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From: peter@mit-amt.MEDIA.MIT.EDU (Peter Schroeder)
Newsgroups: comp.lang.c++
Subject: Re: Constant Question
Keywords: const
Message-ID: <1405@mit-amt.MEDIA.MIT.EDU>
Date: 16 Jan 90 01:05:37 GMT
References: <130286@sun.Eng.Sun.COM>
Reply-To: peter@media-lab.media.mit.edu (Peter Schroeder)
Organization: MIT Media Lab, Cambridge MA
Lines: 30

In article <130286@sun.Eng.Sun.COM>
rbogen%dreams@Sun.COM (Richard Bogen) writes:
>The definition of a constant is an identifier whose
>associated value cannot be modified (even through pointers).
>Yet when the following program is run the value printed is 0.0.
>Can anyone give me a "pointer" to what is happening here?
>
>main()
>{
> const float PI = 3.14159;
> float *const ptr = &PI;
> *ptr = 0.0;
> printf("%PI = %f\n",PI);
>}

What you declared is not a pointer to a const, but a const pointer. Hence
you can not assign to the this const pointer another address, but you can
modify what it points to.

What you want is:
main()
{
 const float PI = 3.14159;
 const float *ptr = &PI;
 *ptr = 0.0; // error
 printf("%PI = %f\n",PI);
}

Peter
peter@media-lab.media.mit.edu