On Sat, Aug 6, 2011 at 1:38 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?
One thing I've done in this situation is the following. Let's say we want to provide 2 different definitions of a member function f within class X<T>, depending on if T satisfies some metafunction cond. Then you can do something like: template< class T > struct X { struct dummy_t { }; typedef /*...*/ f_arg_type; // f's argument type typedef typename boost::mpl::if_< cond<T>, f_arg_type, dummy_t >::type cond_true_f_arg_type; typedef typename boost::mpl::if_< cond<T>, dummy_t, f_arg_type >::type cond_false_f_arg_type; void f(cond_true_f_arg_type) { /*...may assume T does satisfy cond...*/ } void f(cond_false_f_arg_type) { /*...may assume T does not satisfy cond...*/ } }; I think this is fine as long as you don't declare explicit instantiations of X (at least, I don't recall ever having any problems with this technique). Is that kind of what you're looking for? - Jeff