Polymorphism in CPP

Polymorphism

Before getting any deeper into this chapter, you should have a proper understanding of pointers and class inheritance. If you are not really sure of the meaning of any of the following expressions, you should review the indicated sections:

Statement:	Explained in:
int A::b(int c) { }	Classes
a->b	Data structures
class A: public B {};	Friendship and inheritance

Pointers to base class

One of the key features of class inheritance is that a pointer to a derived class is type-compatible with a pointer to its base class. Polymorphism is the art of taking advantage of this simple but powerful and versatile feature.

The example about the rectangle and triangle classes can be rewritten using pointers taking this feature into account:

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// pointers to base class #include <iostream> using namespace std; class Polygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } }; class Rectangle: public Polygon { public: int area() { return width*height; } }; class Triangle: public Polygon { public: int area() { return width*height/2; } }; int main () { Rectangle rect; Triangle trgl; Polygon * ppoly1 = &rect; Polygon * ppoly2 = &trgl; ppoly1->set_values (4,5); ppoly2->set_values (4,5); cout << rect.area() << '\n'; cout << trgl.area() << '\n'; return 0; }

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Function main declares two pointers to Polygon (named ppoly1 and ppoly2). These are assigned the addresses of rect and trgl, respectively, which are objects of type Rectangle and Triangle. Such assignments are valid, since both Rectangle and Triangle are classes derived from Polygon.

Dereferencing ppoly1 and ppoly2 (with *ppoly1 and *ppoly2) is valid and allows us to access the members of their pointed objects. For example, the following two statements would be equivalent in the previous example:

1 2	ppoly1->set_values (4,5); rect.set_values (4,5);

But because the type of ppoly1 and ppoly2 is pointer to Polygon (and not pointer to Rectangle nor pointer to Triangle), only the members inherited from Polygon can be accessed, and not those of the derived classes Rectangle and Triangle. That is why the program above accesses the area members of both objects using rect and trgl directly, instead of the pointers; the pointers to the base class cannot access the area members.

Member area could have been accessed with the pointers to Polygon if area were a member of Polygon instead of a member of its derived classes, but the problem is that Rectangle and Triangle implement different versions of area, therefore there is not a single common version that could be implemented in the base class.

Virtual members

A virtual member is a member function that can be redefined in a derived class, while preserving its calling properties through references. The syntax for a function to become virtual is to precede its declaration with the virtual keyword:

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// virtual members #include <iostream> using namespace std; class Polygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } virtual int area () { return 0; } }; class Rectangle: public Polygon { public: int area () { return width * height; } }; class Triangle: public Polygon { public: int area () { return (width * height / 2); } }; int main () { Rectangle rect; Triangle trgl; Polygon poly; Polygon * ppoly1 = &rect; Polygon * ppoly2 = &trgl; Polygon * ppoly3 = &poly; ppoly1->set_values (4,5); ppoly2->set_values (4,5); ppoly3->set_values (4,5); cout << ppoly1->area() << '\n'; cout << ppoly2->area() << '\n'; cout << ppoly3->area() << '\n'; return 0; }

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In this example, all three classes (Polygon, Rectangle and Triangle) have the same members: width, height, and functions set_values and area.

The member function area has been declared as virtual in the base class because it is later redefined in each of the derived classes. Non-virtual members can also be redefined in derived classes, but non-virtual members of derived classes cannot be accessed through a reference of the base class: i.e., if virtual is removed from the declaration of area in the example above, all three calls to area would return zero, because in all cases, the version of the base class would have been called instead.

Therefore, essentially, what the virtual keyword does is to allow a member of a derived class with the same name as one in the base class to be appropriately called from a pointer, and more precisely when the type of the pointer is a pointer to the base class that is pointing to an object of the derived class, as in the above example.

A class that declares or inherits a virtual function is called a polymorphic class.

Note that despite of the virtuality of one of its members, Polygon was a regular class, of which even an object was instantiated (poly), with its own definition of member area that always returns 0.

Abstract base classes

Abstract base classes are something very similar to the Polygon class in the previous example. They are classes that can only be used as base classes, and thus are allowed to have virtual member functions without definition (known as pure virtual functions). The syntax is to replace their definition by =0 (and equal sign and a zero):

An abstract base Polygon class could look like this:

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// abstract class CPolygon class Polygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } virtual int area () =0; };

Notice that area has no definition; this has been replaced by =0, which makes it a pure virtual function. Classes that contain at least one pure virtual function are known as abstract base classes.

Abstract base classes cannot be used to instantiate objects. Therefore, this last abstract base class version of Polygon could not be used to declare objects like:

Polygon mypolygon; // not working if Polygon is abstract base class

But an abstract base class is not totally useless. It can be used to create pointers to it, and take advantage of all its polymorphic abilities. For example, the following pointer declarations would be valid:

1 2	Polygon * ppoly1; Polygon * ppoly2;

And can actually be dereferenced when pointing to objects of derived (non-abstract) classes. Here is the entire example:

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// abstract base class #include <iostream> using namespace std; class Polygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } virtual int area (void) =0; }; class Rectangle: public Polygon { public: int area (void) { return (width * height); } }; class Triangle: public Polygon { public: int area (void) { return (width * height / 2); } }; int main () { Rectangle rect; Triangle trgl; Polygon * ppoly1 = &rect; Polygon * ppoly2 = &trgl; ppoly1->set_values (4,5); ppoly2->set_values (4,5); cout << ppoly1->area() << '\n'; cout << ppoly2->area() << '\n'; return 0; }

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In this example, objects of different but related types are referred to using a unique type of pointer (Polygon*) and the proper member function is called every time, just because they are virtual. This can be really useful in some circumstances. For example, it is even possible for a member of the abstract base class Polygon to use the special pointer this to access the proper virtual members, even though Polygon itself has no implementation for this function:

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// pure virtual members can be called // from the abstract base class #include <iostream> using namespace std; class Polygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } virtual int area() =0; void printarea() { cout << this->area() << '\n'; } }; class Rectangle: public Polygon { public: int area (void) { return (width * height); } }; class Triangle: public Polygon { public: int area (void) { return (width * height / 2); } }; int main () { Rectangle rect; Triangle trgl; Polygon * ppoly1 = &rect; Polygon * ppoly2 = &trgl; ppoly1->set_values (4,5); ppoly2->set_values (4,5); ppoly1->printarea(); ppoly2->printarea(); return 0; }

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Virtual members and abstract classes grant C++ polymorphic characteristics, most useful for object-oriented projects. Of course, the examples above are very simple use cases, but these features can be applied to arrays of objects or dynamically allocated objects.

Here is an example that combines some of the features in the latest chapters, such as dynamic memory, constructor initializers and polymorphism:

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// dynamic allocation and polymorphism #include <iostream> using namespace std; class Polygon { protected: int width, height; public: Polygon (int a, int b) : width(a), height(b) {} virtual int area (void) =0; void printarea() { cout << this->area() << '\n'; } }; class Rectangle: public Polygon { public: Rectangle(int a,int b) : Polygon(a,b) {} int area() { return width*height; } }; class Triangle: public Polygon { public: Triangle(int a,int b) : Polygon(a,b) {} int area() { return width*height/2; } }; int main () { Polygon * ppoly1 = new Rectangle (4,5); Polygon * ppoly2 = new Triangle (4,5); ppoly1->printarea(); ppoly2->printarea(); delete ppoly1; delete ppoly2; return 0; }

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Notice that the ppoly pointers:

1 2	Polygon * ppoly1 = new Rectangle (4,5); Polygon * ppoly2 = new Triangle (4,5);

are declared being of type "pointer to Polygon", but the objects allocated have been declared having the derived class type directly (Rectangle and Triangle).
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Friendship and inheritance Used In Cpp

Friend functions

In principle, private and protected members of a class cannot be accessed from outside the same class in which they are declared. However, this rule does not affect friends.

Friends are functions or classes declared with the friend keyword.

If we want to declare an external function as friend of a class, thus allowing this function to have access to the private and protected members of this class, we do it by declaring a prototype of this external function within the class, and preceding it with the keyword friend:

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// friend functions #include <iostream> using namespace std; class CRectangle { int width, height; public: void set_values (int, int); int area () {return (width * height);} friend CRectangle duplicate (CRectangle); }; void CRectangle::set_values (int a, int b) { width = a; height = b; } CRectangle duplicate (CRectangle rectparam) { CRectangle rectres; rectres.width = rectparam.width*2; rectres.height = rectparam.height*2; return (rectres); } int main () { CRectangle rect, rectb; rect.set_values (2,3); rectb = duplicate (rect); cout << rectb.area(); return 0; }

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The duplicate function is a friend of CRectangle. From within that function we have been able to access the members width and height of different objects of type CRectangle, which are private members. Notice that neither in the declaration of duplicate() nor in its later use in main() have we considered duplicate a member of classCRectangle. It isn't! It simply has access to its private and protected members without being a member.

The friend functions can serve, for example, to conduct operations between two different classes. Generally, the use of friend functions is out of an object-oriented programming methodology, so whenever possible it is better to use members of the same class to perform operations with them. Such as in the previous example, it would have been shorter to integrate duplicate() within the class CRectangle.

Friend classes

Just as we have the possibility to define a friend function, we can also define a class as friend of another one, granting that first class access to the protected and private members of the second one.

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// friend class #include <iostream> using namespace std; class CSquare; class CRectangle { int width, height; public: int area () {return (width * height);} void convert (CSquare a); }; class CSquare { private: int side; public: void set_side (int a) {side=a;} friend class CRectangle; }; void CRectangle::convert (CSquare a) { width = a.side; height = a.side; } int main () { CSquare sqr; CRectangle rect; sqr.set_side(4); rect.convert(sqr); cout << rect.area(); return 0; }

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In this example, we have declared CRectangle as a friend of CSquare so that CRectangle member functions could have access to the protected and private members of CSquare, more concretely to CSquare::side, which describes the side width of the square.

You may also see something new at the beginning of the program: an empty declaration of class CSquare. This is necessary because within the declaration of CRectangle we refer to CSquare (as a parameter in convert()). The definition of CSquare is included later, so if we did not include a previous empty declaration for CSquare this class would not be visible from within the definition of CRectangle.

Consider that friendships are not corresponded if we do not explicitly specify so. In our example, CRectangle is considered as a friend class by CSquare, but CRectangle does not consider CSquare to be a friend, so CRectanglecan access the protected and private members of CSquare but not the reverse way. Of course, we could have declared also CSquare as friend of CRectangle if we wanted to.

Another property of friendships is that they are not transitive: The friend of a friend is not considered to be a friend unless explicitly specified.

Inheritance between classes

A key feature of C++ classes is inheritance. Inheritance allows to create classes which are derived from other classes, so that they automatically include some of its "parent's" members, plus its own. For example, we are going to suppose that we want to declare a series of classes that describe polygons like our CRectangle, or likeCTriangle. They have certain common properties, such as both can be described by means of only two sides: height and base.

This could be represented in the world of classes with a class CPolygon from which we would derive the two other ones: CRectangle and CTriangle.

 
The class CPolygon would contain members that are common for both types of polygon. In our case: width andheight. And CRectangle and CTriangle would be its derived classes, with specific features that are different from one type of polygon to the other.

Classes that are derived from others inherit all the accessible members of the base class. That means that if a base class includes a member A and we derive it to another class with another member called B, the derived class will contain both members A and B.

In order to derive a class from another, we use a colon (:) in the declaration of the derived class using the following format: 

class derived_class_name: public base_class_name
{ /*...*/ };
Where derived_class_name is the name of the derived class and base_class_name is the name of the class on which it is based. The public access specifier may be replaced by any one of the other access specifiers protected andprivate. This access specifier limits the most accessible level for the members inherited from the base class: The members with a more accessible level are inherited with this level instead, while the members with an equal or more restrictive access level keep their restrictive level in the derived class.

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// derived classes #include <iostream> using namespace std; class CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b;} }; class CRectangle: public CPolygon { public: int area () { return (width * height); } }; class CTriangle: public CPolygon { public: int area () { return (width * height / 2); } }; int main () { CRectangle rect; CTriangle trgl; rect.set_values (4,5); trgl.set_values (4,5); cout << rect.area() << endl; cout << trgl.area() << endl; return 0; }

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The objects of the classes CRectangle and CTriangle each contain members inherited from CPolygon. These are:width, height and set_values().

The protected access specifier is similar to private. Its only difference occurs in fact with inheritance. When a class inherits from another one, the members of the derived class can access the protected members inherited from the base class, but not its private members.

Since we wanted width and height to be accessible from members of the derived classes CRectangle andCTriangle and not only by members of CPolygon, we have used protected access instead of private.

We can summarize the different access types according to who can access them in the following way: 

Access	public	protected	private
members of the same class	yes	yes	yes
members of derived classes	yes	yes	no
not members	yes	no	no

Where "not members" represent any access from outside the class, such as from main(), from another class or from a function.

In our example, the members inherited by CRectangle and CTriangle have the same access permissions as they had in their base class CPolygon:

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CPolygon::width // protected access CRectangle::width // protected access CPolygon::set_values() // public access CRectangle::set_values() // public access

This is because we have used the public keyword to define the inheritance relationship on each of the derived classes:

class CRectangle: public CPolygon { ... }

This public keyword after the colon (:) denotes the most accessible level the members inherited from the class that follows it (in this case CPolygon) will have. Since public is the most accessible level, by specifying this keyword the derived class will inherit all the members with the same levels they had in the base class.

If we specify a more restrictive access level like protected, all public members of the base class are inherited as protected in the derived class. Whereas if we specify the most restricting of all access levels: private, all the base class members are inherited as private.

For example, if daughter was a class derived from mother that we defined as:

class daughter: protected mother;

This would set protected as the maximum access level for the members of daughter that it inherited from mother. That is, all members that were public in mother would become protected in daughter. Of course, this would not restrict daughter to declare its own public members. That maximum access level is only set for the members inherited from mother.

If we do not explicitly specify any access level for the inheritance, the compiler assumes private for classes declared with class keyword and public for those declared with struct.

What is inherited from the base class?

In principle, a derived class inherits every member of a base class except:

• its constructor and its destructor
• its operator=() members
• its friends

Although the constructors and destructors of the base class are not inherited themselves, its default constructor (i.e., its constructor with no parameters) and its destructor are always called when a new object of a derived class is created or destroyed.

If the base class has no default constructor or you want that an overloaded constructor is called when a new derived object is created, you can specify it in each constructor definition of the derived class:

derived_constructor_name (parameters) : base_constructor_name (parameters) {...}
For example: 

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// constructors and derived classes #include <iostream> using namespace std; class mother { public: mother () { cout << "mother: no parameters\n"; } mother (int a) { cout << "mother: int parameter\n"; } }; class daughter : public mother { public: daughter (int a) { cout << "daughter: int parameter\n\n"; } }; class son : public mother { public: son (int a) : mother (a) { cout << "son: int parameter\n\n"; } }; int main () { daughter cynthia (0); son daniel(0); return 0; }

mother: no parameters daughter: int parameter mother: int parameter son: int parameter

Notice the difference between which mother's constructor is called when a new daughter object is created and which when it is a son object. The difference is because the constructor declaration of daughter and son:

1 2	daughter (int a) // nothing specified: call default son (int a) : mother (a) // constructor specified: call this

Multiple inheritance

In C++ it is perfectly possible that a class inherits members from more than one class. This is done by simply separating the different base classes with commas in the derived class declaration. For example, if we had a specific class to print on screen (COutput) and we wanted our classes CRectangle and CTriangle to also inherit its members in addition to those of CPolygon we could write: 

1 2	class CRectangle: public CPolygon, public COutput; class CTriangle: public CPolygon, public COutput;

here is the complete example: 

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// multiple inheritance #include <iostream> using namespace std; class CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b;} }; class COutput { public: void output (int i); }; void COutput::output (int i) { cout << i << endl; } class CRectangle: public CPolygon, public COutput { public: int area () { return (width * height); } }; class CTriangle: public CPolygon, public COutput { public: int area () { return (width * height / 2); } }; int main () { CRectangle rect; CTriangle trgl; rect.set_values (4,5); trgl.set_values (4,5); rect.output (rect.area()); trgl.output (trgl.area()); return 0; }

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Classes (II) Used In Cpp

Overloading operators

C++ incorporates the option to use standard operators to perform operations with classes in addition to with fundamental types. For example:

1 2	int a, b, c; a = b + c;

This is obviously valid code in C++, since the different variables of the addition are all fundamental types. Nevertheless, it is not so obvious that we could perform an operation similar to the following one:

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struct { string product; float price; } a, b, c; a = b + c;

In fact, this will cause a compilation error, since we have not defined the behavior our class should have with addition operations. However, thanks to the C++ feature to overload operators, we can design classes able to perform operations using standard operators. Here is a list of all the operators that can be overloaded:

Overloadable operators
+ - * / = < > += -= *= /= << >> <<= >>= == != <= >= ++ -- % & ^ ! | ~ &= ^= |= && || %= [] () , ->* -> new delete new[] delete[]

To overload an operator in order to use it with classes we declare operator functions, which are regular functions whose names are the operator keyword followed by the operator sign that we want to overload. The format is:

type operator sign (parameters) { /*...*/ }
Here you have an example that overloads the addition operator (+). We are going to create a class to store bidimensional vectors and then we are going to add two of them: a(3,1) and b(1,2). The addition of two bidimensional vectors is an operation as simple as adding the two x coordinates to obtain the resulting xcoordinate and adding the two y coordinates to obtain the resulting y. In this case the result will be (3+1,1+2) = (4,3).

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// vectors: overloading operators example #include <iostream> using namespace std; class CVector { public: int x,y; CVector () {}; CVector (int,int); CVector operator + (CVector); }; CVector::CVector (int a, int b) { x = a; y = b; } CVector CVector::operator+ (CVector param) { CVector temp; temp.x = x + param.x; temp.y = y + param.y; return (temp); } int main () { CVector a (3,1); CVector b (1,2); CVector c; c = a + b; cout << c.x << "," << c.y; return 0; }

4,3

It may be a little confusing to see so many times the CVector identifier. But, consider that some of them refer to the class name (type) CVector and some others are functions with that name (constructors must have the same name as the class). Do not confuse them:

1 2	CVector (int, int); // function name CVector (constructor) CVector operator+ (CVector); // function returns a CVector

The function operator+ of class CVector is the one that is in charge of overloading the addition operator (+). This function can be called either implicitly using the operator, or explicitly using the function name:

1 2	c = a + b; c = a.operator+ (b);

Both expressions are equivalent.

Notice also that we have included the empty constructor (without parameters) and we have defined it with an empty block:

CVector () { };

This is necessary, since we have explicitly declared another constructor:

CVector (int, int);

And when we explicitly declare any constructor, with any number of parameters, the default constructor with no parameters that the compiler can declare automatically is not declared, so we need to declare it ourselves in order to be able to construct objects of this type without parameters. Otherwise, the declaration:

CVector c;

included in main() would not have been valid.

Anyway, I have to warn you that an empty block is a bad implementation for a constructor, since it does not fulfill the minimum functionality that is generally expected from a constructor, which is the initialization of all the member variables in its class. In our case this constructor leaves the variables x and y undefined. Therefore, a more advisable definition would have been something similar to this:

CVector () { x=0; y=0; };

which in order to simplify and show only the point of the code I have not included in the example.

As well as a class includes a default constructor and a copy constructor even if they are not declared, it also includes a default definition for the assignment operator (=) with the class itself as parameter. The behavior which is defined by default is to copy the whole content of the data members of the object passed as argument (the one at the right side of the sign) to the one at the left side:

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CVector d (2,3); CVector e; e = d; // copy assignment operator

The copy assignment operator function is the only operator member function implemented by default. Of course, you can redefine it to any other functionality that you want, like for example, copy only certain class members or perform additional initialization procedures.

The overload of operators does not force its operation to bear a relation to the mathematical or usual meaning of the operator, although it is recommended. For example, the code may not be very intuitive if you use operator + to subtract two classes or operator== to fill with zeros a class, although it is perfectly possible to do so.

Although the prototype of a function operator+ can seem obvious since it takes what is at the right side of the operator as the parameter for the operator member function of the object at its left side, other operators may not be so obvious. Here you have a table with a summary on how the different operator functions have to be declared (replace @ by the operator in each case):

Expression	Operator	Member function	Global function
@a	+ - * & ! ~ ++ --	A::operator@()	operator@(A)
a@	++ --	A::operator@(int)	operator@(A,int)
a@b	+ - * / % ^ & | < > == != <= >= << >> && || ,	A::operator@ (B)	operator@(A,B)
a@b	= += -= *= /= %= ^= &= |= <<= >>= []	A::operator@ (B)	-
a(b, c...)	()	A::operator() (B, C...)	-
a->x	->	A::operator->()	-

Where a is an object of class A, b is an object of class B and c is an object of class C.

You can see in this panel that there are two ways to overload some class operators: as a member function and as a global function. Its use is indistinct, nevertheless I remind you that functions that are not members of a class cannot access the private or protected members of that class unless the global function is its friend (friendship is explained later).

The keyword this

The keyword this represents a pointer to the object whose member function is being executed. It is a pointer to the object itself.

One of its uses can be to check if a parameter passed to a member function is the object itself. For example,

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// this #include <iostream> using namespace std; class CDummy { public: int isitme (CDummy& param); }; int CDummy::isitme (CDummy& param) { if (&param == this) return true; else return false; } int main () { CDummy a; CDummy* b = &a; if ( b->isitme(a) ) cout << "yes, &a is b"; return 0; }

yes, &a is b

It is also frequently used in operator= member functions that return objects by reference (avoiding the use of temporary objects). Following with the vector's examples seen before we could have written an operator= function similar to this one:

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CVector& CVector::operator= (const CVector& param) { x=param.x; y=param.y; return *this; }

In fact this function is very similar to the code that the compiler generates implicitly for this class if we do not include an operator= member function to copy objects of this class.

Static members

A class can contain static members, either data or functions.

Static data members of a class are also known as "class variables", because there is only one unique value for all the objects of that same class. Their content is not different from one object of this class to another. 

For example, it may be used for a variable within a class that can contain a counter with the number of objects of that class that are currently allocated, as in the following example:

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// static members in classes #include <iostream> using namespace std; class CDummy { public: static int n; CDummy () { n++; }; ~CDummy () { n--; }; }; int CDummy::n=0; int main () { CDummy a; CDummy b[5]; CDummy * c = new CDummy; cout << a.n << endl; delete c; cout << CDummy::n << endl; return 0; }

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In fact, static members have the same properties as global variables but they enjoy class scope. For that reason, and to avoid them to be declared several times, we can only include the prototype (its declaration) in the class declaration but not its definition (its initialization). In order to initialize a static data-member we must include a formal definition outside the class, in the global scope, as in the previous example:

int CDummy::n=0;

Because it is a unique variable value for all the objects of the same class, it can be referred to as a member of any object of that class or even directly by the class name (of course this is only valid for static members):

1 2	cout << a.n; cout << CDummy::n;

These two calls included in the previous example are referring to the same variable: the static variable n within class CDummy shared by all objects of this class.

Once again, I remind you that in fact it is a global variable. The only difference is its name and possible access restrictions outside its class.

Just as we may include static data within a class, we can also include static functions. They represent the same: they are global functions that are called as if they were object members of a given class. They can only refer to static data, in no case to non-static members of the class, as well as they do not allow the use of the keywordthis, since it makes reference to an object pointer and these functions in fact are not members of any object but direct members of the class.
Credits:-www.cplusplus.com