Re: Simple question on Pointers

"Robby" <Robby@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:C1962AB0-0055-42E0-B8C8-964B666C7462@xxxxxxxxxxxxxxxx
In a similar set up, if I do:
===============
int main()
{
char a[] = "hello world";
char **ap = &a;

return 0;
}
===============

Why does the latter example give the following warning?

c:\dts_visual_c++\misc_c_samples\misc_c_samples\utility.c(10) : warning
C4047: 'initializing' : 'char **' differs in levels of indirection from
'char
(*)[12]'

I know others have responded, but I'm not sure if they've made it perfectly
clear... and re-reading this just now I think I will not be much help
either!

Though you may find it amusing at least to consider the following:

--------------------

It is in fact not entirely incorrect to think of &a as being a char**. After
all, 'a' is a kind-of pointer or address to some characters. Just try
writing this down:

char a[] = "hello world";
char* pc = a;

No compile warnings. So a is a char* right? WRONG!

Well, sticking to 32 bit pointers, we can cast &a to char** (pointer to
char*), and it WILL work (by coincidence really). I say it will, and it does
work, strangely, "provided that we do not attempt to reference 'a' "!

Why? Well, certainly the address of a (&a) is a "pointer type" like any
address, but 'a' itself is not a pointer, it is a "reference". What that
means is that 'a' merely "represents", or "is an alias for" a memory address
where a char* is located (with data {"hello world\0"}). But the address
where 'a' is located is does not exist. Ok well, it does exists in a sense,
but it's more like a "dummy label" at best (and moreover, it is never used).
For instance try compiling this:

char a[] = "hello world";
char c = (&a+1)[0][0];

warning C4700: uninitialized local variable 'a' used

What do you mean "uninitialized???" I said a[] = "hello world"; !

That's right, 'a' is not initialized, 'a[]' is initialized though.

So we can certainly write down char c = (&a)[0][0] but only because we're
extracting *(&(a[0])) and:

a[0], a[1], .... a[12] are all initialized variables (12 variables). 'a',
however, is not.

- Alan Carre


.

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