[Development] C++11 decltype magic with a container?

[Stephen Kelly]

On Wednesday, May 22, 2013 07:41:12 Thiago Macieira wrote:
> On quarta-feira, 22 de maio de 2013 11.07.44, Stephen Kelly wrote:
> > QSpiRelationArray::value_type is QSpiRelationArrayEntry which, being a
> > QPair,  does have a operator==(), so the decltype gives the result that
> > the
> > std::find should work.
> 
> In other words, QPair has an operator==, but it can't be used because it
> won't compile. Your code detects that it exists, but can't be sure that it
> works?

Correct. The compiler sees that operator==() exists, but it doesn't look into 
the implementation to see that it contains t1 == other.t1, and therefore it 
doesn't check if T1::operator==() exists.

> Then the problem is in QPair.

It's a general problem/limitation in the decltype trick really which affects 
any container.

> You need to modify all operator== in classes with templates to make sure
> they are enable_if for the case where its template(s) is(are) comparable.

See my initial email. Now that we've stepped back it might make more sense to 
you.

Thanks,

-- 
Stephen Kelly <stephen.kelly at kdab.com> | Software Engineer
KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
www.kdab.com || Germany +49-30-521325470 || Sweden (HQ) +46-563-540090
KDAB - Qt Experts - Platform-Independent Software Solutions
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