Re: [boost] [guidelines] why template errors suck

Subject: Re: [boost] [guidelines] why template errors suck
From: Andrew Sutton (asutton.list_at_[hidden])
Date: 2010-09-28 11:00:54

On Tue, Sep 28, 2010 at 9:20 AM, David Abrahams <dave_at_[hidden]> wrote:

> At Tue, 28 Sep 2010 08:39:23 -0500,
> Andrew Sutton wrote:
> >
> > Having done the exercise, I'm satisfied with my results (which agree with
> > Mathias').
>
> Great; let's see your work!
>

This is my interpretation of the type constraints on the template parameters
of the find_if algorithm.

template<typename Iter, typename Pred>
Iter find_if(Iter f, Iter l, Pred p)
{
  for( ; f != l; ++f) if(p(*f)) return f;
  return l;
}

Expressions on Iter:
f != l;
*f;
++f;

Expressions on pred:
p(*f)

and...

is_convertible<decltype(f != l), bool>::value
is_convertible<decltype(p(*f)), bool>::value

I don't know if you'd count the last two as "valid expressions", but they
clearly seem to be requirements on the algorithm. I'm probably missing copy
constructibility requirements.

 It's hard to provide an argument more concrete than an example that
> doesn't work.  But in fairness I guess it's also easy to overlook
> non-workingness when you don't have a computer doing the checking for
> you, or the STL wouldn't have been designed with the "valid
> expression" approach...
>
> OK, I found a good explanation.  See §3.2.3 of
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1758.pdf
>

That is a good explanation, but I don't see why type traits can't be used to
enforce convertability or same-type requirements. Why not describe
LessThanComparable as?

static_assert(is_same<decltype(x < y), bool>::value, "")

Besides the fact that it's a little gross. We can do this now (with C++0x
anyway), and we do use is_same<> to constrain key/value types in
SimpleAssociativeContainers.

It seems to me that writing constraints in terms of valid expressions gives
you flexibility when you want it, but doesn't preclude the strengthening of
requirements when needed.

Andrew Sutton