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Namespaces

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Namespaces provide a method for preventing name conflicts in large projects.

Symbols declared inside a namespace block are placed in a named scope that prevents them from being mistaken for identically-named symbols in other scopes.

Multiple declarations of namespaces with the same name are allowed, resulting in a namespace including all symbols from all such declarations.

Contents

Syntax

namespace ns_name { declarations } (1)
inline namespace ns_name { declarations } (2) (since C++11)
namespace { declarations } (3)
ns_name::name (4)
using namespace ns_name; (5)
using ns_name::name; (6)

Explanation

1) Declaration of the namespace ns_name.
2) Declaration of the namespace ns_name. Definitions will appear both inside ns_name and its enclosing namespace.
3) Unnamed namespace. Definitions have potential scope from their point of declaration to the end of the translation unit.
4) Standard way of accessing namespace content.
5) Making all symbols of a namespace accessible in the scope of the using directive.
6) Making a specific symbols of a namespace accessible in the scope of the using directive.

Example

This example shows how to use a namespace to create a class that already has been named in the std namespace.

#include <vector>
 
namespace vec {
 
    template< typename T >
    class vector {
        // ...
    };
 
} // of vec
 
int main()
{
    std::vector<int> v1; // Standard vector.
    vec::vector<int> v2; // User defined vector.
 
    v1 = v2; // Error: v1 and v2 are different object's type.
 
    {
        using namespace std;
        vector<int> v3; // Same as std::vector
        v1 = v3; // OK
    }
 
    {
        using vec::vector;
        vector<int> v4; // Same as vec::vector
        v2 = v4; // OK
    }
 
    return 0;
}


See also

namespace alias creates an alias of an existing namespace[edit]