C++ typedef for partial templates

Question

Hi, i need to do a typedef like this.

template< class A, class B, class C >
class X
{
};

template< class B, class C >
typedef X< std::vector<B>, B, C >  Y;

I just found that it is not supported in C++. Can someone advise me on how to achieve the same through alternative means?

Thanks, Gokul.

Answer 1

By placing it in a struct. This idea is called template alias and is part of the C++0x standard (the proposal). But a work-around is given by:

template<class B, class C>
struct Y {
  typedef X<std::vector<B>, B, C> type;
};

and use Y<B, C>::type as your desired type.

And you might be inclined to think gcc4.5 or VS2010 might already support it, as is the case with a big subset of C++0x, but I have to disappoint you, it is as we speak still unsupported :).

Answer 2

If you have a C++0x/C++1x compiler, that will be allowed with a slightly different syntax (it seems as if compilers don't still support this feature):

template <typename B, typename C>
using Y = X< std::vector<B>, B, C >;

You can use other techniques, like defining an enclosed type in a templated struct (like Pieter suggests), or abusing inheritance (avoid if possible):

template <typename B, typename C>
class Y : public X< std::vector<B>, B, C > {};