Re: "Mastering C Pointers"....

From: goose (ruse_at_webmail.co.za)
Date: 11/05/03 

Date: 5 Nov 2003 00:03:58 -0800

"Roose" <nospam@nospam.nospam> wrote in message news:<TRIpb.13665$Jn.1970@newssvr14.news.prodigy.com>...
> > After reading the first 1/3 carefully, it seems to me that a pointer is
> very
> > much like a symlink.
>
> Uh, I'm not exactly sure what a symlink is, but if it is anything like a
> Windows shortcut, there is definitely an analogy there.
>
> The windows shortcut just stores the location of a file. A "pointer" to it.
> An address. The address is the _path name_. In C, the "address" is
> basically an integer,
   
no, it isn't *always* an integer type. in x86 real-mode it is *two*
integer types.

> indicating where in a huge array of memory the
> variable is.
>
> You can have as many shortcuts as you want that point to the file. A
> shortcut takes a small amount of storage, but not as much as a (typical)
> file. You can have whole directories of shortcuts that point to files all
> over the place, in order to organize them. Likewise, you can have
> collections of pointers, that point to various different things _scattered_
> all over memory.
>
> For example, on my hard disk, I have a whole bunch of (legal) MP3s organized
> by artist and by album. Now, some of them were ripped poorly and have
> clicks. So I *could* just copy those files to another folder, in order to
> note that I need to rip them again.
>
> However, it is *much* easier and efficient to just copy shortcuts those bad
> files into another folder -- called "bad". That way I don't duplicate any
> data. And I have all the bad files organized in one directory, and can
> access them easily.
>
> You could do the exact same thing with pointers. Suppose I had a whole
> bunch of strings in memory, that listed a ton of song names. I could create
> an array of pointers called "theBeatles" that pointed to every single song
> by the Beatles. Then I could have an array of pointers called "badFiles"
> that pointed to all the bad files, _some of which_ may be by the Beatles.
> Note that I haven't incurred the storage cost of repeating the strings,
> *just* pointers to them. Likewise I could create another array called
> "before1960" that points to all songs stored before 1960 -- you get the
> idea.
>
> Hm, I'm surprised this didn't come up earlier. This is an easy analogy.

but not entirely accurate. this is because you cannot do "shortcut
arithmetic" but you /can/ do pointer arithmetic.

<to OP>

a pointer variable holds an address of an object. for example
a "pointer to char" can hold the addess of a char, and a
"poniter to int" can hold the address of an int. to access
the object itself, one must dereference the pointer.

dereferencing means "get the object at that address". for example:

/* we declare an object of type int */
int foo = 12;

/* we declare a pointer to int */
int *bar;

/* now we set bar to point to foo, using the address of operator */
bar = &foo;

/* foo still contains 12, while bar contains the address of foo.
   now we dereference bar to get the value of foo (namely, 12) */
printf ("bar points to foo which has number %i\n", *bar);
/* that would have printed
   "bar points to foo which has number 12."
   */

hth
goose,
   busy as can bee

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