12.4 enum Type

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The enumerated ( enum ) data type is designed for variables that can contain only a limited set of values. These values are referenced by name ( tag ^[1] ). The compiler assigns each tag an integer value internally, such as the days of the week. You could use the directive const to create values for the days of the week ( day_of_the_week ) as follows :

^[1] Tags are also called "named constants" or "enumerators."

 typedef int day_of_the_week;   // Define the type for days of the week const int SUNDAY    = 0; const int MONDAY    = 1; const int TUESDAY   = 2; const int WEDNESDAY = 3; const int THURSDAY  = 4; const int FRIDAY    = 5; const int SATURDAY  = 6; /* Now to use it */  day_of_the_week today = TUESDAY; 

This method is cumbersome. A better method is to use the enum type:

 enum day_of_the_week {SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY,       FRIDAY, SATURDAY};  /* Now use it */  enum day_of_the_week today = TUESDAY; 

The general form of an enum statement is:

 enum   enum-name   {   tag-1   ,   tag-2   , . . .}   variable-name;   

As with structures, the enum-name or the variable-name may be omitted. The tags may be any valid C++ identifier; however, tags are usually all uppercase.

An additional advantage of using an enum type is that C++ will restrict the values that can be used to the ones listed in the enum declaration. Thus, the following will result in a compiler error:

 today = 5;  // 5 is not a day_of_the_week 

If we want to force the issue, we have to use a static_cast to transform 5 into a day:

 today = static_cast<enum day_of_the_week>(5); 

So far we've let C++ do the mapping from enum tags to integers. For example, our enum declaration:

 enum day_of_the_week {SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY,       FRIDAY, SATURDAY}; 

results in SUNDAY being assigned 0, MONDAY gets 1, and so on. This works great if we don't care about the mapping. But suppose we are interfacing to a device that returns a set of error codes. We would like to define an enum to hold them. The problem is that the device returns error numbers and we need to map them precisely to our enum tags. C++ lets you specify the mapping values in your enum declaration:

 enum ERROR_RETURNS {     MOTOR_FAILURE = 55,     POSITION_ERROR = 58,     OIL_FAILURE = 33 }; 

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