Scope resolution operator: ::

The scope resolution operator :: is used to identify and disambiguate identifiers used in different scopes. For more information about scope, see Scope.

Syntax

qualified-id:
 nested-name-specifier template_opt unqualified-id

nested-name-specifier:
 ::
 type-name ::
 namespace-name ::
 decltype-specifier ::
 nested-name-specifier identifier ::
 nested-name-specifier template_opt simple-template-id ::

unqualified-id:
 identifier
 operator-function-id
 conversion-function-id
 literal-operator-id
 ~ type-name
 ~ decltype-specifier
 template-id

Remarks

The identifier can be a variable, a function, or an enumeration value.

Use :: for classes and namespaces

The following example shows how the scope resolution operator is used with namespaces and classes:

namespace NamespaceA{
    int x;
    class ClassA {
    public:
        int x;
    };
}

int main() {

    // A namespace name used to disambiguate
    NamespaceA::x = 1;

    // A class name used to disambiguate
    NamespaceA::ClassA a1;
    a1.x = 2;
}

A scope resolution operator without a scope qualifier refers to the global namespace.

namespace NamespaceA{
    int x;
}

int x;

int main() {
    int x;

    // the x in main()
    x = 0;
    // The x in the global namespace
    ::x = 1;

    // The x in the A namespace
    NamespaceA::x = 2;
}

You can use the scope resolution operator to identify a member of a namespace, or to identify a namespace that nominates the member's namespace in a using directive. In the example below, you can use NamespaceC to qualify ClassB, even though ClassB was declared in namespace NamespaceB, because NamespaceB was nominated in NamespaceC by a using directive.

namespace NamespaceB {
    class ClassB {
    public:
        int x;
    };
}

namespace NamespaceC{
    using namespace NamespaceB;
}

int main() {
    NamespaceB::ClassB b_b;
    NamespaceC::ClassB c_b;

    b_b.x = 3;
    c_b.x = 4;
}

You can use chains of scope resolution operators. In the following example, NamespaceD::NamespaceD1 identifies the nested namespace NamespaceD1, and NamespaceE::ClassE::ClassE1 identifies the nested class ClassE1.

namespace NamespaceD{
    namespace NamespaceD1{
        int x;
    }
}

namespace NamespaceE{
    class ClassE{
    public:
        class ClassE1{
        public:
            int x;
        };
    };
}

int main() {
    NamespaceD:: NamespaceD1::x = 6;
    NamespaceE::ClassE::ClassE1 e1;
    e1.x = 7  ;
}

Use :: for static members

You must use the scope resolution operator to call static members of classes.

class ClassG {
public:
    static int get_x() { return x;}
    static int x;
};

int ClassG::x = 6;

int main() {

    int gx1 = ClassG::x;
    int gx2 = ClassG::get_x();
}

Use :: for scoped enumerations

The scoped resolution operator is also used with the values of a scoped enumeration Enumeration declarations, as in the following example:

enum class EnumA{
    First,
    Second,
    Third
};

int main() {
    EnumA enum_value = EnumA::First;
}

See also

C++ built-in operators, precedence, and associativity
Namespaces