March 10, 2023

Why you should use enum class.

TLDR: enum class is safer and less bug prone than enums. Here are the reasons why this is the case:

They don't implicitly convert to other types

Take for instance a plain enum:

enum Shape {SQUARE, CIRCLE, TRIANGLE};

int main() {
  Shape square = SQUARE;  // OK
  int circle = CIRCLE; // Ouch! OK
}

Now if as enum class:

enum class Shape {SQUARE, CIRCLE, TRIANGLE};

int main() {
  Shape square = SQUARE;  // error: undeclared identifier CIRCLE
  int circle = Shape::CIRCLE; // error: can't initialise type
}

And here's from Bjarne Stroustrup's C++11 FAQ:

The enum classes ("new enums", "strong enums") address three problems with traditional C++ enumerations:

• conventional enums implicitly convert to int, causing errors when someone does not want an enumeration to act as an integer.
• conventional enums export their enumerators to the surrounding scope, causing name clashes.
• the underlying type of an enum cannot be specified, causing confusion, compatibility problems, and makes forward declaration impossible.

The new enums are "enum class" because they combine aspects of traditional enumerations (names values) with aspects of classes (scoped members and absence of conversions).

Assembly code

It's always interesting to have a look at the generated Assembly code to verify if there's anything else to it.

For a plain enum we have the following:

enum Vehicle {BYCICLE, MOTORBIKE, ELECTRIC_CAR};

Vehicle bike = BYCICLE;
Vehicle motorbike = MOTORBIKE;

Assembly:

bike:
        .zero   4
motorbike:
        .long   1

Very straightforward, just two 4 byte labels, one containing only zeros, and another containing the value 1.

As for the enum class:

enum class Vehicle {BYCICLE, MOTORBIKE, ELECTRIC_CAR};

Vehicle bike = Vehicle::BYCICLE;
Vehicle motorbike = Vehicle::MOTORBIKE;

The assembly code is effectively the same:

bike:
        .zero   4
motorbike:
        .long   1

No surprises :)