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Thinking in C++
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The using declaration

You can inject names one at a time into the current scope with a using declaration. Unlike the using directive, which treats names as if they were declared globally to the scope, a using declaration is a declaration within the current scope. This means it can override names from a using directive:

//: C10:UsingDeclaration.h
namespace U {
  inline void f() {}
  inline void g() {}
namespace V {
  inline void f() {}
  inline void g() {}
#endif // USINGDECLARATION_H ///:~
//: C10:UsingDeclaration1.cpp
#include "UsingDeclaration.h"
void h() {
  using namespace U; // Using directive
  using V::f; // Using declaration
  f(); // Calls V::f();
  U::f(); // Must fully qualify to call
int main() {} ///:~

The using declaration just gives the fully specified name of the identifier, but no type information. This means that if the namespace contains a set of overloaded functions with the same name, the using declaration declares all the functions in the overloaded set.

You can put a using declaration anywhere a normal declaration can occur. A using declaration works like a normal declaration in all ways but one: because you don’t give an argument list, it’s possible for a using declaration to cause the overload of a function with the same argument types (which isn’t allowed with normal overloading). This ambiguity, however, doesn’t show up until the point of use, rather than the point of declaration.

A using declaration can also appear within a namespace, and it has the same effect as anywhere else – that name is declared within the space:

//: C10:UsingDeclaration2.cpp
#include "UsingDeclaration.h"
namespace Q {
  using U::f;
  using V::g;
  // ...
void m() {
  using namespace Q;
  f(); // Calls U::f();
  g(); // Calls V::g();
int main() {} ///:~

A using declaration is an alias, and it allows you to declare the same function in separate namespaces. If you end up re-declaring the same function by importing different namespaces, it’s OK – there won’t be any ambiguities or duplications.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire