C++ Enums

An enumeration is a type whose values are a fixed list of names: the days of the week, the levels of a game, the states of an order. C++ has two kinds; prefer enum class.

enum class

#include <iostream>

enum class Level { Low, Medium, High };

int main() {
    Level fan = Level::Medium;
    if (fan == Level::Medium) {
        std::cout << "Medium speed\n";
    }
    std::cout << "Medium is number " << static_cast<int>(Level::Medium) << "\n";
    return 0;
}

Medium speed
Medium is number 1

The names are written with the enum's name in front (Level::Medium), so two enums can both have a value called Low. An enum class value does not turn into a number by itself; static_cast<int> converts it when you need the number.

Choosing the values

#include <iostream>

enum class HttpStatus { Ok = 200, NotFound = 404, ServerError = 500 };

int main() {
    HttpStatus s = HttpStatus::NotFound;
    std::cout << static_cast<int>(s) << "\n";
    return 0;
}

404

Enums and switch

#include <iostream>
#include <string>

enum class Size { Small, Medium, Large };

int price(Size s) {
    switch (s) {
        case Size::Small:  return 120;
        case Size::Medium: return 150;
        case Size::Large:  return 180;
    }
    return 0;
}

std::string name(Size s) {
    switch (s) {
        case Size::Small:  return "small";
        case Size::Medium: return "medium";
        case Size::Large:  return "large";
    }
    return "?";
}

int main() {
    Size order[] = {Size::Large, Size::Small, Size::Medium};
    int total = 0;
    for (Size s : order) {
        std::cout << name(s) << ": " << price(s) << "\n";
        total += price(s);
    }
    std::cout << "total: " << total << "\n";
    return 0;
}

large: 180
small: 120
medium: 150
total: 450

A switch over all values of an enum gets a warning when a value is forgotten, which helps when new values are added later.

The older plain enum

Without class, enum Color { Red, Green }; puts Red and Green directly into the surrounding scope and lets them convert to int silently. That is how C works and you will see it in older code, but the names can clash and mistakes slip through; enum class avoids both.

Why not just use numbers?

setSpeed(Level::High) explains itself; setSpeed(2) does not. With an enum class, a function that takes a Level cannot be called with a random number or with a value of another enum by mistake: the compiler stops it. Enums also make the set of possible values visible in one place, so a reader knows every case the code must handle.

Practice exercises

1. Make an enum class Season and print a sentence for each season with a switch.
2. Make an enum class Direction and a function that returns the opposite direction.
3. Try to print an enum class value directly with cout and read the error; fix it with static_cast.