ccoding-stylec-preprocessor

Why do most C developers use define instead of const?

In many programs, a #define serves the same purpose as a constant. For example.

#define FIELD_WIDTH 10
const int fieldWidth = 10;

I commonly see the first form preferred over the other, relying on the preprocessor to handle what is basically an application decision. Is there a reason for this tradition?

Solution

  • There is a very solid reason for this: const in C does not mean something is constant. It just means a variable is read-only.

    In places where the compiler requires a true constant (such as for array sizes for non-VLA arrays), using a const variable, such as fieldWidth is just not possible.