weidagang2046的专栏

Syntax

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

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

A using-declaration can be used in a class definition. For example:

class B
{
    void f(char);
    void g(char);
};

class D : B
{
    using B::f;
    void f(int) { f('c'); }        // calls B::f(char)
    void g(int) { g('c'); }        // recursively calls D::g(int)
                                   // only B::f is being used
};

When used to declare a member, a using-declaration must refer to a member of a base class. For example:

class C
{
    int g();
};

class D2 : public B
{
    using B::f;          // ok: B is a base of D2
    using C::g;          // error: C isn't a base of D2
};

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

void f();

namespace A
{
    void g();
}

namespace X
{
    using ::f;        // global f
    using A::g;       // A's g
}

void h()
{
    X::f();           // calls ::f
    X::g();           // calls A::g
}

Just as with any declaration, a using-declaration can be used repeatedly only where multiple declarations are allowed. For example:

namespace A
{
    int i;
}

void f()
{
    using A::i;
    using A::i;        // ok: double declaration
}

class B
{
protected:
    int i;
};

class X : public B
{
public:
    using B::i;
    using B::i;        // error: class members cannot be multiply declared
};

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. For example:

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);
}

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.

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. For example:

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.

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

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);                // error: ambiguous: B::f(int) or C::f(int)?
    void f(int);         // error: conflicts with B::f(int) and C::f(int)
}

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. For example:

struct B
{
    virtual void f(int);
    virtual void f(char);
    void g(int);
    void h(int);
};

struct D : B
{
    using B::f;
    void f(int);         // ok: D::f(int) overrides B::f(int)

    using B::g;
    void g(char);        // ok: there is no B::g(char)

    using B::h;
    void h(int);         // error: D::h(int) conflicts with B::h(int)
                         // B::h(int) is not virtual
};

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)
}

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. For example:

class A
{
private:
    void f(char);
public:
    void f(int);
protected:
    void g();
};

class B : public A
{
    using A::f;          // error: A::f(char) is inaccessible
public:
    using A::g;          // B::g is a public synonym for A::g
};


from: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vccelng/htm/tions_43.asp