2012/10/10 Matt Calabrese <rivorus@gmail.com>
On Wed, Oct 10, 2012 at 5:06 PM, Vicente J. Botet Escriba < vicente.botet@wanadoo.fr> wrote:
- Usage-patterns show valid expressions and properties of expressions
Usage-patterns: that
deal with the concept's arguments (mostly subjective if this is pro or con)
On simple cases this is not an advantage but there are cases that writing the pseudo-signatures is quite complex(or even impossible) if we don't want to say more than what the usage pattern says.
for example
f(a)
requires that there is an unique overload of f that can be found for f(a)
With pseudo signatures we could state that there is a function f with a parameter T and that a is implicitly convertible to T.
void f(T) requires (Convertible<decltype(a), T>
Note that a model that defines
int f(A)
satisfies the usage patter f(a) but not the pseudo signature. I don't see how to introduce the type R if it is not part of the concept parameters.
Actually, this all works with pseudo-signatures of N2914 and Boost.Generic. Note that when you make a pseudo-signature such as
void f( T );
you are NOT requiring that the model in question has a signature that matches exactly, nor that the return type must be void. The match is loose, including convertibility of arguments and results, and if a return type is specified as void in a pseudo-signature, it will match fine with functions that do not return void (just don't try to use the return type as though it were something other than void when in a constrained context).
"The match is loose" -- I believe that this "loose match" may be a problem in itself. The pseudo-signatures look like any other signature so one might expect that they also work as any other signature. One would be surprised to learn they are only similar with other signatures on the surface, but differ in semantics due to this "loose match". In contrast, with usage patterns you are already alerted with the unusual syntax that the meaning of the declarations is unusual. My comment may be very superficial though. I do not know pseudo-signatures in-depth. And I do not find pseudo-signatures nferior. I only argue with this "loose match" idea. Regards, &rzej