on Thu Oct 11 2012, Lorenzo Caminiti <lorcaminiti-AT-gmail.com> wrote:
On Thu, Oct 11, 2012 at 5:25 AM, Andrew Sutton <asutton.list@gmail.com> wrote:
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)
Can you produce those examples? You've made me curious.
Andrew, what were the reasons in N3351 to move away from pseudo-signatures and toward usage-patterns?
I red N3351 but if I had to summarize the rationale for the usage-pattern approach instead of C++0x concept's pseudo-signature then I'd say that N3351's argument is that its concept design for STL algorithms is "simpler" using usage-pattern than using C++0x pseudo-signatures. However, "simpler" is a subjective metric... I'm sure I'm missing something maybe reading N3351 one more time will clarify my thinking.
One other thing about this rationale: it doesn't appear to consider what's simple for someone trying to write a generic algorithm. It's easy to maintain that rationale when you don't have an implementation of algorithm type-checking to force rigor on you. When you start working with an actual implementation, and it refuses to compile "obviously right" things like f(*p) because in fact, the requirements as stated do not guarantee that it *should* compile, then your idea of what's simple may change. A usable system has to be simple both for programmers modeling concepts and for programmers using those concepts in generic code. -- Dave Abrahams BoostPro Computing Software Development Training http://www.boostpro.com Clang/LLVM/EDG Compilers C++ Boost