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From: "Richard Smith"<richard@metafoo.co.uk>
Newsgroups: comp.std.c++
Subject: Defect report: constexpr and mutable members of literal types
Date: Wed, 19 Oct 2011 15:23:47 -0700 (PDT)
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Xref: x330-a1.tempe.blueboxinc.net comp.std.c++:328

Hi,

The C++11 IS allows literal types to have mutable members. In particular,
programs like the following are legal:

struct MM {
   mutable int n;
};

template<int n>  struct S { int k = n; };

int f(int k) {
   constexpr MM m = { 0 }; // ok, MM is a literal type
   m.n = k;                // ok, m.n is mutable
   return S<m.n>().k;      // ok, an lvalue-to-rvalue conversion of
                           // a subobject of a constexpr object is
                           // a constant expression [expr.const]/2
}

There seem to be two natural ways to fix this. Either (a) an lvalue-to-rvalue
conversion on a mutable member of a constexpr object should be considered to
not be a constant expression, or (b) literal types should not be allowed to
have mutable members.

(b) seems like the better solution, since it allows constexpr objects to be
ROM-able, which the constexpr papers note as an explicit goal. The clang
compiler currently implements (b).

Thanks,
Richard


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