Scope resolution operator in C++

Difficulty Level : Medium
Last Updated : 06 Aug, 2019

In C++, scope resolution operator is ::. It is used for following purposes.

1) To access a global variable when there is a local variable with same name:

// C++ program to show that we can access a global variable
// using scope resolution operator :: when there is a local 
// variable with same name 
#include<iostream> 
using namespace std;
   
int x;  // Global x
   
int main()
{
  int x = 10; // Local x
  cout << "Value of global x is " << ::x;
  cout << "\nValue of local x is " << x;  
  return 0;
}

Output:

Value of global x is 0
Value of local x is 10

2) To define a function outside a class.

// C++ program to show that scope resolution operator :: is used
// to define a function outside a class
#include<iostream> 
using namespace std;
  
class A 
{
public: 
  
   // Only declaration
   void fun();
};
  
// Definition outside class using ::
void A::fun()
{
   cout << "fun() called";
}
  
int main()
{
   A a;
   a.fun();
   return 0;
}

Output:

fun() called

3) To access a class’s static variables.

// C++ program to show that :: can be used to access static
// members when there is a local variable with same name
#include<iostream>
using namespace std;
   
class Test
{
    static int x;  
public:
    static int y;   
  
    // Local parameter 'a' hides class member
    // 'a', but we can access it using ::
    void func(int x)  
    { 
       // We can access class's static variable
       // even if there is a local variable
       cout << "Value of static x is " << Test::x;
  
       cout << "\nValue of local x is " << x;  
    }
};
   
// In C++, static members must be explicitly defined 
// like this
int Test::x = 1;
int Test::y = 2;
   
int main()
{
    Test obj;
    int x = 3 ;
    obj.func(x);
   
    cout << "\nTest::y = " << Test::y;
  
    return 0;
}

Output:

Value of static x is 1
Value of local x is 3
Test::y = 2;

4) In case of multiple Inheritance:
If same variable name exists in two ancestor classes, we can use scope resolution operator to distinguish.

// Use of scope resolution operator in multiple inheritance.
#include<iostream>
using namespace std;
  
class A
{
protected:
    int x;
public:
    A() { x = 10; }
};
  
class B
{
protected:
    int x;
public:
    B() { x = 20; }
};
  
class C: public A, public B
{
public:
   void fun()
   {
      cout << "A's x is " << A::x;
      cout << "\nB's x is " << B::x;
   }
};
  
int main()
{
    C c;
    c.fun();
    return 0;
}

Output:

A's x is 10
B's x is 20

5) For namespace
If a class having the same name exists inside two namespace we can use the namespace name with the scope resolution operator to refer that class without any conflicts

// Use of scope resolution operator for namespace.
#include<iostream>
  
  
int main(){
    std::cout << "Hello" << std::endl;
  
}

Here, cout and endl belong to the std namespace.

6) Refer to a class inside another class:
If a class exists inside another class we can use the nesting class to refer the nested class using the scope resolution operator

// Use of scope resolution class inside another class.
#include<iostream>
using namespace std;
  
class outside
{
public:
      int x;
      class inside
      {
      public:
            int x;
            static int y; 
            int foo();
  
      };
};
int outside::inside::y = 5; 
  
int main(){
    outside A;
    outside::inside B;
  
}

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