Question

This code:

template <class T>
class Foo {};

typedef Foo<void*> Bar;

template <class T>
class Foo<T*> : public Bar {};

// use Foo<int*> somewhere.

Compiles and works fine in MSVC 9.0, but doesn’t compile in GCC 4.1.1 or GCC 4.3.4, with the error:

error: invalid use of undefined type 'class Bar'

Is this illegal C++ that MSVC accepts incorrectly, or a limitation of GCC?

Either way, how can I work around this get the desired behaviour: pointer specialisations of Foo that inherit from unspecialised Foo<void*>?

Answer 1

You cannot do that, except by writing the specialization for all T*, except when T is void. Otherwise, you will derive the class from itself, which for obvious reasons can’t work.

Instantiating the primary class template for arguments that have an explicit or partial specialization is not possible. If you try to, by provoking an instantiation before the explicit or partial specialization is visible (note that your code did not provoke such an instantiation), your program is ill-formed, with no diagnostic being required (which effectively renders the behavior undefined).

To achieve the above work-around, you can use SFINAE

template <class T>
struct isnt_void { typedef void type; };

template<> struct isnt_void<void> { };

template <class T, class = void>
class Foo {};

template <class T>
class Foo<T*, typename isnt_void<T>::type> : public Foo<void*> {};