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. Anyone (or 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 - code can't silently modify something important.
• A well-named constant reads better: const double PI = 3.14159; is far clearer than a naked 3.14159 in a formula.

A note about qualifiers

const is a qualifier - a word that modifies how a type behaves. There are others, like volatile (tells the compiler a value may change outside the program). Qualifiers sit before or after the type; we'll meet more later.

constexpr: constants the compiler knows

constexpr (C++11) means "this is a constant whose value is known at compile time". The compiler can compute and bake the value in before the program even runs:

constexpr int daysPerWeek = 7;

For fixed values used everywhere, constexpr is often the best choice: it's fast and it guarantees the value is set at compile time.

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.
• Qualifiers like const modify a type's behaviour.
• constexpr is for values known at compile time.