What is the difference between macro constants and constant variables in C?

Answer 1

Macros are handled by the pre-processor - the pre-processor does text replacement in your source file, replacing all occurances of 'A' with the literal 8.

Constants are handled by the compiler. They have the added benefit of type safety.

For the actual compiled code, with any modern compiler, there should be zero performance difference between the two.

Answer 2

For one thing, the first will cause the preprocessor to replace all occurrences of A with 8 before the compiler does anything whereas the second doesn't involve the preprocessor

Answer 3

You can write

#define A 8
int arr[A];

but not

const int A = 8;
int arr[A];

if I recall the rules correctly.

Answer 4

Macro-defined constants are replaced by the preprocessor. Constant 'variables' are managed just like regular variables.

For example, the following code:

#define A 8
int b = A + 10;

Would appear to the actual compiler as

int b = 8 + 10;

However, this code:

const int A = 8;
int b = A + 10;

Would appear as:

const int A = 8;
int b = A + 10;

:)

In practice, the main thing that changes is scope: constant variables obey the same scoping rules as standard variables in C, meaning that they can be restricted, or possibly redefined, within a specific block, without it leaking out - it's similar to the local vs. global variables situation.