[Development] C++11 for Examples

[Thiago Macieira]

On Monday 02 March 2015 16:20:29 Blasche Alexander wrote:
> The suggestion was/is to permit the usage of C++11 in Qt examples. Here is a
> concrete set of suggestions to address the open issues and consequences:
> 
> 1.) Use C++11 for examples starting with Qt 5.6

I'd say "use C++11 where it makes for cleaner code".

Don't go using things you don't have to just because they're there.

> 2.) Coding convention for C++11 constructs and set of permitted features:
> https://wiki.qt.io/index.php?title=Coding_Conventions#Conventions_for_C.2B.2
> B11_usage

I didn't know this existed. That's a nice summary and a good start.

I guess we'll expand it as necessary.

> 3.) The coding conventions above imply a set of base line compilers for Qt
> examples (gcc 4.5+, VS2010+, OSX 10.7+)

10.7 is gone for Qt 5.5, so we only need for XCode 5 and up.

> 4.) Don't use any of the macros such as Q_DECL_OVERRIDE (assuming that
> override would be part of the accepted set of features). The examples
> should be clean C++11 code.

Agreed, if we're going to write C++11 code to teach people, we should use the 
actual  keywords, not the Qt replacement.

> The suggestions 2 & 3 above are based on current Qt Creator policy.
> 
> You might ask why just lambda and auto as suggestion? The biggest problem is
> that various compilers have differing support for C++11. Therefore each new
> C++11 feature is likely to enforce even stricter/newer example compilers.
> If you want your pet C++11 feature please ensure that the compilers in item
> 3 support this feature. Alternatively we may discuss whether the suggested
> list of "example base line" compilers is correct.

I'd say we should think of raising the bar a little. How about making the 
examples require GCC 4.7 and VS 2012? Those are 4-year-old compilers by the 
time Qt 5.6 is released.

Also, I don't expect us to do a wholesale replacement, so some examples should 
still work for a while in older compilers.

The common subset of features supported by GCC 4.7, VS 2012 and Clang 3.3 
(should match Apple Clang 5.0) is exactly VS2012.

Q_COMPILER_ATOMICS
Q_COMPILER_AUTO_FUNCTION
Q_COMPILER_AUTO_TYPE
Q_COMPILER_CLASS_ENUM
Q_COMPILER_DECLTYPE
Q_COMPILER_EXPLICIT_OVERRIDES
Q_COMPILER_EXTERN_TEMPLATES
Q_COMPILER_LAMBDA
Q_COMPILER_NULLPTR
Q_COMPILER_RVALUE_REFS
Q_COMPILER_STATIC_ASSERT

Of those, atomics and extern templates should not happen in example code. As 
for rvalue refs, while they will work, std::move will be a problem for QNX.

> contains(QT_CONFIG, c++11)
> 
> is a very coarse qmake check for a given set of C++11 features it is
> unlikely to be useful to serve as enabler for converted examples.
> Workarounds might be to not compile examples on unsupported platforms in
> the CI anymore or we define a new qmake flag that precisely covers the
> required set of C++ features. The latter would certainly be more user
> friendly.
> 
> Comments, suggestions and desires?

Make it unconditional and let people disable with -nomake tests or 
-no-compile-examples if they have older compilers.

I'd also propose dropping the VS 2010 packages for either 5.5 or 5.6 and 
replace them with VS 2015.

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