Re: [boost] BOOST_NO_CXX11_CONSTEXPR and msvc-14.0

Subject: Re: [boost] BOOST_NO_CXX11_CONSTEXPR and msvc-14.0
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2015-07-22 20:08:10

On 21 Jul 2015 at 23:48, Peter Dimov wrote:

> I see that BOOST_NO_CXX11_CONSTEXPR is no longer being defined for 
> msvc-14.0. Even the RTM version has spotty support for constexpr though; the 
> techniques in my metaprogramming article, for example, don't work.

I turned on constexpr for VS2015 for all my own code to see what 
happens.

VS2015 RC couldn't compile my code with constexpr on, but I'm glad to 
report that with a bit of minor rejigging of code VS2015 successfully 
compiles everything I've got with C++ 11 constexpr enabled.

The minor rejigging was a bit silly. For example, I had a constructor 
like this:

class A;

class B : public A
{
  B() = default;
  constexpr B(int) : B() { }
};

... and VS2015 barfed about the B(int) not properly initialising all 
base classes, even though it is via a delegation to the defaulted 
constructor. The simple fix was:

  constexpr B(int) : A() { }

Another example was this:

template<class T> class Foo
{
  union
  {
    T a;
    std::error_code b;
    std::exception_ptr c;
  }
  int d;
  constexpr Foo() : d(5) { }
};

VS2015 didn't like that the union wasn't being initialised, despite 
that clang and GCC were happy with it. I think VS2015 is being 
correct here actually, the standard seems pretty clear I should have 
been initialising something. I worked around it like this:

template<class T> class Foo
{
  struct constexpr_standin_type { };
  union
  {
    T a;
    std::error_code b;
    std::exception_ptr c;
    constexpr_standin_type standin;
  }
  int d;
  constexpr Foo() : standin(), d(5) { }
};

... and now VS2015 is happy.

All these are minor annoyances really, but until Boost code is 
similarly rejigged to fit with VS2015's interpretation of constexpr I 
wouldn't enable constexpr by default for 1.59. Straight after release 
is the right time I think.

And my hat is off to the Visual Studio team for doing such a great 
job with VS2015 which really is a very conforming C++ 11 compiler 
now. I'm finally beginning to run into things older GCCs and clangs 
won't compile that VS2015 will, and that's *amazing*.

Niall

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