enum class: Named Choices

Sometimes a value can only be one of a few choices: a traffic light is red, yellow or green. Instead of using numbers like 0, 1 and 2 (and remembering what they mean), give the choices names with enum class:

#include <iostream>
using namespace std;

enum class Light { Red, Yellow, Green };

int main() {
    Light now = Light::Red;
    if (now == Light::Red) {
        cout << "Stop!\n";
    }
    now = Light::Green;
    if (now == Light::Green) {
        cout << "Go!\n";
    }
    return 0;
}

Output

Stop!
Go!

• Light is a new type with exactly three possible values.
• You write the values with the type name in front: Light::Red.
• You can't accidentally put any other value into a Light variable.

enum and switch: a perfect pair

#include <iostream>
#include <string>
using namespace std;

enum class Level { Easy, Medium, Hard };

string describe(Level level) {
    switch (level) {
        case Level::Easy:   return "Relaxed and fun";
        case Level::Medium: return "A fair challenge";
        case Level::Hard:   return "Good luck!";
    }
    return "";
}

int main() {
    cout << describe(Level::Medium) << "\n";
    cout << describe(Level::Hard) << "\n";
    return 0;
}

Output

A fair challenge
Good luck!

Getting the number behind a value

Each choice is stored as a number, starting at 0. To see it, use static_cast<int>:

#include <iostream>
using namespace std;

enum class Day { Mon, Tue, Wed, Thu, Fri };

int main() {
    Day d = Day::Wed;
    cout << static_cast<int>(d) << "\n";
    return 0;
}

Output

2

Remember: Older code uses plain enum (without class). It works, but its names leak out and mix with numbers too easily. Prefer enum class.