Re: does this have undefined behaviour

From:
"Victor Bazarov" <v.Abazarov@comAcast.net>
Newsgroups:
comp.lang.c++
Date:
Fri, 21 Dec 2007 09:10:28 -0500
Message-ID:
<fkghgl$qk6$1@news.datemas.de>
werasm wrote:

On Dec 21, 5:04 am, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

subramanian10...@yahoo.com wrote:

Consider the following code:

#include <iostream>
#include <cstdlib>

using namespace std;

int main()
{
const double& ref = 100;
double& d = const_cast<double&>(ref);
cout << d << endl;
d = 1.1;
cout << ref << endl;

return EXIT_SUCCESS;
}

In both g++ and VC++ 2005 Express Edition, the output is
100
1.1

But my doubt is: "does using 'd' invoke undefined behavior or is it
valid."

double& d = const_cast<double&>(ref);


What you're doing here is changing the value of the temporary
bound to a const reference. Such temporary is not const, so,
changing its value by casting away const-ness should be OK.
I can't find any place in the Standard that would prohibit
such functionality. Then my guess is that it's allowed.


I think the OP might have thought that he actually changed the value
of "ref" (the constant one). I don't think he realized that a
temporary was created - therefore changing the value of "d" does
not change the value of "ref".


No, that's incorrect, IMO. First off, you can't change the value
of ref because it's a reference, it doesn't really have a value
(aside from the fact that it refers to some particular object).
The temporary created is of type 'double', it has initial value
of 100.00 and it starts its lifetime just before 'ref' is bound
to it. The integral expression '100' is the initialiser for that
temporary.

The temporary to which 'ref' is bound is not a const object. It
is perfectly OK to change its value. Whether it makes sense to
do so or not is another question, but the idea is the same as in

    void foo(int const& rci)
    {
        int& ri = const_cast<int&>(rci);
        ri = 666; /// Is this OK? -- I say, yes, it is!
    }

    #include <iostream>
    #include <ostream>
    int main() {
        int a = 42;
        std::cout << "Before a = " << a << std::endl;
        foo(a);
        std::cout << "After a = " << a << std::endl;
    }

V
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