Proposal: constexpr, non-const member functions

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From	"Richard Smith"<richard@metafoo.co.uk>
Newsgroups	comp.std.c++
Subject	Proposal: constexpr, non-const member functions
Date	Tue, 8 Nov 2011 11:32:36 -0800 (PST)
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Hi,

Pointers and references to non-const objects can generally be used within
constant expressions in C++11. However, there is a roadblock in the way of
anyone who wants to use them as the 'this' pointer, because constexpr member
functions are implicitly const. Consider a trivial class like this:

   template<typename T>
   class Wrapper {
     T v;
   public:
     constexpr Wrapper(const T&v) : v(v) {}
     T&get() { return v; } // note, can't be constexpr
     constexpr const T&get() { return v; }
     // ...
   };

This seems fine until you try to use a temporary of this type in a constant
expression:

   constexpr int n = Wrapper<int>(0).get(); // ill-formed!

This doesn't work, because the non-const (and thus non-constexpr) get()
overload  is selected.

Since C++11 is already out of the gate, it's too late to remove the 'constexpr
implies const' rule, so instead we could consider a syntax like this:

   constexpr T&get() mutable { return v; }

Does this seem like a useful extension?
-- 
Richard


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Proposal: constexpr, non-const member functions "Richard Smith"<richard@metafoo.co.uk> - 2011-11-08 11:32 -0800
  Re: Proposal: constexpr, non-const member functions Dave Abrahams <dave@boostpro.com> - 2011-11-08 14:45 -0800
  Re: Proposal: constexpr, non-const member functions Daniel Krügler <daniel.kruegler@googlemail.com> - 2011-11-08 14:47 -0800
  Re: Proposal: constexpr, non-const member functions Marc <marc.glisse@gmail.com> - 2011-11-08 14:47 -0800
    Re: Proposal: constexpr, non-const member functions Daniel Krügler <daniel.kruegler@googlemail.com> - 2011-11-09 21:54 -0800
      Re: Proposal: constexpr, non-const member functions Daniel Krügler<daniel.kruegler@googlemail.com> - 2011-11-09 23:14 -0800

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