c++ - GCC issue: using a member of a base class that depends on a template argument

Question

The following code doesn't compile with gcc, but does with Visual Studio:

template <typename T> class A {
public:
    T foo;
};

template <typename T> class B: public A <T> {
public:
    void bar() { cout << foo << endl; }
};

I get the error:

test.cpp: In member function ‘void B::bar()’:

test.cpp:11: error: ‘foo’ was not declared in this scope

But it should be! If I change bar to

void bar() { cout << this->foo << endl; }

then it does compile, but I don't think I have to do this. Is there something in the official specs of C++ that GCC is following here, or is it just a quirk?

Answer 1

David Joyner had the history, here is the reason.

The problem when compiling B<T> is that its base class A<T> is unknown from the compiler, being a template class, so no way for the compiler to know any members from the base class.

Earlier versions did some inference by actually parsing the base template class, but ISO C++ stated that this inference can lead to conflicts where there should not be.

The solution to reference a base class member in a template is to use this (like you did) or specifically name the base class:

template <typename T> class A {
public:
    T foo;
};

template <typename T> class B: public A <T> {
public:
    void bar() { cout << A<T>::foo << endl; }
};

More information in gcc manual.