C++ | Using declaration

A using declaration introduces a single name into the current scope that was previously declared elsewhere.

Importing names individually from a namespace

Once using is used to introduce the name cout from the namespace std into the scope of the main function, the std::cout object can be referred to as cout alone.

#include <iostream>
int main() {
    using std::cout;
    cout << "Hello, world!\n";
}

Redeclaring members from a base class to avoid name hiding

If a using-declaration occurs at class scope, it is only allowed to redeclare a member of a base class. For example, using std::cout is not allowed at class scope.

Often, the name redeclared is one that would otherwise be hidden. For example, in the below code, d1.foo only refers to Derived1::foo(const char*) and a compilation error will occur. The function Base::foo(int) is hidden not considered at all. However, d2.foo(42) is fine because the using-declaration brings Base::foo(int) into the set of entities named foo in Derived2. Name lookup then finds both foos and overload resolution selects Base::foo.

struct Base {
    void foo(int);
};
struct Derived1 : Base {
    void foo(const char*);
};
struct Derived2 : Base {
    using Base::foo;
    void foo(const char*);
};
int main() {
    Derived1 d1;
    d1.foo(42);  // error
    Derived2 d2;
    d2.foo(42);  // OK
}

Inheriting constructors

As a special case, a using-declaration at class scope can refer to the constructors of a direct base class. Those constructors are then inherited by the derived class and can be used to initialize the derived class.

struct Base {
    Base(int x, const char* s);
};
struct Derived1 : Base {
    Derived1(int x, const char* s) : Base(x, s) {}
};
struct Derived2 : Base {
    using Base::Base;
};
int main() {
    Derived1 d1(42, "Hello, world");
    Derived2 d2(42, "Hello, world");
}

In the above code, both Derived1 and Derived2 have constructors that forward the arguments directly to the corresponding constructor of Base. Derived1 performs the forwarding explicitly, while Derived2, using the C++11 feature of inheriting constructors, does so implicitly.

Syntax

• using typename(opt) nested-name-specifier unqualified-id;
• using :: unqualified-id;

Remarks

A using-declaration is distinct from a using directive, which tells the compiler to look in a particular namespace when looking up any name. A using-directive begins with using namespace.

A using-declaration is also distinct from an alias declaration, which gives a new name to an existing type in the same manner as typedef. An alias declaration contains an equals sign.