Re: [boost] [contract] concept error messages

Subject: Re: [boost] [contract] concept error messages
From: Andrew Sutton (asutton.list_at_[hidden])
Date: 2012-10-06 14:29:23

> template< typename T >
> typename std::enable_if<
>     AlwaysTrue<T>::value && contract::std::EqualityComparable<T>::value,
> bool>::type equal1 ( T const& a, T const& b )
> {
>     return a == b;
> }

Just curious, but what is the purpose of AlwaysTrue? If it's always
true, why not omit it?

> And given this, I personally don't see the need for specifying
> concepts in place of typename or using requires. In other words, I
> think the above use of enable_if is sufficient and the following
> syntax is not necessary (even if it's part of N3351):
>
> template< AlwaysTrue T >
>     requires contract::std::EqualityComparable<T>
> bool equal1 ( T const& a, T const& b )
> {
>     return a == b;
> }

Lots of reasons, but mostly it clearly delineates what is required
from what is returned.

>From a technical perspective, how would you specify this behavior for
the standard? Can you do it in a way that guarantees the same behavior
for all of the following, equivalent declarations?

// std::enable_if
template <typename T>
  typename std::enable_if<Eq<T>::value, bool>::type
f(T x)

// Boost enable_if
template <typename T>
  typename boost::enable_if<Eq<T>, bool>::type
f(T x)

// Origin style
template <typename T>
  Requires<Eq<T>(), bool>
f(T x)

Should the compiler know that all of these are the same? How could you
guarantee that? Would the behavior be different for the
std::enable_if? If Requires was an alias to std::enable_if (it is),
does that get the same behavior?

Libraries are great for helping figure out what you want, but you're
going to hit a wall at some point. I think "requires" is one of those
walls.

Andrew