New feature in C++11 - "enum class"

Recently, I stumbled upon a new feature in C++11, “enum class”es. A simple feature, but a much need relief. Stroustrup has described the feature briefly in his C++11 FAQ here.

Anyone who has worked with C or C++ knows about enumerations, which was really a half-baked feature in the language. The “old” enumerations had the following problems:

Bad scoping rules - Enumerations had a global scope, which was not a great idea in the first place. So, if you had two enumerations with same enumerated value, you will need to adopt some sort of coding convention to avoid name collisions.

enum BookColor { RED, BLUE, GREEN };
enum PenColor { RED }; // error, RED is already declared

Implicit conversion - Since the C days, enumerations are known to convert to integers at the drop of a hat. So, if you have

enum BookColor { RED, BLUE, GREEN };
int color = BookColor::RED;   // color = 0 :(
if ( BookColor::RED < 5 )     // perfectly legal with implicit conversion

Inability to specify size of enumeration - Size of enumerations and even signed-ness is implementation defined, leading to all sort of confusions and portability problems. So, the above enumeration may be treated as a char on one machine/compiler and an integer on another machine.

Enter C++11 “enum class”

Scoping problem is resolved. So, the enumeration names do not conflict with other enums. This reflects the strong type safety provided by these enum classes. Following is legal in C++11:

enum class BookColor { RED, BLUE, GREEN };
enum class PenColor { RED }; // OK

No surprises with implicit conversions

if ( BookColor::RED > 5 ) // error, not legal in C++11 with enum class

You can specify the size of an enum now. Taking an example from the proposal document:

enum class E : unsigned long { E1 = 1, E2 = 2, Ebig = 0xFFFFFFF0U };
// ability to specify an unsigned type for unsigned enum value

Much needed relief

The feature eliminates surprises for new users and ensure type safety, which was missing for so long in C and C++ languages. I guess, I should read the Stroustrup FAQ more often and try to blog everytime I find something new or maybe write something on every topic.