Declaring type of pointers?

Question

I just read that we need to give the type of pointers while declaring them in C (or C++) i.e.:

int *point ; 

As far as I know, pointers store the address of variables, and address occupies same amount of memory whatever may be the type. So, why do we need to declare its type?

Answer 1

Type-safety. If you don't know what p is supposed to point to, then there'd be nothing to prevent category errors like

*p = "Nonsense"; int i = *p; 

Static type checking is a very powerful tool for preventing all kinds of errors like that.

C and C++ also support pointer arithmetic, which only works if the size of the target type is known.

address occupies same amount of memory whatever my be the type

That's true for today's popular platforms. But there have been platforms for which that wasn't the case. For example, a pointer to a multi-byte word could be smaller than a pointer to a single byte, since it doesn't need to represent the byte's offset within the word.

Answer 2

Because:

1. addresses to different types don't need to have the same size. The standard explicitly specifies so (C 2011 standard (online draft), 6.2.5/28).
2. type-safety: this allows the compiler to detect when you provide an incompatible pointer to a function, or in an assignment. This in turn prevents ugly situations where you mess up the argument order to a function.
3. the compiler needs to know the type when the pointer is dereferenced.
4. to do pointer arithmetic the size of the object pointed to needs to be known and thus its type.

The last two points don't apply to void pointers, which is why they cannot by dereferenced and no pointer arithmetic may be done on them. The standard specifies that a void pointer must be big enough to hold any kind of pointer (except function pointers, which are a different story altogether) and that assignment to and from void pointers may be made without casts (at least in C, in C++ casts are always needed).