A using declaration in a definition of a class A allows you to introduce a name of a data member or member function from a base class of A into the scope of A.
You would need a using declaration in a class definition if you want to create a set of overload a member functions from base and derived classes, or you want to change the access of a class member.
using declaration syntax >>-using--+-+----------+--+----+--nested_name_specifier--unqualified_id--;-+->< | '-typename-' '-::-' | '-::--unqualified_id--;------------------------------------------'
A using declaration in a class A may name one of the following:
The following example demonstrates this:
struct Z {
int g();
};
struct A {
void f();
enum E { e };
union { int u; };
};
struct B : A {
using A::f;
using A::e;
using A::u;
// using Z::g;
};The compiler would not allow the using declaration using Z::g because Z is not a base class of A.
A using declaration cannot name a template. For example, the compiler will not allow the following:
struct A {
template<class T> void f(T);
};
struct B : A {
using A::f<int>;
};Every instance of the name mentioned in a using declaration must be accessible. The following example demonstrates this:
struct A {
private:
void f(int);
public:
int f();
protected:
void g();
};
struct B : A {
// using A::f;
using A::g;
};The compiler would not allow the using declaration using A::f because void A::f(int) is not accessible from B even though int A::f() is accessible.
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