Question

I’m trying to use a typedef from a subclass in my project, I’ve isolated my problem in the example below.

Does anyone know where I’m going wrong?

template<typename Subclass> class A {     public:         //Why doesn't it like this?         void action(typename Subclass::mytype var) {             (static_cast<Subclass*>(this))->do_action(var);         } };  class B : public A<B> {     public:         typedef int mytype;          B() {}          void do_action(mytype var) {             // Do stuff         } };  int main(int argc, char** argv) {     B myInstance;     return 0; } 

This is the output I get:

sean@SEAN-PC:~/Documents/LucadeStudios/experiments$ g++ -o test test.cpp test.cpp: In instantiation of ‘A<B>’: test.cpp:10:   instantiated from here test.cpp:5: error: invalid use of incomplete type ‘class B’ test.cpp:10: error: forward declaration of ‘class B’ 

Answer 1

The reason is that when instantiating a class template, all its declarations (not the definitions) of its member functions are instantiated too. The class template is instantiated precisely when the full definition of a specialization is required. That is the case when it is used as a base class for example, as in your case.

So what happens is that A<B> is instantiated at

class B : public A<B> 

at which point B is not a complete type yet (it is after the closing brace of the class definition). However, A<B>::action‘s declaration requires B to be complete, because it is crawling in the scope of it:

Subclass::mytype 

What you need to do is delaying the instantiation to some point at which B is complete. One way of doing this is to modify the declaration of action to make it a member template.

template<typename T> void action(T var) {     (static_cast<Subclass*>(this))->do_action(var); } 

It is still type-safe because if var is not of the right type, passing var to do_action will fail.