C++ Using declaration Redeclaring members from a base class to avoid name hiding


Example

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
}