Pointer Declarations

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A pointer must be declared as pointing to some particular type, even if that type is void (which really means a pointer to anything). Once declared, though, a pointer can usually be reassigned so that it points to an object of another type. The compiler lets you reassign pointers like this without typecasting, but the compiler will warn you unless the pointer was originally declared to be of type pointer to void. In C, but not C++, you can assign a void* pointer to a non-void* pointer. See void for details.

Warning: You need to initialize pointers before using them.

If type is any predefined or user-defined type, including void, the declaration

type *ptr;   /* Uninitialized pointer */

declares ptr to be of type "pointer to type." All the scoping, duration, and visibility rules apply to the ptr object just declared.

A null pointer value is an address that is guaranteed to be different from any valid pointer in use in a program. Assigning the integer constant 0 to a pointer assigns a null pointer value to it.

The mnemonic NULL (defined in the standard library header files, such as stdio.h) can be used for legibility. All pointers can be successfully tested for equality or inequality to NULL.

The pointer type "pointer to void" must not be confused with the null pointer. The declaration

void *vptr;

declares that vptr is a generic pointer capable of being assigned to by any "pointer to type" value, including null, without complaint. Assignments without proper casting between a "pointer to type1" and a "pointer to type2," where type1 and type2 are different types, can invoke a compiler warning or error. If type1 is a function and type2 isn't (or vice versa), pointer assignments are illegal. If type1 is a pointer to void, no cast is needed. Under C, if type2 is a pointer to void, no cast is needed.