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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