Re: [boost] [utility] Proposal to extract some components from Boost.Log

Subject: Re: [boost] [utility] Proposal to extract some components from Boost.Log
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2013-09-01 07:36:20

On Sunday 01 September 2013 11:06:16 Daryle Walker wrote:

> I don’t think you need a “constexpr_if.”  You can unconditionally put
> BOOST_CONSTEXPR in the definition.  This will unconditionally add
> “constexpr” to the member operator (for C++11 systems), which will be
> ignored if the compiler determines that the attached function (or a
> particular call) is disqualified from being used in constant expressions. 
> So any extra preprocessor work, or a variant macro, is unnecessary.

No, that doesn't work.

struct foo
{
  void* m_p;

  constexpr explicit operator bool () const
  {
    return !this->operator!();
  }

  bool operator! () const
  {
    return !m_p;
  }
};

g++ -std=gnu++11 constexpr_operator.cpp -o constexpr_operator
constexpr_operator.cpp: In member function ‘constexpr foo::operator bool() 
const’:
constexpr_operator.cpp:9:27: error: call to non-constexpr function ‘bool 
foo::operator!() const’