Function overloading with polymorphic templates

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 hap开发者_运维技巧pening 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.

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.

Either add a using declaration like the other answers show or make a call using a qualified name

e.Base<float>::foo();

The foo from Extension is hiding the foo from Base.

Adding a using clause to Extension solves the compilation error.

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

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

int main()
{
    Extension<float> e;
    e.foo();
    e.foo(1);
    return 0;
}

it looks like Base::foo() is private.