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

[Daniel Teske]

Hi,

Standard C++ is evolving in a unprecedented pace at the moment. Both C++11 and 
C++14 added a lot of new good features. C++17 is planned to be a big step 
again. 

Qt needs to evolve together with C++ or it will be a outdated toolkit stuck in 
a C++98 world.

As an example, Qt's container classes and C++11 range based for loop do not 
mix very well.[1] And for the same reason, the upcoming Ranges TS will not be 
too useful for Qt's container. 

We have started using some parts of C++11 in Creator a year ago and our 
experience is that lambdas (and std::function) are useful everywhere. Today we 
have more than 400 lambdas in Creator's source and have several interfaces 
that take a std::function. 

I would expect that allowing C++11 in Qt would similarly lead to a wider 
understanding on how to leverage the new features for better code and better 
APIs.

We need to start now and deprecate old compilers that do not support any C++11 
features at all. I I suggest requiring support for lambda as 
supported by MSVC 2010, g++ 4.5 and clang 3.1 in Qt 5.7 and deprecating all 
platforms that do not in Qt 5.6.

daniel

[1] ranged based for uses std::begin(container), which if not overloaded calls 
container.begin(), which detaches. 

So using range-based can be used:
- If the container is const or
- If the container is unshared or
- To actually change the container's contents