C++ Core Guidelines: Rules for Enumerations

The section to enumerations has eight rules. Since C++11, we have scoped enumerations which overcome a lot of the drawbacks of classical enumerations. 

Enumerations are sets of integer values which behave like a type. Here is the summary of the rules:

• Enum.1: Prefer enumerations over macros
• Enum.2: Use enumerations to represent sets of related named constants
• Enum.3: Prefer enum classes over “plain” enums
• Enum.4: Define operations on enumerations for safe and simple use
• Enum.5: Don’t use ALL_CAPS for enumerators
• Enum.6: Avoid unnamed enumerations
• Enum.7: Specify the underlying type of an enumeration only when necessary
• Enum.8: Specify enumerator values only when necessary

As I mentioned in the opening of this post: classical enumerations have a lot of drawbacks. Let me explicitly compare classical (unscoped) enumerations and scoped enumerations (sometimes called strongly-typed enumerations) because this important comparison is not explicitly described in the rules.

Here is a classical enumeration:

enum Colour{
  red,
  blue,
  green
};

Here are the drawbacks of the classical enumerations:

• The enumerators have no scope
• The enumerators implicitly convert to int
• The enumerators pollute the global namespace
• The type of enumerator is not defined. It just has to be big enough to hold the enumerator.

By using the keyword class or struct, the classical enumeration becomes a scoped enumeration (enum class):

enum class ColourScoped{
  red,
  blue,
  green
};

Now, you must use the scope operator to access the enumerators: ColourScoped::red. ColourScoped::red will not implicitly convert to int and will not pollute the global namespace. Additionally, the underlying type is per default int. 

After providing the background information, we can directly jump into the rules.

Enum.1: Prefer enumerations over macros

Macros don't respect scope and have no type. This means you can override a previously set macro that specifies a color.

// webcolors.h 
#define RED   0xFF0000

// productinfo.h
#define RED    0

int webcolor = RED;   // should be 0xFF0000

With ColourScoped, this will not happen because you have to use the scope operator: ColourScoped webcolour = ColourScoped::red;

Enum.2: Use enumerations to represent sets of related named constants

This rule is quite evident because the enumerators are a set of integers that create a type.  

Enum.3: Prefer enum classes over “plain” enums

The enumerators of a scoped enum (enum class) will not automatically convert to int. You have to access them with the scope operator.

// scopedEnum.cpp

#include <iostream>

enum class ColourScoped{
  red,
  blue,
  green
};

void useMe(ColourScoped color){

  switch(color){
  case ColourScoped::red:
    std::cout << "ColourScoped::red" << std::endl;
    break;
  case ColourScoped::blue:
    std::cout << "ColourScoped::blue" << std::endl;
    break;
  case ColourScoped::green:
    std::cout << "ColourScoped::green" << std::endl;
    break;
  }
}

int main(){

  std::cout <<  static_cast<int>(ColourScoped::red) << std::endl;   // 0
  std::cout <<  static_cast<int>(ColourScoped::red) << std::endl;   // 0

  std::cout << std::endl;

  ColourScoped colour{ColourScoped::red};
  useMe(colour);                                                     // ColourScoped::red

}

Enum.4: Define operations on enumerations for safe and simple use

The rules define an enumeration Day that supports the increment operation.

enum Day { mon, tue, wed, thu, fri, sat, sun };

Day& operator++(Day& d)
{
    return d = (d == Day::sun) ? Day::mon : static_cast<Day>(static_cast<int>(d)+1);
}

Day today = Day::sat;
Day tomorrow = ++today;

The static_cast is necessary in this example because applying the increment operator inside the increment operator would cause an infinite recursion:

Day& operator++(Day& d)
{
    return d = (d == Day::sun) ? Day::mon : Day{++d};    // error
}

Enum.5: Don’t use ALL_CAPS for enumerators

If you use ALL_CAPS for enumerators, you may get a conflict with macros because they are typically written in ALL_CAPS.

#define RED 0xFF0000

enum class ColourScoped{ RED };  // error

Enum.6: Avoid unnamed enumerations

If you can't find a name for the enumerations, the enumerations maybe not be related. In this case, you should use a constexpr value.

// bad
enum { red = 0xFF0000, scale = 4, is_signed = 1 };

// good
constexpr int red = 0xFF0000;
constexpr short scale = 4;
constexpr bool is_signed = true;

Enum.7: Specify the underlying type of an enumeration only when necessary

Since C++11, you can specify the underlying type of enumeration and save memory. Per default, the type of a scoped enum is int and, therefore, you can forward declare an enum.

// typeEnum.cpp

#include <iostream>

enum class Colour1{
  red,
  blue,
  green
};
 
enum struct Colour2: char {
  red,
  blue,
  green
};

int main(){

  std::cout << sizeof(Colour1) << std::endl;  // 4
  std::cout << sizeof(Colour2) << std::endl;  // 1

}

Enum.8: Specify enumerator values only when necessary

By specifying the enumerator values, you may set a value twice. The following enumeration Col2 has this issue.

enum class Col1 { red, yellow, blue };
enum class Col2 { red = 1, yellow = 2, blue = 2 };    // typo
enum class Month { jan = 1, feb, mar, apr, may, jun,
                   jul, august, sep, oct, nov, dec }; // starting with 1 is conventional

What's next?

I made it relatively short in this post. The meta-rule you should keep in mind is: use scoped enums.

The next section of the C++ core guidelines deals with about 35 rules for resource management. This means we dive into the next post right into the heart of C++.