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using Declaration

 

The latest version of this topic can be found at using Declaration.

The using declaration introduces a name into the declarative region in which the using declaration appears.

Syntax

  
      using [typename][::] nested-name-specifier unqualified-id  
using :: unqualified-id  

Remarks

The name becomes a synonym for an entity declared elsewhere. It allows an individual name from a specific namespace to be used without explicit qualification. This is in contrast to the using directive, which allows all the names in a namespace to be used without qualification. See using Directive for more information. This keyword is also used for type aliases.

Example

A using declaration can be used in a class definition.

// using_declaration1.cpp  
#include <stdio.h>  
class B {  
public:  
   void f(char) {  
      printf_s("In B::f()\n");  
   }  
  
   void g(char) {  
      printf_s("In B::g()\n");  
   }  
};  
  
class D : B {  
public:  
   using B::f;  
   using B::g;  
   void f(int) {  
      printf_s("In D::f()\n");  
      f('c');  
   }  
  
   void g(int) {  
      printf_s("In D::g()\n");  
      g('c');  
   }  
};  
  
int main() {  
   D myD;  
   myD.f(1);  
   myD.g('a');  
}  
In D::f
()  
In B::f
()  
In B::g
()  

Example

When used to declare a member, a using declaration must refer to a member of a base class.

// using_declaration2.cpp  
#include <stdio.h>  
  
class B {  
public:  
   void f(char) {  
      printf_s("In B::f()\n");  
   }  
  
   void g(char) {  
      printf_s("In B::g()\n");  
   }  
};  
  
class C {  
public:  
   int g();  
};  
  
class D2 : public B {  
public:  
   using B::f;   // ok: B is a base of D2  
   // using C::g;   // error: C isn't a base of D2  
};  
  
int main() {  
   D2 MyD2;  
   MyD2.f('a');  
}  
In B::f
()  

Example

Members declared with a using declaration can be referenced using explicit qualification. The :: prefix refers to the global namespace.

// using_declaration3.cpp  
#include <stdio.h>  
  
void f() {  
   printf_s("In f\n");  
}  
  
namespace A {  
   void g() {  
      printf_s("In A::g\n");  
   }  
}  
  
namespace X {  
   using ::f;   // global f  
   using A::g;   // A's g  
}  
  
void h() {  
   printf_s("In h\n");  
   X::f();   // calls ::f  
   X::g();   // calls A::g  
}  
  
int main() {  
   h();  
}  
In h  
In f  
In A::g  

Example

When a using declaration is made, the synonym created by the declaration refers only to definitions that are valid at the point of the using declaration. Definitions added to a namespace after the using declaration are not valid synonyms.

A name defined by a using declaration is an alias for its original name. It does not affect the type, linkage or other attributes of the original declaration.

// post_declaration_namespace_additions.cpp  
// compile with: /c  
namespace A {  
   void f(int) {}  
}  
  
using A::f;   // f is a synonym for A::f(int) only  
  
namespace A {  
   void f(char) {}  
}  
  
void f() {  
   f('a');   // refers to A::f(int), even though A::f(char) exists  
}  
  
void b() {  
   using A::f;   // refers to A::f(int) AND A::f(char)  
   f('a');   // calls A::f(char);  
}  

Example

With respect to functions in namespaces, if a set of local declarations and using declarations for a single name are given in a declarative region, they must all refer to the same entity, or they must all refer to functions.

// functions_in_namespaces1.cpp  
// C2874 expected  
namespace B {  
    int i;  
    void f(int);  
    void f(double);  
}  
  
void g() {  
    int i;  
    using B::i;   // error: i declared twice  
    void f(char);  
    using B::f;   // ok: each f is a function  
}  

In the example above, the using B::i statement causes a second int i to be declared in the g() function. The using B::f statement does not conflict with the f(char) function because the function names introduced by B::f have different parameter types.

Example

A local function declaration cannot have the same name and type as a function introduced by using declaration. For example:

// functions_in_namespaces2.cpp  
// C2668 expected  
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) and B::f(double)  
    using C::f;          // C::f(int), C::f(double), and C::f(char)  
    f('h');              // calls C::f(char)  
    f(1);                // C2668 ambiguous: B::f(int) or C::f(int)?  
    void f(int);         // C2883 conflicts with B::f(int) and C::f(int)  
}  

Example

With respect to inheritance, when a using declaration introduces a name from a base class into a derived class scope, member functions in the derived class override virtual member functions with the same name and argument types in the base class.

// using_declaration_inheritance1.cpp  
#include <stdio.h>  
struct B {  
   virtual void f(int) {  
      printf_s("In B::f(int)\n");  
   }  
  
   virtual void f(char) {  
      printf_s("In B::f(char)\n");  
   }  
  
   void g(int) {  
      printf_s("In B::g\n");  
   }  
  
   void h(int);  
};  
  
struct D : B {  
   using B::f;  
   void f(int) {   // ok: D::f(int) overrides B::f(int)  
      printf_s("In D::f(int)\n");  
   }  
  
   using B::g;  
   void g(char) {   // ok: there is no B::g(char)  
      printf_s("In D::g(char)\n");  
   }  
  
   using B::h;  
   void h(int) {}   // Note: D::h(int) hides non-virtual B::h(int)  
};  
  
void f(D* pd) {  
   pd->f(1);   // calls D::f(int)  
   pd->f('a');   // calls B::f(char)  
   pd->g(1);   // calls B::g(int)  
   pd->g('a');   // calls D::g(char)  
}  
  
int main() {  
   D * myd = new D();  
   f(myd);  
}  
In D::f(int)  
In B::f(char)  
In B::g  
In D::g(char)  

Example

All instances of a name mentioned in a using declaration must be accessible. In particular, if a derived class uses a using declaration to access a member of a base class, the member name must be accessible. If the name is that of an overloaded member function, then all functions named must be accessible.

See Member-Access Control, for more information on accessibility of members.

// using_declaration_inheritance2.cpp  
// C2876 expected  
class A {  
private:  
   void f(char);  
public:  
   void f(int);  
protected:  
   void g();  
};  
  
class B : public A {  
   using A::f;   // C2876: A::f(char) is inaccessible  
public:  
   using A::g;   // B::g is a public synonym for A::g  
};  

See Also

Namespaces
Keywords