Constants

Constants

Some values should never change: the speed of light, the number of days in a week, the tax rate. C lets you lock a value with const.

const

Put const before a declaration and the variable becomes read-only. Any later code that tries to change it gets a compile error:

const int x = 5;   // x can never change
x = 10;            // ERROR: x is const

• The value is fixed at creation.
• The compiler checks that nothing ever writes to it again.
• It prevents whole classes of "who changed this?" bugs.

Why use constants?

• They make your intent clear: "this is a fixed value, by design".
• They stop accidents.
• A well-named constant reads better: const double PI = 3.14159; is far clearer than a naked 3.14159.

The #define alternative

You may also see the older #define:

#define PI 3.14159

The preprocessor replaces every PI with 3.14159 before compiling. It has no type, so modern style prefers const.

TL;DR

• const makes a variable read-only: const int x = 5;.
• Trying to change a const value is a compile error.
• Constants express intent and prevent accidental changes.
• #define is the older, type-less way.