On Sat, Aug 6, 2011 at 4:38 AM, John Maddock <boost.regex@virgin.net> wrote:
Folks, I have a problem with enable_if: it works just dandy for template member functions in my class, but I have some *non-template* member functions and constructors that I want to enable only if the template is instantiated with template args that meet certain criteria. Seems like I can't do this with enable_if as it then generates an invalid class definition. I guess I could use base classes, and/or partial specialization, but both involve a lot of code duplication I'd rather not have. Anyone any ideas?
Thanks, John.
C++0x has a simple answer to this -- default function template template arguments. This works with constructors and even conversion operators. I added documentation for this kind of usage to trunk back in April or so, but I guess it never made it into Release. A quick example of what you can do in 0x: /////////////////////////// class foo { typedef double value_type; template< class A = int , typename enable_if_c < sizeof( A ) != 0 && your_arbitrary_condition< value_type >::value , int >::type = 0 > foo() { /* some constructor enabled only if value_type meets the condition */ } }; /////////////////////////// There are two sort of subtle things here. First, the sizeof( A ) != 0 part is to force the condition to be dependent on a template argument. The other subtle thing is that the second template argument is a non-type template argument. It was suggested that a macro should be made to hide the subtleties and make things more concise. In this situation it seems like it would simplify things a lot. I'll put something together and make sure it all gets to Release. -- -Matt Calabrese