Why should I use the "using" keyword to access my base class method?

Question

I wrote the below code in order to explain my issue. If I comment the line 11 (with the keyword "using"), the compiler does not compile the file and displays this error: invalid conversion from 'char' to 'const char*'. It seems to not see the method void action(char) of the Parent class in the Son class.

Why the compiler behave this way? Or have I done something wrong?

class Parent
{
    public:
        virtual void action( const char how ){ this->action( &how ); }
        virtual void action( const char * how ) = 0;
};

class Son : public Parent
{
    public:
        using Parent::action; // Why should i write this line?
        void action( const char * how ){ printf( "Action: %c\n", *how ); }
};

int main( int argc, char** argv )
{
    Son s = Son();
    s.action( 'a' );
    return 0;
}

Answer 1

The action declared in the derived class hides the action declared in the base class. If you use action on a Son object the compiler will search in the methods declared in Son, find one called action, and use that. It won't go on to search in the base class's methods, since it already found a matching name.

Then that method doesn't match the parameters of the call and you get an error.

See also the C++ FAQ for more explanations on this topic.