Niall’s virtual diary archives – Friday 8th May 2015

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Friday 8th May 2015: 10.27am. Link shared: http://cppnow2015.sched.org/event/37beb4ec955c082f70729e4f6d1a1a05#.VUuMqvkUUuU

As part of publicising my C++ Now 2015 talk next week, here is part 3 of 20 from its accompanying Handbook of Examples of Best Practice for C++ 11/14 (Boost) libraries:

3. PORTABILITY: Strongly consider trying your library on Microsoft Visual Studio 2015

More than half the libraries reviewed had no support for Microsoft Visual Studio, and only supported GCC and clang. When the authors were asked why, in many cases it was simply assumed that MSVC didn't implement much C++ 11/14 and the authors hadn't attempted MSVC support.

This is in fact untrue. Here is a complete list of C++ 11/14 features which VS2015 does NOT support (everything else you can assume it supports, including a full and complete C++ 14 STL):

* Expression SFINAE (there are workarounds. Note the STL "turns on" magic Expression SFINAE support for those parts of the STL requiring it, so any Expression SFINAE done by the STL for you works as expected).

* Any serious constexpr use (try "#define constexpr" i.e. disable it completely. Most of the time your code will compile and work. Consider using a BOOST_LIBNAME_CONSTEXPR macro thereafter). It is claimed by Microsoft that full C++ 11 constexpr conformance is present, but to be honest in my own code I don't find anything less than C++ 14 constexpr useful in practice.

* No two phase lookup. Reordering decls to make the compiler look up in a way which doesn't produce non-conforming outcomes is a straightforward, if annoying, workaround.

* MSVC's C99 support is still less than complete, but it's significantly more complete than before.

* MSVC's preprocessor is still non-conforming, but it's less broken than it has ever been.

* Variable templates.

* Non-static data member initialisers for aggregates.

VS2015 is a very highly conforming C++ 11/14 compiler. It meets or exceeds clang 3.3 on every C++ 11/14 feature, so if your library can be compiled by clang 3.3 then it is highly likely it should compile, without too much work, on VS2015 assuming the missing items above are not showstoppers. VS2015 even has some support for C++ 1z (C++ 17) matching about what clang 3.5 provides minus C++ 14 relaxed constexpr and C++ 14 variable templates. See  http://blogs.msdn.com/b/vcblog/archive/2015/04/29/c-11-14-17-features-in-vs-2015-rc.aspx.

I am not claiming that you won't get surprises when you try getting MSVC to compile your code which you thought was standards compliant. MSVC is not an AST based compiler and uses heuristics to trigger partial local AST compilation, and therefore has a unique processing model which exposes your assumptions about what you think or thought is valid C++. This is exactly why it is worthwhile getting your C++ 11/14 library working on MSVC because you will get a better quality, more standards conforming C++ library out of it.

A good example of just how much C++ 14 support VS2015 provides is in Boost.DI. When I first contacted the author about the lack of VS2015 support, he proceeded to port his entirely C++ 14 codebase to VS2015 successfully, though he had to do a bit of refactoring to make the port work. Interestingly because he didn't push constexpr use past C++ 11 capacity in DI, VS2015's constexpr support was enough for DI to work as expected on VS2015 for the most part.

* https://krzysztof-jusiak.github.io/di/boost/libs/di/doc/html/

#cpp  #cplusplus #cppnow   #cppnow2015   #c ++ #boostcpp   #c ++11 #c ++14

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