Re: [xml] what should to be 'const'?

[Rush Manbert <rush manbert com>]

massimo morara wrote:
> On Mon, May 21, 2007 at 10:00:03AM -0700, Rush Manbert wrote:
>
> > massimo morara wrote:
> >
> > > 	In the function xmlCopyNode(const xmlNodePtr node, int 
> > > extended) [tree.c], what is supposed be 'const'?
>
>
> > Write the declaration out, then read it backwards to determine what is 
> > const. In this case, I believe the declaration would be "const xmlNode 
> > *", so reading it backward says that it's a pointer to a xmlNode that is 
> > const. A const pointer would be "xmlNodePtr const node"
>
>
>         Sorry, but i'm pretty sure that "const xmlNodePtr node" is (in
> "tree.h") "xmlNode * const node", so is the pointer that is costant;
> next i transcribe a little trivial program to verify a similar case.
>         My question is: what we wish costant: the pointer or the data?
>         If we wish that is costant the structure pointed, we should
> write "xmlNode const * node" or, defining in "tree.h" something like
>
> typedef  xmlNode const *  xmlConstNodePtr;
>
> write "xmlConstNodePtr node".
>         For xmlCopyNode() and for other functions.
>
>                 massimo morara
>
>
> ps: sorry for my bad english
>
>
> ---- test program ---- 
>
>
> #include <stdio.h>
>
> typedef struct
>  {
>    int foo;
>  } fooStruct ;
>
>
> typedef  fooStruct *  fooStrPtr;
>
>
> void  changeFooData (const fooStrPtr  pfd)
>  {
>    pfd->foo = 2;
>
>    /* pfd++; forbidden */
>  }
>
>
> int main (void)
>  {
>    fooStruct  fooData;
>
>    fooData.foo = 1;
>
>    changeFooData(&fooData);
>
>    printf("foo val: %i\n", fooData.foo);
>
>    return(0);
>  }

Massimo,

Sorry, my mistake. I thought that "const xmlNodePtr node" meant the same 
thing as "const xmlNode * node", as if xmlNodePtr were a macro 
definition that expanded to "xmlNode *", but the typedef changes things, 
so the const modifies the pointer itself.

I took your example and changed changeFooData to this:

void  changeFooData (const fooStruct *  pfd)
{
        pfd->foo = 2; /* <--- Forbidden */
        
        pfd++;
}

and the compiler complains about the assignment, but not the increment, 
so now it makes sense to me.

You are correct and I have learned something. The declaration means that 
the pointer is const, not the data. If what you are asking for is the 
intent, then Daniel will need to answer.

- Rush