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

Thiago Macieira thiago.macieira at intel.com
Thu Feb 19 20:36:04 CET 2015

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;

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.

Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center

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