer (no chance a user will mess up the invariants and linkages between the various nodes), easier to use (users don’t need to expend extra effort keeping the node-count equal to the actual number of nodes, or any other infrastructure stuff), and more flexible (by changing a single typedef, users could change their code from using LinkedList to some other list-like class and the bulk of their code would compile cleanly and hopefully with improved performance characteristics).

#include // Poor man’s exception handling class LinkedListIterator;

class LinkedList;

class Node {

// No public members; this is a “private class” friend class LinkedListIterator; // A friend class friend class LinkedList;

Node* next_; int elem_;

};

class LinkedListIterator { public:

bool operator== (LinkedListIterator i) const; bool operator!= (LinkedListIterator i) const; void operator++ (); // Go to the next element int& operator* (); // Access the current element private:

LinkedListIterator(Node* p); Node* p_;

friend class LinkedList; // so LinkedList can construct a LinkedListIterator

};

class LinkedList { public:

void append(int elem); // Adds elem after the end

void prepend(int elem); // Adds elem before the beginning

LinkedListIterator begin(); LinkedListIterator end();

private: Node* first_;

};

Here are the methods that are obviously inlinable (probably in the same header file):

inline bool LinkedListIterator::operator== (LinkedListIterator i) const

{

return p_ == i.p_;

}

inline bool LinkedListIterator::operator!= (LinkedListIterator i) const

{

return p_ != i.p_;

}

inline void LinkedListIterator::operator++()

{

assert(p_ != NULL); // or if (p_==NULL) throw … p_ = p_->next_;

}

inline int& LinkedListIterator::operator*()

{

assert(p_ != NULL); // or if (p_==NULL) throw … return p_->elem_;

}

inline LinkedListIterator::LinkedListIterator(Node* p)

: p_(p)

{ }

inline LinkedListIterator LinkedList::begin()

{

return first_;

}

inline LinkedListIterator LinkedList::end()

{

return NULL;

}

Conclusion: The linked list had two different kinds of data. The values of the elements stored in the linked list are the responsibility of the user of the linked list (and only the user; the linked list itself makes no attempt to prohibit users from changing the third element to 5), and the linked list’s infrastructure data (next pointers, etc.), whose values are the responsibility of the linked list (and only the linked list; e.g., the linked list does not let users change (or even look at!) the various next pointers).

Thus the only get()/set() methods were to get and set the elements of the linked list, but not the infrastructure of the linked list. Since the linked list hides the infrastructure pointers/etc., it is

able to make very strong promises regarding that infrastructure (e.g., if it were a doubly linked list, it might guarantee that every forward pointer was matched by a backwards pointer from the next Node).

So, we see here an example of where the values of some of a class’s data is the responsibility of users (in which case the class needs to have get()/set() methods for that data) but the data that the class wants to control does not necessarily have get()/set() methods.

Note: the purpose of this example is not to show you how to write a linked-list class. In fact you should not “roll your own” linked-list class since you should use one of the “container classes” provided with your compiler. Ideally you’ll use one of the standard container classes such as the std::list template.

Q: What is the difference between new/delete and malloc/free?

A: Malloc/free do not know about constructors and destructors. New and delete create and destroy objects, while malloc and free allocate and deallocate memory.

Q: What is difference between new and malloc?

A: Both malloc and new functions are used for dynamic memory allocations and the basic difference is: malloc requires a special “typecasting” when it allocates memory for eg. if the pointer used is the char pointer then after the processor allocates memory then this allocated memory needs to be typecasted to char pointer i.e (char*).but new does not requires any typecasting. Also, free is the keyword used to free the memory while using malloc and delete the keyword to free memory while using new, otherwise this will lead the memory leak.

Q: What is the difference between delete and delete[]?

A: Whenever you allocate memory with new[], you have to free the memory using delete[]. When you allocate memory with ‘new’, then use ‘delete’ without the brackets. You use new[] to allocate an array of values (always starting at the index 0).

Q: What is difference between malloc()/free() and new/delete?

A: malloc allocates memory for object in heap but doesn’t invoke object’s constructor to initialize the object. new allocates memory and also invokes constructor to initialize the object. malloc() and free() do not support object semantics, does not construct and destruct objects

Eg. string * ptr = (string *)(malloc (sizeof(string))) Are not safe, and does not calculate the size of the objects that it construct

The following return a pointer to void int *p = (int *) (malloc(sizeof(int))); int *p = new int;

Are not extensible

new and delete can be overloaded in a class

“delete” first calls the object’s termination routine (i.e. its destructor) and then releases the space the object occupied on the heap memory. If an array of objects was created using new, then delete must be told that it is dealing with an array by preceding the name with an empty []:- Int_t *my_ints = new Int_t[10];

delete []my_ints;

Q: What is the difference between “new” and “operator new” ?

A:“operator new” works like malloc.

Q: What is Memory alignment??

A: The term alignment primarily means the tendency of an address pointer value to be a multiple of some power of two. So a pointer with two byte alignment has a zero in the least signi_cant bit. And a pointer with four byte alignment has a zero in both the two least signi_cant bits. And so on. More alignment means a longer sequence of zero bits in the lowest bits of a pointer.

Q: Is there a way to force new to allocate memory from a specific memory area?

A: Yes. The good news is that these “memory pools” are useful in a number of situations. The bad news is that I’ll have to drag you through the mire of how it works before we discuss all the uses. But if you don’t know about memory pools, it might be worthwhile to slog through this FAQ you might learn something useful!

First of all, recall that a memory allocator is simply supposed to return uninitialized bits of memory; it is not supposed to produce “objects.” In particular, the memory allocator is not supposed to set the virtual-pointer or any other part of the object, as that is the job of the constructor which runs after the memory allocator. Starting with a simple memory allocator function, allocate(), you would use placement new to construct an object in that memory. In other words, the following is morally equivalent to new Foo():

void* raw = allocate(sizeof(Foo)); // line 1 Foo* p = new(raw) Foo(); // line 2

Okay, assuming you’ve used placement new and have survived the above two lines of code, the next step is to turn your memory allocator into an object. This kind of object is called a “memory pool” or a “memory arena.” This lets your users have more than one “pool” or “arena” from which memory will be allocated. Each of these memory pool objects will allocate a big chunk of memory using some specific system call (e.g., shared memory, persistent memory, stack memory, etc.; see below), and will dole it out in little chunks as needed. Your memory-pool class might look something like this:

class Pool { public:

void* alloc(size_t nbytes); void dealloc(void* p); private:

…data members used in your pool object…

};

void* Pool::alloc(size_t nbytes)

{

…your algorithm goes here…

}

void Pool::dealloc(void* p)

{

…your algorithm goes here…

}

Now one of your users might have a Pool called pool, from which they could allocate objects like this:

Pool pool;

void* raw = pool.alloc(sizeof(Foo)); Foo* p = new(raw) Foo();

Or simply:

Foo* p = new(pool.alloc(sizeof(Foo))) Foo();

The reason it’s good to turn Pool into a class is because it lets users create N different pools of memory rather than having one massive pool shared by all users. That allows users to do lots of funky things. For example, if they have a chunk of the system that allocates memory like crazy then goes away, they could allocate all their memory from a Pool, then not even bother doing any deletes on the little pieces: just deallocate the entire pool at once. Or they could set up a “shared memory” area (where the operating system specifically provides memory that is shared between multiple processes) and have the pool dole out chunks of shared memory rather than process-local memory. Another angle: many systems support a non-standard function often called alloca() which allocates a block of memory from the stack rather than the heap. Naturally this block of memory automatically goes away when the function returns, eliminating the need for explicit deletes. Someone could use alloca() to give the Pool its big chunk of memory, then all the little pieces allocated from that Pool act like they’re local: they automatically vanish when the function returns. Of course the destructors don’t get called in some of these cases, and if the destructors do something nontrivial you won’t be able to use these techniques, but in cases where the destructor merely deallocates memory, these sorts of techniques can be useful.

Okay, assuming you survived the 6 or 8 lines of code needed to wrap your allocate function as a method of a Pool class, the next step is to change the syntax for allocating objects. The goal is to change from the rather clunky syntax new(pool.alloc(sizeof(Foo))) Foo() to the simpler syntax new(pool) Foo(). To make this happen, you need to add the following two lines of code just below the definition of your Pool class:

inline void* operator new(size_t nbytes, Pool& pool)

{

return pool.alloc(nbytes);

}

Now when the compiler sees new(pool) Foo(), it calls the above operator new and passes sizeof(Foo) and pool as parameters, and the only function that ends up using the funky pool.alloc(nbytes) method is your own operator new.

Now to the issue of how to destruct/deallocate the Foo objects. Recall that the brute force approach sometimes used with placement new is to explicitly call the destructor then explicitly deallocate the memory:

void sample(Pool& pool)

{

Foo* p = new(pool) Foo();

p->~Foo(); // explicitly call dtor pool.dealloc(p); // explicitly release the memory

}

This has several problems, all of which are fixable:

The memory will leak if Foo::Foo() throws an exception.

The destruction/deallocation syntax is different from what most programmers are used to, so they’ll probably screw it up.

Users must somehow remember which pool goes with which object. Since the code that allocates is often in a different function from the code that deallocates, programmers will have to pass around two pointers (a Foo* and a Pool*), which gets ugly fast (example, what if they had an array of Foos each of which potentially came from a different Pool; ugh).

We will fix them in the above order.

Problem #1: plugging the memory leak. When you use the “normal” new operator, e.g., Foo* p = new Foo(), the compiler generates some special code to handle the case when the constructor throws an exception. The actual code generated by the compiler is functionally similar to this:

// This is functionally what happens with Foo* p = new Foo() Foo* p;

// don’t catch exceptions thrown by the allocator itself void* raw = operator new(sizeof(Foo));

// catch any exceptions thrown by the ctor try {

p = new(raw) Foo(); // call the ctor with raw as this

}

catch (…) {

// oops, ctor threw an exception operator delete(raw);

throw; // rethrow the ctor’s exception

}

The point is that the compiler deallocates the memory if the ctor throws an exception. But in the case of the “new with parameter” syntax (commonly called “placement new”), the compiler won’t know what to do if the exception occurs so by default it does nothing:

// This is functionally what happens with Foo* p = new(pool) Foo(): void* raw = operator new(sizeof(Foo), pool);

// the above function simply returns “pool.alloc(sizeof(Foo))”

Foo* p = new(raw) Foo();

// if the above line “throws”, pool.dealloc(raw) is NOT called

So the goal is to force the compiler to do something similar to what it does with the global new operator. Fortunately it’s simple: when the compiler sees new(pool) Foo(), it looks for a corresponding operator delete. If it finds one, it does the equivalent of wrapping the ctor call in a try block as shown above. So we would simply provide an operator delete with the following signature (be careful to get this right; if the second parameter has a different type from the second parameter of the operator new(size_t, Pool&), the compiler doesn’t complain; it simply bypasses the try block when your users say new(pool) Foo()):

void operator delete(void* p, Pool& pool)

{

pool.dealloc(p);

}

After this, the compiler will automatically wrap the ctor calls of your new expressions in a try block:

// This is functionally what happens with Foo* p = new(pool) Foo() Foo* p;

// don’t catch exceptions thrown by the allocator itself void* raw = operator new(sizeof(Foo), pool);

// the above simply returns “pool.alloc(sizeof(Foo))”

// catch any exceptions thrown by the ctor try {

p = new(raw) Foo(); // call the ctor with raw as this

}

catch (…) {

// oops, ctor threw an exception

operator delete(raw, pool); // that’s the magical line!! throw; // rethrow the ctor’s exception

}

In other words, the one-liner function operator delete(void* p, Pool& pool) causes the compiler to automagically plug the memory leak. Of course that function can be, but doesn’t have to be, inline.

Problems #2 (“ugly therefore error prone”) and #3 (“users must manually associate pool-pointers with the object that allocated them, which is error prone”) are solved simultaneously with an additional 10-20 lines of code in one place. In other words, we add 10-20 lines of code in one place (your Pool header file) and simplify an arbitrarily large number of other places (every piece of code that uses your Pool class).

The idea is to implicitly associate a Pool* with every allocation. The Pool* associated with the global allocator would be NULL, but at least conceptually you could say every allocation has an associated Pool*. Then you replace the global operator delete so it looks up the associated Pool*, and if non-NULL, calls that Pool’s deallocate function. For example, if(!) the normal deallocator used free(), the replacment for the global operator delete would look something like this:

void operator delete(void* p)

{

if (p != NULL) {

Pool* pool = /* somehow get the associated ‘Pool*’ */; if (pool == null)

free(p); else

pool->dealloc(p);

}

}

If you’re not sure if the normal deallocator was free(), the easiest approach is also replace the global operator new with something that uses malloc(). The replacement for the global operator new would look something like this (note: this definition ignores a few details such as the new_handler loop and the throw std::bad_alloc() that happens if we run out of memory):

void* operator new(size_t nbytes)

{

if (nbytes == 0)

nbytes = 1; // so all alloc’s get a distinct address void* raw = malloc(nbytes);

…somehow associate the NULL ‘Pool*’ with ‘raw’… return raw;

}

The only remaining problem is to associate a Pool* with an allocation. One approach, used in at least one commercial product, is to use a std::map. In other words, build a look-up table whose keys are the allocation-pointer and whose values are the associated Pool*. For reasons I’ll describe in a moment, it is essential that you insert a key/value pair into the map only in operator new(size_t,Pool&). In particular, you must not insert a key/value pair from the global operator new (e.g., you must not say, poolMap[p] = NULL in the global operator new). Reason: doing that would create a nasty chicken-and-egg problem since std::map probably uses the global operator new, it ends up inserting a new entry every time inserts a new entry, leading to infinite recursion bang you’re dead.

Even though this technique requires a std::map look-up for each deallocation, it seems to have acceptable performance, at least in many cases.

Another approach that is faster but might use more memory and is a little trickier is to prepend a

Pool* just before all allocations. For example, if nbytes was 24, meaning the caller was asking to allocate 24 bytes, we would allocate 28 (or 32 if you think the machine requires 8-byte alignment for things like doubles and/or long longs), stuff the Pool* into the first 4 bytes, and return the pointer 4 (or 8) bytes from the beginning of what you allocated. Then your global operator delete backs off the 4 (or 8) bytes, finds the Pool*, and if NULL, uses free() otherwise calls pool-

>dealloc(). The parameter passed to free() and pool->dealloc() would be the pointer 4 (or 8) bytes to the left of the original parameter, p. If(!) you decide on 4 byte alignment, your code would look something like this (although as before, the following operator new code elides the usual out-of-memory handlers):

void* operator new(size_t nbytes)

{

if (nbytes == 0)

nbytes = 1; // so all alloc’s get a distinct address

void* ans = malloc(nbytes + 4); // overallocate by 4 bytes

*(Pool**)ans = NULL; // use NULL in the global new return (char*)ans + 4; // don’t let users see the Pool*

}

void* operator new(size_t nbytes, Pool& pool)

{

if (nbytes == 0)

nbytes = 1; // so all alloc’s get a distinct address

void* ans = pool.alloc(nbytes + 4); // overallocate by 4 bytes

*(Pool**)ans = &pool; // put the Pool* here

return (char*)ans + 4; // don’t let users see the Pool*

}

void operator delete(void* p)

{

if (p != NULL) {

p = (char*)p – 4; // back off to the Pool* Pool* pool = *(Pool**)p;

if (pool == null)

free(p); // note: 4 bytes left of the original p else

pool->dealloc(p); // note: 4 bytes left of the original p

}

}

Naturally the last few paragraphs of this FAQ are viable only when you are allowed to change the global operator new and operator delete. If you are not allowed to change these global functions, the first three quarters of this FAQ is still applicable.

Q: How does free know the size of memory to be deleted.?

int *i = (int *)malloc(12); followed by free(i); how did free function call know how much of memory to delete?

A: It depends on the implementation, but there is usually a malloc header added to all the memory allocated through malloc. on Linux its 4 bytes of memory preceding the memory returned to you, whihc contains the number of bytes allocated + 4(itself).

so when you say,

int *i = (int *)malloc(12); it allocates 16 bytes.

——————-

17 | | | |

——————-

^

| i

As you can see above total of 16 bytes are allocated, first 4 bytes stores the number of bytes allocated(offset to the next memory from the start of this memory).

address of the 5th byte is returned to i. now i can access 12 bytes from this byte.

Q: How do I allocate multidimensional arrays using new?

A: There are many ways to do this, depending on how flexible you want the array sizing to be. On one extreme, if you know all the dimensions at compile-time, you can allocate multidimensional arrays statically (as in C):

class Fred { /*…*/ };

void someFunction(Fred& fred);

void manipulateArray()

{

const unsigned nrows = 10; // Num rows is a compile-time constant const unsigned ncols = 20; // Num columns is a compile-time constant Fred matrix[nrows][ncols];

for (unsigned i = 0; i < nrows; ++i) { for (unsigned j = 0; j < ncols; ++j) {

// Here’s the way you access the (i,j) element: someFunction( matrix[i][j] );

// You can safely “return” without any special delete code: if (today == “Tuesday” && moon.isFull())

return; // Quit early on Tuesdays when the moon is full

}

}

// No explicit delete code at the end of the function either

}

More commonly, the size of the matrix isn’t known until run-time but you know that it will be rectangular. In this case you need to use the heap (“freestore”), but at least you are able to allocate all the elements in one freestore chunk.

void manipulateArray(unsigned nrows, unsigned ncols)

{

Fred* matrix = new Fred[nrows * ncols];

// Since we used a simple pointer above, we need to be VERY

// careful to avoid skipping over the delete code.

// That’s why we catch all exceptions: try {

// Here’s how to access the (i,j) element: for (unsigned i = 0; i < nrows; ++i) { for (unsigned j = 0; j < ncols; ++j) { someFunction( matrix[i*ncols + j] );

}

}

// If you want to quit early on Tuesdays when the moon is full,

// make sure to do the delete along ALL return paths: if (today == “Tuesday” && moon.isFull()) {

delete[] matrix; return;

}

…insert code here to fiddle with the matrix…

}

catch (…) {

// Make sure to do the delete when an exception is thrown: delete[] matrix;

throw; // Re-throw the current exception

}

// Make sure to do the delete at the end of the function too: delete[] matrix;

}

Finally at the other extreme, you may not even be guaranteed that the matrix is rectangular. For example, if each row could have a different length, you’ll need to allocate each row individually. In the following function, ncols[i] is the number of columns in row number i, where i varies between 0 and nrows-1 inclusive.

void manipulateArray(unsigned nrows, unsigned ncols[])

{

typedef Fred* FredPtr;

// There will not be a leak if the following throws an exception: FredPtr* matrix = new FredPtr[nrows];

// Set each element to NULL in case there is an exception later.

// (See comments at the top of the try block for rationale.) for (unsigned i = 0; i < nrows; ++i)

matrix[i] = NULL;

// Since we used a simple pointer above, we need to be

// VERY careful to avoid skipping over the delete code.

// That’s why we catch all exceptions: try {

// Next we populate the array. If one of these throws, all

// the allocated elements will be deleted (see catch below). for (unsigned i = 0; i < nrows; ++i)

matrix[i] = new Fred[ ncols[i] ];

// Here’s how to access the (i,j) element: for (unsigned i = 0; i < nrows; ++i) { for (unsigned j = 0; j < ncols[i]; ++j) { someFunction( matrix[i][j] );

}

}

// If you want to quit early on Tuesdays when the moon is full,

// make sure to do the delete along ALL return paths: if (today == “Tuesday” && moon.isFull()) {

for (unsigned i = nrows; i > 0; –i) delete[] matrix[i-1];

delete[] matrix; return;

}

…insert code here to fiddle with the matrix…

}

catch (…) {

// Make sure to do the delete when an exception is thrown:

// Note that some of these matrix[…] pointers might be

// NULL, but that’s okay since it’s legal to delete NULL. for (unsigned i = nrows; i > 0; –i)

delete[] matrix[i-1]; delete[] matrix;

throw; // Re-throw the current exception

}

// Make sure to do the delete at the end of the function too.

// Note that deletion is the opposite order of allocation: for (unsigned i = nrows; i > 0; –i)

delete[] matrix[i-1]; delete[] matrix;

}

Note the funny use of matrix[i-1] in the deletion process. This prevents wrap-around of the unsigned value when i goes one step below zero.

Finally, note that pointers and arrays are evil. It is normally much better to encapsulate your pointers in a class that has a safe and simple interface. The following FAQ shows how to do this.

Q: Can I free() pointers allocated with new? Can I delete pointers allocated with malloc()?

A: No!

It is perfectly legal, moral, and wholesome to use malloc() and delete in the same program, or to use new and free() in the same program. But it is illegal, immoral, and despicable to call free() with a pointer allocated via new, or to call delete on a pointer allocated via malloc().

Beware! I occasionally get e-mail from people telling me that it works OK for them on machine X and compiler Y. Just because they don’t see bad symptoms in a simple test case doesn’t mean it won’t crash in the field. Even if they know it won’t crash on their particular compiler doesn’t mean it will work safely on another compiler, another platform, or even another version of the same compiler.

Beware! Sometimes people say, “But I’m just working with an array of char.” Nonetheless do not mix malloc() and delete on the same pointer, or new and free() on the same pointer! If you allocated via p = new char[n], you must use delete[] p; you must not use free(p). Or if you allocated via p = malloc(n), you must use free(p); you must not use delete[] p or delete p! Mixing these up could cause a catastrophic failure at runtime if the code was ported to a new machine, a new compiler, or even a new version of the same compiler.

You have been warned.

Q: Why should I use new instead of trustworthy old malloc()?

A: Constructors/destructors, type safety, overridability.

Constructors/destructors: unlike malloc(sizeof(Fred)), new Fred() calls Fred’s constructor. Similarly, delete p calls *p’s destructor.

Type safety: malloc() returns a void* which isn’t type safe. new Fred() returns a pointer of the right type (a Fred*).

Overridability: new is an operator that can be overridden by a class, while malloc() is not overridable on a per-class basis.

Q: Can I use realloc() on pointers allocated via new?

A: No!

When realloc() has to copy the allocation, it uses a bitwise copy operation, which will tear many C++ objects to shreds. C++ objects should be allowed to copy themselves. They use their own copy constructor or assignment operator.

Besides all that, the heap that new uses may not be the same as the heap that malloc() and realloc() use!

Q: Do I need to check for NULL after p = new Fred()?

A: No! (But if you have an old compiler, you may have to force the new operator to throw an exception if it runs out of memory.)

It turns out to be a real pain to always write explicit NULL tests after every new allocation. Code like the following is very tedious:

Fred* p = new Fred(); if (p == NULL)

throw std::bad_alloc();

If your compiler doesn’t support (or if you refuse to use) exceptions, your code might be even more tedious:

Fred* p = new Fred(); if (p == NULL) {

std::cerr << “Couldn’t allocate memory for a Fred” << std::endl; abort();

}

Take heart. In C++, if the runtime system cannot allocate sizeof(Fred) bytes of memory during p

= new Fred(), a std::bad_alloc exception will be thrown. Unlike malloc(), new never returns NULL!

Therefore you should simply write:

Fred* p = new Fred(); // No need to check if p is NULL

However, if your compiler is old, it may not yet support this. Find out by checking your compiler’s documentation under “new”. If you have an old compiler, you may have to force the compiler to have this behavior.

Note: If you are using Microsoft Visual C++, to get new to throw an exception when it fails you must #include some standard header in at least one of your .cpp files. For example, you could

#include (or or or …).

Q: How can I convince my (older) compiler to automatically check new to see if it returns NULL?

A: Eventually your compiler will.

If you have an old compiler that doesn’t automagically perform the NULL test, you can force the runtime system to do the test by installing a “new handler” function. Your “new handler” function can do anything you want, such as throw an exception, delete some objects and return (in which case operator new will retry the allocation), print a message and abort() the program, etc.

Here’s a sample “new handler” that prints a message and throws an exception. The handler is installed using std::set_new_handler():

#include // To get std::set_new_handler

#include // To get abort()

#include // To get std::cerr

class alloc_error : public std::exception { public:

alloc_error() : exception() { }

};

void myNewHandler()

{

// This is your own handler. It can do anything you want. throw alloc_error();

}

int main()

{

std::set_new_handler(myNewHandler); // Install your “new handler”

}

After the std::set_new_handler() line is executed, operator new will call your myNewHandler() if/when it runs out of memory. This means that new will never return NULL:

Fred* p = new Fred(); // No need to check if p is NULL

Note: If your compiler doesn’t support exception handling, you can, as a last resort, change the line throw …; to:

std::cerr << “Attempt to allocate memory failed!” << std::endl; abort();

Note: If some global/static object’s constructor uses new, it won’t use the myNewHandler() function since that constructor will get called before main() begins. Unfortunately there’s no convenient way to guarantee that the std::set_new_handler() will be called before the first use of new. For example, even if you put the std::set_new_handler() call in the constructor of a global object, you still don’t know if the module (“compilation unit”) that contains that global object will be elaborated first or last or somewhere inbetween. Therefore you still don’t have any guarantee that your call of std::set_new_handler() will happen before any other global’s constructor gets invoked.

Q: Do I need to check for NULL before delete p?

A: No!

The C++ language guarantees that delete p will do nothing if p is equal to NULL. Since you might get the test backwards, and since most testing methodologies force you to explicitly test every branch point, you should not put in the redundant if test.

Wrong:

if (p != NULL)

delete p; Right: delete p;

Q: What are the two steps that happen when I say delete p?

A: N delete p is a two-step process: it calls the destructor, then releases the memory. The code generated for delete p is functionally similar to this (assuming p is of type Fred*):

// Original code: delete p; if (p != NULL) {

p->~Fred(); operator delete(p);

}

The statement p->~Fred() calls the destructor for the Fred object pointed to by p.

The statement operator delete(p) calls the memory deallocation primitive, void operator

delete(void* p). This primitive is similar in spirit to free(void* p). (Note, however, that these two are not interchangeable; e.g., there is no guarantee that the two memory deallocation primitives even use the same heap!)

Q: In p = new Fred(), does the Fred memory “leak” if the Fred constructor throws an exception?

A: No.

If an exception occurs during the Fred constructor of p = new Fred(), the C++ language guarantees that the memory sizeof(Fred) bytes that were allocated will automagically be released back to the heap.

Here are the details: new Fred() is a two-step process:

sizeof(Fred) bytes of memory are allocated using the primitive void* operator new(size_t nbytes). This primitive is similar in spirit to malloc(size_t nbytes). (Note, however, that these two are not interchangeable; e.g., there is no guarantee that the two memory allocation primitives even use the same heap!).

It constructs an object in that memory by calling the Fred constructor. The pointer returned from the first step is passed as the this parameter to the constructor. This step is wrapped in a try … catch block to handle the case when an exception is thrown during this step.

Thus the actual generated code is functionally similar to:

// Original code: Fred* p = new Fred();

Fred* p = (Fred*) operator new(sizeof(Fred)); try {

new(p) Fred(); // Placement new

}

catch (…) {

operator delete(p); // Deallocate the memory throw; // Re-throw the exception

}

The statement marked “Placement new” calls the Fred constructor. The pointer p becomes the this pointer inside the constructor, Fred::Fred().

Q: How do I allocate / unallocate an array of things?

A: Use p = new T[n] and delete[] p:

Fred* p = new Fred[100];

delete[] p;

Any time you allocate an array of objects via new (usually with the [n] in the new expression), you must use [] in the delete statement. This syntax is necessary because there is no syntactic difference between a pointer to a thing and a pointer to an array of things (something we inherited from C).

Q: What if I forget the [] when deleteing array allocated via new T[n]?

A: All life comes to a catastrophic end.

It is the programmer’s not the compiler’s responsibility to get the connection between new T[n] and delete[] p correct. If you get it wrong, neither a compile-time nor a run-time error message will be generated by the compiler. Heap corruption is a likely result. Or worse. Your program will probably die.

Q: Can I drop the [] when deleteing array of some built-in type (char, int, etc)?

A: No!

Sometimes programmers think that the [] in the delete[] p only exists so the compiler will call the appropriate destructors for all elements in the array. Because of this reasoning, they assume that an array of some built-in type such as char or int can be deleted without the []. E.g., they assume the following is valid code:

void userCode(int n)

{

char* p = new char[n];

delete p; // ERROR! Should be delete[] p !

}

But the above code is wrong, and it can cause a disaster at runtime. In particular, the code that’s called for delete p is operator delete(void*), but the code that’s called for delete[] p is operator delete[](void*). The default behavior for the latter is to call the former, but users are allowed to replace the latter with a different behavior (in which case they would normally also replace the corresponding new code in operator new[](size_t)). If they replaced the delete[] code so it wasn’t compatible with the delete code, and you called the wrong one (i.e., if you said delete p rather than delete[] p), you could end up with a disaster at runtime.

Q: After p = new Fred[n], how does the compiler know there are n objects to be destructed during delete[] p?

A: Short answer: Magic.

Long answer: The run-time system stores the number of objects, n, somewhere where it can be retrieved if you only know the pointer, p. There are two popular techniques that do this. Both these techniques are in use by commercial-grade compilers, both have tradeoffs, and neither is perfect. These techniques are:

Over-allocate the array and put n just to the left of the first Fred object. Use an associative array with p as the key and n as the value.

Q: Is it legal (and moral) for a member function to say delete this?

A: As long as you’re careful, it’s OK for an object to commit suicide (delete this).

Here’s how I define “careful”:

You must be absolutely 100% positive sure that this object was allocated via new (not by new[], nor by placement new, nor a local object on the stack, nor a global, nor a member of another object; but by plain ordinary new).

You must be absolutely 100% positive sure that your member function will be the last member function invoked on this object.

You must be absolutely 100% positive sure that the rest of your member function (after the delete this line) doesn’t touch any piece of this object (including calling any other member functions or touching any data members).

You must be absolutely 100% positive sure that no one even touches the this pointer itself after the delete this line. In other words, you must not examine it, compare it with another pointer, compare it with NULL, print it, cast it, do anything with it.

Naturally the usual caveats apply in cases where your this pointer is a pointer to a base class when you don’t have a virtual destructor.

Advertisements