Why this CRTP does not compile?

Question

Possible Duplicate:
“Inherited” types using CRTP and typedef

template<typename T>
struct A {
  typename T::Sub  s;
};

struct B : A<B> {
  struct Sub {};
};

Clang is reporting this error:

todo.cc:3:15: error: no type named 'Sub' in 'B'
  typename T::Sub  s;
  ~~~~~~~~~~~~^~~
todo.cc:6:12: note: in instantiation of template class 'A<B>' requested here
struct B : A<B> {
           ^

How can I get this to work?

How can `B` inherit from `A` when `B` contains the definition of something needed in `A`? — chrisaycock, Jul 23 '12 at 17:06

score 2 · Answer 1 · answered Jul 23 '12 at 17:07

2

B is an incomplete class at the point it requests the instantiation of A<B>.

This means that you can only refer to B within template methods on A, as the instantiation of those will be delayed until B is complete.

answered Jul 23 '12 at 17:07

ecatmur

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score 1 · Answer 2 · answered Jul 23 '12 at 17:15

Since B inherits from A<B>, A<B> must be constructed before B is.

Since you are using the CRTP, this means that B is an incomplete class when A<B> is being constructed. Due to this, the compiler cannot tell if B has a member field Sub, thus failing at compiling.

Note that, this time, gcc gives a more meaningful error:

$ cat crtp.cpp

template<typename T>
struct A {
      typename T::Sub  s;
};

struct B : A<B> {
      struct Sub {};
};

$ g++ -c crtp.cpp -o crtp.o

crtp.cpp: In instantiation of ‘A<B>’:
crtp.cpp:6:17:   instantiated from here
crtp.cpp:3:21: error: invalid use of incomplete type ‘struct B’
crtp.cpp:6:8: error: forward declaration of ‘struct B’

Why this CRTP does not compile?

2 Answers2

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