[Dune-devel] PSA: please avoid implicit conversion integral_constant<bool, ...> to bool

[Jö Fahlke]

Am Mon, 20. Feb 2017, 21:12:43 +0100 schrieb Carsten Gräser:
> Hi Jö,
> 
> Am 20.02.2017 um 13:10 schrieb Jö Fahlke:
> > Hi!
> > 
> > I've seen expression like `std::is_same<T1, T2>()` (or any other type
> > predicate) where the result is implicitly converted to `bool` pop up
> > throughout Dune for some time now.  Please stop that; they can't work in all
> > cases, and when they break they lead to hard-to-diagnose errors.
> the guideline to alway avoid this implicit conversion seems
> to be a little to general to me. I don't see a problem if
> you already have an object of type integral_constant<bool, ...>.

The problem with making exceptions to guidelines is that it makes them more
complicated.  The distinction between a type trait and its BaseCharacteristic
is easy to overlook -- picking your personal favorite and sticking with it for
all `integral_constant`-like thing seems simpler to me.

> By the same argument one would not be allowed to use
> v[Indices::_0] for e.g. a std::vector v because this
> involves such an implicit conversion.

If I know it is a `std::vector` I cannot think of any reason to prefer an
index constant literal over an integer literal for the subscript.  So I'm
assuming you're thinking about code that does not know whether the container
is a `std::vector` or actually something that needs to be subcripted with an
index constant.

In that case you were right in calling it a guideline, and the rule about
guidelines applies: you sometimes need to ignore them.  I can see no way
around relying on the implicit conversion there.

> Isn't the real guideline: Never use the conversion if the
> expression may involve incomplete types?

That would work, but then you have to determine whether the expression may
involve incomplete types.  Most of the time in template code you can't be sure
that the passed-in types are sufficiently complete.

Regards,
Jö.

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