From: Erik Max Francis <max AT alcyone DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Constant pointers help.
Date: Wed, 29 Apr 1998 08:47:39 -0700
Organization: Alcyone Systems
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Jakub wrote:

> Could someone help me out here.
> 
> example:
> 
> const char *name1 = "John";    // Value cannot be changed
> 
> Is the above the same as?:
>    const char something = "John";
>    char *name1;
>    name1 = &something;
> 
> What would be the expansion of the following?
>  char *const name2 = "John";    // Pointer cannot be changed

Here and the different meanings of the const keyword:

    const int ki = 0; // means "constant int"
    ki = 2; // illegal

    const int aki[3] = { 1, 2, 3 }; // means "constant array of int"
    aki = ...; // illegal in any case; array name is not an lvalue
    aki[0] = 0; // illegal; array is const

    const int *pki = &ki; // means "pointer to constant int"
    pki = &aki[0]; // legal; pointer can be changed
    *pki = 0; // illegal; pointer is to constant int

    int *const kpi = &ki; // means "constant pointer to int"
    pki = &aki[0]; // illegal; pointer is const
    *pki = 0; // legal; pointer is const, but not the int it points to

    const int *const kpki = &ki; // means "constant pointer to 
                                 // constant int"
    kpki = &aki[0]; // illegal; pointer is const
    *kpki = 0; // also illegal; pointed-to int is also const

So in summary:  a const near the pointed-to type means that the value
pointed to, or the values in the array, cannot be changed.  In the case
of pointers, a const after the * means that the point itself is const,
which is independent of whether the point-to type is.

Remember to always read C declarations inside-out.  For instance:

    (const int (*(const (kpki))))

means kpki is a const pointer to const int.

-- 
         Erik Max Francis, &tSftDotIotE / mailto:max AT alcyone DOT com
                       Alcyone Systems / http://www.alcyone.com/max/
  San Jose, California, United States / icbm:+37.20.07/-121.53.38
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