Question

Why is the following code not compiling and how would it be possible to use the function from the base class?

template<typename K> struct Base
{
    K foo() { return (K)0; }
};

template<typename K> struct Extension
: public Base<K>
{
    K foo(int a) { return (K)a; }
};

int main()
{
    Extension<float> e;
    e.foo();
    return 0;
}

Edit: Ok, I thought that this is only happening with template classes … What is the idea behind the design decision to hide the base class version by its overloaded version from the child class? I mean, declaring both functions in the same class works just fine.

Answer 1

Extension::foo is hiding Base::foo. You can use a using delaration to bring it back:

template<typename K> struct Extension
: public Base<K>
{
    using Base<K>::foo;
    K foo(int a) { return (K)a; }
};

Item #33 (“Avoid hiding inherited names”) in Scott Meyers’s “Effective C++” is about this issue.