[Development] Proposal: Deprecating platforms in Qt 5.6 that don't support lambda

[Mathias Hasselmann]

NO, please. Just use std::cref(). The feature is there already in the STL.

Am 19.02.2015 um 20:36 schrieb Thiago Macieira:
> On Thursday 19 February 2015 12:07:04 Matthew Woehlke wrote:
>> On 2015-02-19 07:29, Daniel Teske wrote:
>>> Qt's container classes and C++11 range based for loop do not mix very
>>> well.
>>> Ranged based for uses std::begin(container), which if not overloaded calls
>>> container.begin(), which detaches.
>>
>> As an aside, the "correct" fix for this IMHO is for range-based for to
>> support a mechanism for marking the RHS 'const', whether or not it
>> otherwise would be so.
>>
>> Worst case, Qt could (should?) implement something like:
>>
>>    struct QConstWrapper<ContainerType>
>>    {
>>      ContainerType::const_iterator begin() const;
>>      ContainerType::const_iterator end() const;
>>      // remaining "magic" elided
>>    };
>>
>>    QConstWrapper<ContainerType> qConst(ContainerType const&);
>>
>> That said, note that range-based for also differs from foreach in that
>> the former operates on the original container, whereas the latter
>> operates on a copy.
>
> It actually needs to be:
>
> template <typename T> const T qConst(const T &t) { return t; }
>
> It needs to create a copy and return it. There's a gotcha with range-based for
> that it is defined in such a way that your temporaries may be destroyed before
> the iteration. The standard defines it as:
>
> {
> 	auto && __range = <your range>;
> 	for ( auto __begin = std::begin(__range),
> 			__end = std::end(__range);
> 			__begin != __end;
> 			++__begin ) {
> 		<for-range-declaration> = *__begin;
> 		<statement>
> 	}
> }
>
> If you have a temporary in your right hand of the ':' it will get lifetime-
> extended by that __range reference. Any other temporaries will get destroyed.
>
> That means a cast works and a copy works. Returning a reference doesn't.
>