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From: thp@cs.ucr.edu
Newsgroups: comp.std.c++
Subject: Re: Are literals const?
Date: Thu, 23 Aug 2001 17:48:17 GMT
Organization: University of California, Riverside
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Peter Dimov <pdimov@mmltd.net> wrote:
: What do you think about this piece of code:

: int main()
: {
:   int const & r = 5;
:   const_cast<int &>(r) = 6;
: }

: Is this legal? I think that it is. Literals aren't const (they are
: about 90% const, but not 100% const.)

: Similarly:

: int f()
: {
:   return 5;
: }

: int main()
: {
:   int const & r = f();
:   const_cast<int &>(r) = 6;
: }

AFIK, they are both legal, and so is:

   int main()
   {
     int x = 5;
     int const & r = x;
     const_cast<int &>(r) = 6;
   }

IIRC, a const reference can refer to a non-const object but can't be
used to modify it.

: Do we _really_ need non-const literals and rvalues (of built-in
: types)?

Until recently I though that only lvalues could be const, but rvalues
of struct/class types have a this-pointer by which their member
functions can modify them.  So:

struct Stuff {
 void bar() const;
 void baz();
};

const Stuff foo();

int main() {
  foo().bar(); // cool
  foo().baz(); // error -- violates constness of rvalue foo().
}

That wedge opens the door for both const and volatile rvalues of all
types.  (I'm still getting used to the notion.)

Tom Payne

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