on Wed Oct 10 2012, Lorenzo Caminiti <lorcaminiti-AT-gmail.com> wrote:
On Tue, Oct 9, 2012 at 7:16 PM, Dave Abrahams <dave@boostpro.com> wrote:
on Tue Oct 09 2012, Lorenzo Caminiti <lorcaminiti-AT-gmail.com> wrote:
I made a first attempt to sketch how N3351-like concepts will look into Boost.Contract (just for the find algorithm and its concepts for now): [...] Please tell me what you think now
Since you asked what I think,
"pseudo-signatures > usage patterns"
Hello all,
Can we write down pros and cons for concepts implemented via pseudo-signatures (C++0x-like and Boost.Generic) vs. usage patterns (N3351 and Boost.Contract)?
Who wants to start? Matt, Dave, Andrzej, ... I can compile a qbk table with what we discuss.
Matt C. already mentioned the ease of producing a new model of a given concept. I will mention that usage patterns tend to leave unstated assumptions about intermediate associated types that make it very hard to be sure you're saying what you really mean, and tend to lead to semantically-vague requirements. Look for example at http://www.sgi.com/tech/stl/find_if.html. I can easily satisfy all the stated requirements and produce an example that won't compile. I can also easily fail to satisfy the stated requirements and produce an example that *will* compile. That's because there's a great deal of potential mischief hiding in expressions such as f(*p) [For example, what is the relationship between the type of *p and the iterator's value type? What is the relationship between that type and the argument type of the function object? Even if the reference type of p is convertible to the argument type of f, the call can still be ambiguous... etc...] This is a perfect example of what you get when you deal in usage patterns, and the C++98 (and even the '03) standard is full of these problems. One of the most important features of the pseudo-signature approach is that it injects implicit conversions to (explicitly-stated) associated types, so these issues don't arise. I don't know how to make a tidy little bullet out of this issue for your list of pros and cons, but I think it's the most important one on the list. -- Dave Abrahams BoostPro Computing Software Development Training http://www.boostpro.com Clang/LLVM/EDG Compilers C++ Boost