Using-declaration

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

Explanation

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.

In namespace and block scope

Using-declaration introduces a member of another namespace into current namespace or block scope.

#include <iostream>
#include <string>
using std::string;
int main()
{
    string str = "Example";
    using std::cout;
    cout << str;
}

See namespace for details.

In class definition

Using-declaration introduces a member of a base class into the derived class definition, such as to expose a protected member of base as public member of derived. In this case, nested-name-specifier must name a base class of the one being defined. If the name is the name of an overloaded member function of the base class, all base class member functions with that name are introduced. If the derived class already has a member with the same name, parameter list, and qualifications, the derived class member hides or overrides (doesn't conflict with) the member that is introduced from the base class.

#include <iostream>
struct B {
    virtual void f(int) { std::cout << "B::f\n"; }
    void g(char)        { std::cout << "B::g\n"; }
    void h(int)         { std::cout << "B::h\n"; }
 protected:
    int m; // B::m is protected
    typedef int value_type;
};
 
struct D : B {
    using B::m; // D::m is public
    using B::value_type; // D::value_type is public
 
    using B::f;
    void f(int) { std::cout << "D::f\n"; } // D::f(int) overrides B::f(int)
    using B::g;
    void g(int) { std::cout << "D::g\n"; } // both g(int) and g(char) are visible
                                           // as members of D
    using B::h;
    void h(int) { std::cout << "D::h\n"; } // D::h(int) hides B::h(int)
};
 
int main()
{
    D d;
    B& b = d;
 
//    b.m = 2; // error, B::m is protected
    d.m = 1; // protected B::m is accessible as public D::m
    b.f(1); // calls derived f()
    d.f(1); // calls derived f()
    d.g(1); // calls derived g(int)
    d.g('a'); // calls base g(char)
    b.h(1); // calls base h()
    d.h(1); // calls derived h()
}

D::f
D::f
D::g
B::g
B::h
D::h

Inheriting constructors

struct B1 {
    B1(int);
};
struct D1 : B1 {
    using B1::B1;
// The set of candidate inherited constructors is 
// 1. B1(const B1&)
// 2. B1(B1&&)
// 3. B1(int)
 
// D1 has the following constructors:
// 1. D1() = delete
// 2. D1(const D1&) 
// 3. D1(D1&&)
// 4. D1(int) <- inherited
};
 
struct B2 {
    B2(int = 13, int = 42);
};
struct D2 : B2 {
    using B2::B2;
// The set of candidate inherited constructors is
// 1. B2(const B2&)
// 2. B2(B2&&)
// 3. B2(int = 13, int = 42)
// 4. B2(int = 13)
// 5. B2()
 
// D2 has the following constructors:
// 1. D2()
// 2. D2(const D2&)
// 3. D2(D2&&)
// 4. D2(int, int) <- inherited
// 5. D2(int) <- inherited
};

struct B1 {  B1(int, ...) { } };
struct B2 {  B2(double)   { } };
 
int get();
 
struct D1 : B1 {
  using B1::B1;  // inherits B1(int, ...)
  int x;
  int y = get();
};
 
void test() {
  D1 d(2, 3, 4); // OK: B1 is initialized by calling B1(2, 3, 4),
                 // then d.x is default-initialized (no initialization is performed),
                 // then d.y is initialized by calling get()
  D1 e;          // Error: D1 has no default constructor
}
 
struct D2 : B2 {
  using B2::B2; // inherits B2(double)
  B1 b;
};
 
D2 f(1.0);       // error: B1 has no default constructor

struct W { W(int); };
struct X : virtual W {
 using W::W;   // inherits W(int)
 X() = delete;
};
struct Y : X {
 using X::X;
};
struct Z : Y, virtual W {
  using Y::Y;
};
Z z(0); // OK: initialization of Y does not invoke default constructor of X

struct A { A(int); };
struct B : A { using A::A; };
struct C1 : B { using B::B; };
struct C2 : B { using B::B; };
 
struct D1 : C1, C2 {
  using C1::C1;
  using C2::C2;
};
D1 d1(0); // ill-formed: constructor inherited from different B base subobjects
 
struct V1 : virtual B { using B::B; };
struct V2 : virtual B { using B::B; };
 
struct D2 : V1, V2 {
  using V1::V1;
  using V2::V2;
};
D2 d2(0); // OK: there is only one B subobject.
          // This initializes the virtual B base class,
          //  which initializes the A base class
          // then initializes the V1 and V2 base classes
          //  as if by a defaulted default constructor

struct B1 {   B1(int); };
struct B2 {   B2(int); };
 
struct D2 : B1, B2 {
  using B1::B1;
  using B2::B2;
  D2(int);   // OK: D2::D2(int) hides both B1::B1(int) and B2::B2(int)
};
D2 d2(0);    // calls D2::D2(int)

Notes

Only the name explicitly mentioned in the using-declaration is transferred into the declarative scope: in particular, enumerators are not transferred when the enumeration type name is using-declared.

A using-declaration cannot refer to a namespace, to a scoped enumerator, to a destructor of a base class or to a specialization of a member template for a user-defined conversion function.

Using-declaration can refer to a member template, but cannot refer to a member template specialization (template-id is not permitted).

struct B { template<class T> void f(); };
struct D : B {
      using B::f;      // OK: names a template
//    using B::f<int>; // Error: names a template specialization
      void g() { f<int>(); }
};

Using-declaration also can't refer to a dependent member template (the template disambiguator for dependent names is not permitted).

template<class X> struct B { template<class T> void f(); };
template<class Y> struct D : B<Y> {
//  using B<Y>::template f; // Error: disambiguator not allowed
  using B<Y>::f;            // compiles, but declares f as non-type non-template
  void g() { f<int>(); }    // Error here
};

If a using-declaration brings the base class assignment operator into derived class, whose signature happens to match the derived class's copy-assignment or move-assignment operator, that operator is hidden by the implicitly-declared copy/move assignment operator of the derived class. Same applies to a using-declaration that inherits a base class constructor that happens to match the derived class copy/move constructor (since C++17).

template <typename... Ts>
struct Overloader : Ts... {
    using Ts::operator()...; // exposes operator() from every base
};
 
template <typename... T>
Overloader(T...) -> Overloader<T...>; // C++17 deduction guide
 
int main() {
    auto o = Overloader{ [] (auto const& a) {std::cout << a;},
                         [] (float f) {std::cout << std::setprecision(3) << f;} };
}