C++ template class and inheritance [duplicate]

Question

Possible Duplicates:
[FAQ] Why doesn't a derived template class have access to a base template class' identifiers? Problem with protected fields in base class in c++
cannot access data member in a class template

Following code gives me compilation error. What is wrong?

struct Base {
   int amount;
};

template<class T> struct D1 : public Base {
};

template<class T>
struct D2 : D1<T> {
  void foo() { amount=amount*2; /* I am trying to access base class data member */ };
};

int main() {
  D2<int> data;
};


test.cpp: In member function 'void D2<T>::foo()':
test.cpp:11: error: 'amount' was not declared in this scope

How to fix this?

thanks

Answer 1

The problem here has to do with how names are looked up in template classes that inherit from template base classes. The actual rules behind it are pretty arcane and I don't know them off the top of my head; I usually have to consult a reference to learn precisely why this doesn't work.

The way to fix this is to explicitly prefix the member you're accessing with this->:

void foo() { 
    this->amount = this->amount * 2; // Or: this->amount *= 2;
}

This gives the compiler an unambiguous hint about where the name amount comes from and should resolve the compiler error.

If someone wants to give a more detailed description of why this error occurs I'd love to see a good explanation.