Pointers and Numbers

Pointers are not integer types, even though computers typically handle addresses as integers. Conceptually, pointers are distinct types from integers. Integers are numbers you can add, subtract, divide, and so on. But a pointer describes a location, and it doesn’t make sense, for example, to multiply two locations by each other. In terms of the operations you can perform with them, pointers and integers are different from each other. Consequently, you can’t simply assign an integer to a pointer:

int * pt;
pt = 0xB8000000;  // type mismatch

Here, the left side is a pointer to int, so you can assign it an address, but the right side is just an integer. You might know that 0xB8000000 is the combined segment-offset address of video memory on your aging computer system, but nothing in the statement tells the program that this number is an address. C prior to C99 lets you make assignments like this. But C++ more stringently enforces type agreement, and the compiler will give you an error message saying you have a type mismatch. If you want to use a numeric value as an address, you should use a type cast to convert the number to the appropriate address type:

int * pt;
pt = (int *) 0xB8000000; // types now match

Now both sides of the assignment statement represent addresses of integers, so the assignment is valid. Note that just because it is the address of a type int value doesn’t mean that pt itself is type int. For example, one might have a platform for which type int is a 2-byte value and the addresses are 4-byte values.

Pointers have some other interesting properties that we’ll discuss as they become relevant. Meanwhile, let’s look at how pointers can be used to manage runtime allocation of memory space.