Scope Resolution Operator: ::

Artikel
01/03/2018

The latest version of this topic can be found at 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

:: identifier  
class-name :: identifier  
namespace :: identifier  
enum class :: identifier  
enum struct :: identifier

Remarks

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

With 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 B;  
  
}  
int main() {  
    NamespaceB::ClassB c_b;  
    NamespaceC::ClassB c_c;  
  
    c_b.x = 3;  
    c_c.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  ;  
}

With 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();   
}

With 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;  
}

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