Constants (const)

Sometimes a value should never change once it's set — think of the number of days in a week, or the maximum number of players allowed in a game. C++ lets you mark a variable as a constant using the const keyword, which tells the compiler this variable's value must never change after it is initialized.

const double PI = 3.14159;
std::cout << PI << std::endl;

By convention, constant names are often written in UPPER_SNAKE_CASE: every letter capitalized, with underscores separating words, such as MAX_PLAYERS or DAYS_IN_WEEK. This makes constants instantly recognizable, setting them apart at a glance from regular camelCase variables.

const int MAX_PLAYERS = 4;
std::cout << MAX_PLAYERS << std::endl;

If you try to reassign a const variable after it has been initialized, the code will not compile — the compiler catches the mistake before the program ever runs. This is exactly why constants are so useful: they replace unexplained 'magic numbers' scattered through your code with a single, clearly named, protected value. If that value ever needs to change, you update it in exactly one place.

const int MAX_LIVES = 3;
MAX_LIVES = 5; // Compile error: assignment of read-only variable

The ____ keyword tells C++ that a variable's value can never be changed after it is initialized.

What happens if you try to reassign a variable declared with const?

A const variable must be given its value at the moment it is declared, since it can never be assigned afterward.