Namespaces

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 namespace blocks with the same name are allowed. All declarations within those blocks are declared in the named scope.

Syntax

Explanation

Namespaces

Namespace definitions are only allowed at namespace scope, including the global scope.

To reopen an existing namespace (formally, to be an extension-namespace-definition), the lookup for the identifier used in the namespace definition must resolve to a namespace name (not a namespace alias), that was declared as a member of the enclosing namespace or of an inline namespace within an enclosing namespace.

The namespace-body defines a namespace scope, which affects name lookup.

All names introduced by the declarations that appear within namespace-body (including nested namespace definitions) become members of the namespace identifier, whether this namespace definition is the original namespace definition (which introduced identifier), or an extension namespace definition (which "reopened" the already defined namespace).

A namespace member that was declared within a namespace body may be defined or redeclared outside of it using explicit qualification.

namespace Q
{
    namespace V   // V is a member of Q, and is fully defined within Q
    { // namespace Q::V { // C++17 alternative to the lines above
        class C { void m(); }; // C is a member of V and is fully defined within V
                               // C::m is only declared
        void f(); // f is a member of V, but is only declared here
    }
 
    void V::f() // definition of V's member f outside of V
                // f's enclosing namespaces are still the global namespace, Q, and Q::V
    {
        extern void h(); // This declares ::Q::V::h
    }
 
    void V::C::m() // definition of V::C::m outside of the namespace (and the class body)
                   // enclosing namespaces are the global namespace, Q, and Q::V
    {}
}

Out-of-namespace definitions and redeclarations are only allowed.

• after the point of declaration,
• at namespace scope, and
• in namespaces that enclose the original namespace (including the global namespace).

namespace Q
{
    namespace V    // original-namespace-definition for V
    {
        void f();  // declaration of Q::V::f
    }
 
    void V::f() {} // OK
    void V::g() {} // Error: g() is not yet a member of V
 
    namespace V    // extension-namespace-definition for V
    {
        void g();  // declaration of Q::V::g
    }
}
 
namespace R           // not an enclosing namespace for Q
{
    void Q::V::g() {} // Error: cannot define Q::V::g inside R
}
 
void Q::V::g() {}     // OK: global namespace encloses Q

Names introduced by friend declarations within a non-local class X become members of the innermost enclosing namespace of X, but they do not become visible to ordinary name lookup (neither unqualified nor qualified) unless a matching declaration is provided at namespace scope, either before or after the class definition. Such name may be found through ADL which considers both namespaces and classes.

Only the innermost enclosing namespace is considered by such friend declaration when deciding whether the name would conflict with a previously declared name.

void h(int);
namespace A
{
    class X
    {
        friend void f(X);       // A::f is a friend
 
        class Y
        {
            friend void g();    // A::g is a friend
            friend void h(int); // A::h is a friend, no conflict with ::h
        };
    };
    // A::f, A::g and A::h are not visible at namespace scope
    // even though they are members of the namespace A
 
    X x;
    void g()  // definition of A::g
    {
        f(x); // A::X::f is found through ADL
    }
 
    void f(X) {}   // definition of A::f
    void h(int) {} // definition of A::h
    // A::f, A::g and A::h are now visible at namespace scope
    // and they are also friends of A::X and A::X::Y
}

// in C++14, std::literals and its member namespaces are inline
{
    using namespace std::string_literals; // makes visible operator""s 
                                          // from std::literals::string_literals
    auto str = "abc"s;
}
 
{
    using namespace std::literals; // makes visible both
                                   // std::literals::string_literals::operator""s
                                   // and std::literals::chrono_literals::operator""s
    auto str = "abc"s;
    auto min = 60s;
}
 
{
    using std::operator""s; // makes both std::literals::string_literals::operator""s
                            // and std::literals::chrono_literals::operator""s visible
    auto str = "abc"s;
    auto min = 60s;
}

namespace Lib
{
    inline namespace Lib_1
    {
        template<typename T> class A; 
    }
 
    template<typename T> void g(T) { /* ... */ }
}
/* ... */
struct MyClass { /* ... */ };
namespace Lib
{
    template<> class A<MyClass> { /* ... */ };
}
 
int main()
{
    Lib::A<MyClass> a;
    g(a);  // ok, Lib is an associated namespace of A
}

Unnamed namespaces

The unnamed-namespace-definition is a namespace definition of the form.

This definition is treated as a definition of a namespace with unique name and a using-directive in the current scope that nominates this unnamed namespace (Note: implicitly added using directive makes namespace available for the qualified name lookup and unqualified name lookup, but not for the argument-dependent lookup). The unique name is unique over the entire program, but within a translation unit each unnamed namespace definition maps to the same unique name: multiple unnamed namespace definitions in the same scope denote the same unnamed namespace.

namespace
{
    int i; // defines ::(unique)::i
}
 
void f()
{
    i++;   // increments ::(unique)::i
}
 
namespace A
{
    namespace
    {
        int i;        // A::(unique)::i
        int j;        // A::(unique)::j
    }
 
    void g() { i++; } // A::(unique)::i++
}
 
using namespace A; // introduces all names from A into global namespace
 
void h()
{
    i++;    // error: ::(unique)::i and ::A::(unique)::i are both in scope
    A::i++; // ok, increments ::A::(unique)::i
    j++;    // ok, increments ::A::(unique)::j
}

Using-declarations

Introduces a name that is defined elsewhere into the declarative region where this using-declaration appears.

Using-declarations can be used to introduce namespace members into other namespaces and block scopes, or to introduce base class members into derived class definitions, or to introduce enumerators into namespaces, block, and class scopes (since C++20).

For the use in derived class definitions, see using declaration.

Names introduced into a namespace scope by a using-declaration can be used just like any other names, including qualified lookup from other scopes:

void f();
namespace A
{
    void g();
}
 
namespace X
{
    using ::f;        // global f is now visible as ::X::f
    using A::g;       // A::g is now visible as ::X::g
    using A::g, A::g; // (C++17) OK: double declaration allowed at namespace scope
}
 
void h()
{
    X::f(); // calls ::f
    X::g(); // calls A::g
}

If, after the using-declaration was used to take a member from a namespace, the namespace is extended and additional declarations for the same name are introduced, those additional declarations do not become visible through the using-declaration (in contrast with using-directive). One exception is when a using-declaration names a class template: partial specializations introduced later are effectively visible, because their lookup proceeds through the primary template.

namespace A
{
    void f(int);
}
using A::f; // ::f is now a synonym for A::f(int)
 
namespace A       // namespace extension
{
    void f(char); // does not change what ::f means
}
 
void foo()
{
    f('a'); // calls f(int), even though f(char) exists.
}
 
void bar()
{
    using A::f; // this f is a synonym for both A::f(int) and A::f(char)
    f('a');     // calls f(char)
}

Using-declarations cannot name template-id, or namespace, or a scoped enumerator (until C++20). Each declarator in a using-declaration introduces one and only one name, for example using-declaration for an enumeration does not introduce any of its enumerators.

All restrictions on regular declarations of the same names, hiding, and overloading rules apply to using-declarations:

namespace A
{
    int x;
}
 
namespace B
{
    int i;
    struct g {};
    struct x {};
 
    void f(int);
    void f(double);
    void g(char); // OK: function name g hides struct g
}
 
void func()
{
    int i;
    using B::i;   // error: i declared twice
 
    void f(char);
    using B::f;   // OK: f(char), f(int), f(double) are overloads
    f(3.5);       // calls B::f(double)
 
    using B::g;
    g('a');       // calls B::g(char)
    struct g g1;  // declares g1 to have type struct B::g
 
    using B::x;
    using A::x;   // OK: hides struct B::x
    x = 99;       // assigns to A::x
    struct x x1;  // declares x1 to have type struct B::x
}

If a function was introduced by a using-declaration, declaring a function with the same name and parameter list is ill-formed (unless the declaration is for the same function). If a function template was introduced by a using-declaration, declaring a function template with the same name, parameter type list, return type, and template parameter list is ill-formed. Two using-declarations can introduce functions with the same name and parameter list, but if a call to that function is attempted, the program is ill-formed.

namespace B
{
    void f(int);
    void f(double);
}
 
namespace C
{
    void f(int);
    void f(double);
    void f(char);
}
 
void h()
{
    using B::f;  // introduces B::f(int), B::f(double)
    using C::f;  // introduces C::f(int), C::f(double), and C::f(char)
    f('h');      // calls C::f(char)
    f(1);        // error: B::f(int) or C::f(int)?
    void f(int); // error: f(int) conflicts with C::f(int) and B::f(int)
}

If an entity is declared, but not defined in some inner namespace, and then declared through using-declaration in the outer namespace, and then a definition appears in the outer namespace with the same unqualified name, that definition is a member of the outer namespace and conflicts with the using-declaration:

namespace X
{
    namespace M
    {
        void g(); // declares, but doesn't define X::M::g()
    }
    using M::g;
 
    void g();     // Error: attempt to declare X::g which conflicts with X::M::g()
}

More generally, a declaration that appears in any namespace scope and introduces a name using an unqualified identifier always introduces a member into the namespace it's in and not to any other namespace. The exceptions are explicit instantiations and explicit specializations of a primary template that is defined in an inline namespace: because they do not introduce a new name, they may use unqualified-id in an enclosing namespace.

Using-directives

A using-directive is a block-declaration with the following syntax:

Using-directives are allowed only in namespace scope and in block scope. From the point of view of unqualified name lookup of any name after a using-directive and until the end of the scope in which it appears, every name from namespace-name is visible as if it were declared in the nearest enclosing namespace which contains both the using-directive and namespace-name.

Using-directive does not add any names to the declarative region in which it appears (unlike the using-declaration), and thus does not prevent identical names from being declared.

Using-directives are transitive for the purposes of unqualified lookup: if a scope contains a using-directive that nominates a namespace-name, which itself contains using-directive for some namespace-name-2, the effect is as if the using directives from the second namespace appear within the first. The order in which these transitive namespaces occur does not influence name lookup.

namespace A
{
    int i;
}
 
namespace B
{
    int i;
    int j;
 
    namespace C
    {
        namespace D
        {
            using namespace A; // all names from A injected into global namespace
 
            int j;
            int k;
            int a = i;         // i is B::i, because A::i is hidden by B::i
        }
 
        using namespace D; // names from D are injected into C
                           // names from A are injected into global namespace
 
        int k = 89; // OK to declare name identical to one introduced by a using
        int l = k;  // ambiguous: C::k or D::k
        int m = i;  // ok: B::i hides A::i
        int n = j;  // ok: D::j hides B::j
    }
}

If, after a using-directive was used to nominate some namespace, the namespace is extended and additional members and/or using-directives are added to it, those additional members and the additional namespaces are visible through the using-directive (in contrast with using-declaration).

namespace D
{
    int d1;
    void f(char);
}
using namespace D; // introduces D::d1, D::f, D::d2, D::f,
                   // E::e, and E::f into global namespace!
 
int d1;            // OK: no conflict with D::d1 when declaring
 
namespace E
{
    int e;
    void f(int);
}
 
namespace D            // namespace extension
{
    int d2;
    using namespace E; // transitive using-directive
    void f(int);
}
 
void f()
{
    d1++;    // error: ambiguous ::d1 or D::d1?
    ::d1++;  // OK
    D::d1++; // OK
    d2++;    // OK, d2 is D::d2
 
    e++;     // OK: e is E::e due to transitive using
 
    f(1);    // error: ambiguous: D::f(int) or E::f(int)?
    f('a');  // OK: the only f(char) is D::f(char)
}

Notes

The using-directive using namespace std; at any namespace scope introduces every name from the namespace std into the global namespace (since the global namespace is the nearest namespace that contains both std and any user-declared namespace), which may lead to undesirable name collisions. This, and other using directives are generally considered bad practice at file scope of a header file (SF.7: Don’t write using namespace at global scope in a header file).

Example

#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
    }
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also