Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!etnibsd!vsh
From: vsh@etnibsd.UUCP (Steve Harris)
Newsgroups: comp.lang.c++
Subject: Re: Constant Question
Keywords: const
Message-ID: <1103@etnibsd.UUCP>
Date: 19 Jan 90 18:38:53 GMT
References: <130286@sun.Eng.Sun.COM> <1405@mit-amt.MEDIA.MIT.EDU>
Reply-To: vsh@etnibsd.UUCP (Steve Harris)
Organization: Eaton Corp, Semiconductor Equipment Div., Beverly, MA
Lines: 54

In article <1405@mit-amt.MEDIA.MIT.EDU> peter@media-lab.media.mit.edu (Peter Schroeder) writes:
 > 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

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Peter corrected the code, but didn't answer the question!

Isn't the real problem in line 2, not line 3?

Clearly, ptr is a const pointer to a (variable) float.  The pointer is
constant, but the thing pointed to is modifyable.

In line 2, you declare this pointer and initialize it to point to PI.
However, PI is a const float, not modifyable.

Shouldn't the compiler refuse to allow you to initalize (or assign) a
pointer (const or otherwise) to a const float?

Shouldn't the compiler insist that the pointer be of type "const float *"?

My g++ generates a warning at line 2, and creates a second object for ptr
to point to, so PI remains unchanged and *ptr is initalized to 3.14159 but
then is changed to 0.0.

-- 
Steve Harris - Eaton Corp. - Beverly, MA - uunet!etnibsd!vsh