(1) The implementation of Unary and Binary (FunctionConcepts) assume that the return type is assignable ("r = f(arg)" tries to assign a value to r). But what if the return type is a reference? Won't the function concept fail to compile? Is this a bug?

That looks like a bug. Here's a challenge for you: how would you fix this bug?

In my own functor concept implementation, I used the function-pointer trick to make it work: ==== CODE ==== template struct special_UnaryFunctionConcept { void constraints() { Ret (Func::*x) (Arg) = Func::operator(); } }; ==============

(2) I'm implementing an NaryFunctionConcept for functors taking more than 2 arguments. The BOOST_CLASS_REQUIREx macro, however, appears to be limited to 4 types. This limits the NaryFunctionConcept to functors with up to two arguments. Is there a way to expand the BOOST_CLASS_REQUIREx macro to handle more than 4 types?

I'm sure there is a way. Can you think of a way to do it? Perhaps the boost preprocessor library would help.

Well, I was thinking that it BOOST_CLASS_REQUIREx for x > 4 could be added to the concept_check library in the same way that it is for x <= 4. Is this possible?

(3) I ran into a compiler problem when using BOOST_CLASS_REQUIREx; when any of the type macros have spaces or non-alphanumeric characters in them, the compiler reports an error. In the following example, I try to use the macro to require a UnaryFunctor taking a const std::string & and returning an int.

Yes, you need to create a typedef for such types. This restriction should be documented. I've checked in a fix to the docs.

Ok, gotcha. Interesting... Thanks, --Steve