static member functions in C++

Static data members in C++

Predict the output of following C++ program:

#include <iostream>
using namespace std;

class A
{
public:
    A() { cout << "A's Constructor Called " << endl;  }
};

class B
{
    static A a;
public:
    B() { cout << "B's Constructor Called " << endl; }
};

int main()
{
    B b;
    return 0;
}

Output:

B's Constructor Called

The above program calls only B’s constructor, it doesn’t call A’s constructor. The reason for this is simple, static members are only declared in class declaration, not defined. They must be explicitly defined outside the class using scope resolution operator.
If we try to access static member ‘a’ without explicit definition of it, we will get compilation error. For example, following program fails in compilation.

#include <iostream>
using namespace std;

class A
{
    int x;
public:
    A() { cout << "A's constructor called " << endl;  }
};

class B
{
    static A a;
public:
    B() { cout << "B's constructor called " << endl; }
    static A getA() { return a; }
};

int main()
{
    B b;
    A a = b.getA();
    return 0;
}

Output:

Compiler Error: undefined reference to `B::a' 

If we add definition of a, the program will works fine and will call A’s constructor. See the following program.

#include <iostream>
using namespace std;

class A
{
    int x;
public:
    A() { cout << "A's constructor called " << endl;  }
};

class B
{
    static A a;
public:
    B() { cout << "B's constructor called " << endl; }
    static A getA() { return a; }
};

A B::a;  // definition of a

int main()
{
    B b1, b2, b3;
    A a = b1.getA();

    return 0;
}

Output:

A's constructor called
B's constructor called
B's constructor called
B's constructor called

Note that the above program calls B’s constructor 3 times for 3 objects (b1, b2 and b3), but calls A’s constructor only once. The reason is, static members are shared among all objects. That is why they are also known as class members or class fields. Also, static members can be accessed without any object, see the below program where static member ‘a’ is accessed without any object.

#include <iostream>
using namespace std;

class A
{
    int x;
public:
    A() { cout << "A's constructor called " << endl;  }
};

class B
{
    static A a;
public:
    B() { cout << "B's constructor called " << endl; }
    static A getA() { return a; }
};

A B::a;  // definition of a

int main()
{
    // static member 'a' is accessed without any object of B
    A a = B::getA();

    return 0;
}

Output:

A's constructor called

Some interesting facts about static member functions in C++

1) static member functions do not have this pointer.
For example following program fails in compilation with error “`this’ is unavailable for static member functions “

#include<iostream>
class Test {      
   static Test * fun() {
     return this; // compiler error
   }
};
 
int main()
{
   getchar();
   return 0;
}

2) A static member function cannot be virtual (See this G-Fact)
3) Member function declarations with the same name and the name parameter-type-list cannot be overloaded if any of them is a static member function declaration.
For example, following program fails in compilation with error “‘void Test::fun()’ and `static void Test::fun()’ cannot be overloaded ”

#include<iostream>
class Test {
   static void fun() {}
   void fun() {} // compiler error
};
 
int main()
{
   getchar();
   return 0;
}

4) A static member function can not be declared const, volatile, or const volatile.
For example, following program fails in compilation with error “static member function `static void Test::fun()’ cannot have `const’ method qualifier ”

#include<iostream>
class Test {      
   static void fun() const { // compiler error
     return;
   }
};
 
int main()
{
   getchar();
   return 0;
}