Velocity Reviews - Array and Pointer Tutorial

Some programmers treat arrays just like pointers (and some even think that
they're exactly equivalent). I'm going to demonstrate the differences.

Firstly, let's assume that we're working on a platform which has the
following properties:

1) char's are 8-Bit. ( "char" is synomonous with "byte" ).
2) int's are 32-Bit. ( sizeof(int) == 4 ).
3) Pointers are 64-Bit. ( sizeof(int*) == 8 ).

First let's make two declarations:

int main(void)
{
int array[5];

int* const pointer = (int*)malloc( 5 * sizeof(int) );
}

Now I'll demonstrate how "array" and "pointer" are different:

I'll start off with simple analgous expressions:

================================================== ==========================
| Expression | Type and Access Specifiers | That in English |
================================================== ==========================
| | | |
| array | int[5] | An array of five int's.|
| | | |
|---------------------------------------------------------------------------
| | |A const pointer which |
| pointer | int* const |points to a modifiable |
| | |int. |
|--------------------------------------------------------------------------|
| | |A const pointer which |
| &array | int (* const)[5] |points to a modifiable |
| | |array of five int's. |
|--------------------------------------------------------------------------|
| | |A const pointer, which |
| &pointer | int* const* const |points to a const |
| | |pointer, which points to|
| | |a modifiable int. |
================================================== ==========================

Here's how "sizeof" works with them:

================================================== =========
| Expression | sizeof( exp ) | But Why? |
================================================== =========
| | | |
| array | 20 | It's five int's. |
| | (5 * 4) | |
|---------------------------------------------------------|
| | | |
| pointer | 8 | It's just a pointer.|
| | (just 8) | |
|---------------------------------------------------------|
| | | |
| &array | 8 | It's just a pointer.|
| | (just 8) | |
|----------------------------------------------------------
| | | |
| &pointer | 8 | It's just a pointer.|
| | | |
| | (just 8) | |
================================================== =========

Okay next thing to discuss is the usage of square brackets, and the
dereference operator. The two of these are to be used by pointers only. So
how come we can use them with arrays, as follows?:

array[0] = 4;

*array = 6;

The reason is that an expression of the following type:

int[5]

can implicitly convert to an expression of the following type:

int* const

What it does is convert to a pointer to the first element of the array.
Therefore, the first example:

array[0] = 4;

implicitly converts "array" to a normal pointer, then uses chain brackets to
access memory at a certain offset from the original address.

Also the second example:

*array = 6;

implicitly converts "array" to a normal pointer, then dereferences it.

NOTE: You must remember that an array implicitly converts to a pointer to
its first element, NOT to a pointer to the array. This fact has a few
implications. Here's one such implication:

*(array + 3) = 6;

What the above line of code does is the following:

1) Implicitly converts "array" to an: int* const
2) Adds 3 * sizeof(int) to the address.
3) Dereferences the resulting pointer, and assigns 6 to it.

If "array" implicitly converted to: int (*)[5]
rather than a pointer to the first element, then Step 2 above would be
different, specifically:

2) Adds 3 * sizeof( int[5] ) to the address.

And we know that sizeof(int[5]) is 20 on this platform (not 8!).

So you may ask, "What's the point in having a pointer to an array?" -- well
here's where it may come in handy:

void SomeFunc ( int (* const p_array)[5] )
{
(*p_array)[0] = 99;
(*p_array)[1] = 98;
(*p_array)[2] = 97;
(*p_array)[3] = 96;
(*p_array)[4] = 95;

/* This function won't accept an array of any
other size! */
}

And here's a C++-specific example with references:

void SomeFunc ( int (&array)[5] )
{
array[0] = 99;
array[1] = 98;
array[2] = 97;
array[3] = 96;
array[4] = 95;

/* This function won't accept an array of any
other size! */
}

Also in C++, you can exploit the use of templates:

template<class T, unsigned long len>
void KeepCopy( const T (&array)[len] )
{
static my_array[len];

/* Do some other stuff */
}

I've posted this to a few newsgroups, so if you'd like to reply, please post
to comp.lang.c because it's the common denominator. If your post is C++-
specific, the please post to comp.lang.c++.

Did I leave anything out?

-Tomás

[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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