On 7/26/2011 2:25 PM, Edward Diener wrote:
On 7/26/2011 5:03 PM, Jeffrey Lee Hellrung, Jr. wrote:
On Tue, Jul 26, 2011 at 1:19 PM, Noah Roberts<roberts.noah@gmail.com>wrote: [...]
One thing that I think is missing is the option to check for a named callable within a type. For example, one might want to use enable_if style concept checking. So long as a type has function 'xxx' that takes arguments of type x,y,and z, and returns something convertible to type A, the concept is obeyed. I don't believe this can be done with C++03, but with C++1X I was able to do something using decltype.
[...]
Actually, you can approximate it pretty well in C++03 (using derivation and using tricks) to the point that I think it will work pretty much as you expect except for nullary member functions, where your queries must be more restrictive. This is a useful query, but I believe Edward has decided it to be outside the scope of TTI.
I am not sure what the OP means by 'named callable". If it can be introspected within the type I can consider whether it can be done or not.
What I mean by "callable" is that the expression may not be just a function call but could also be a template instantiation. For example: Concept C z = c.fun(x) | x is an int | result convertible to double This class obeys: struct class_a { double fun(int); }; As does this one: struct class_b { template < typename T > double fun(T); }; As does this one: struct class_c { template < typename T > converts_to_double_type fun(T); }; And this: struct class_d { double fun(has_implicit_int_constructor); }; etc... The question being asked here is, can I call something called X within type T that accepts parameters x,y,z and returns something I can assign to type A? Answering this question is often more interesting than the question, "Does type T have function X with signature Y?" It also cannot be correctly answered with such a check because it could be implemented by template, a function that has default arguments, or a function that takes other types but they are convertible. I would consider this within the scope of "introspection", but I can see why it might be omitted for a future version or something.