C++ template gotchas

Question

just now I had to dig through the website to find out why template class template member function was giving syntax errors:

template class F00         


        

Answer 1

I got tripped up the first time I inherited a templated class from another templated class:

template
class Base {
    int a;
};

template
class Derived : public Base {
    void func() {
        a++; // error! 'a' has not been declared
    }
};

The problem is that the compiler doesn't know if Base is going to be the default template or a specialized one. A specialized version may not have int a as a member, so the compiler doesn't assume that it's available. But you can tell the compiler that it's going to be there with the using directive:

template
class Derived : public Base {
    using Base::a;
    void func() {
        a++; // OK!
    }
};

Alternatively, you can make it explicit that you are using a member of T:

void func {
    T::a++; // OK!
}