Enumerations Enumerations Enumerations

Scoped Enumerations C++11

enum class name { enumerator1, enumerator2, … enumeratorN };

default: each enumerator is mapped to a whole number from 0 to N-1

enum class day { mon, tue, wed, thu, fri, sat, sun };
day d = day::mon;      
d = day::tue;  // 
d = wed;  //  COMPILER ERROR: 'wed' only known in day's scope

enumerators confined to a named scope
cannot query properties of enumeration as in some other languages

Unscoped enumerations

enum name { enumerator1, enumerator2, … enumeratorN };

note the absence of keyword class

enum day { mon, tue, wed, thu, fri, sat, sun };
day d = mon;  // OK!, enumerator "mon" unscoped
int i = wed;  // OK!, i = 2
enum stars { sun, … };  //  COMPILER ERROR: name collision

enumerators not confined to a scope ⇒ name collisions
dangerous implicit conversion to underlying type
cannot query properties of enumeration as in some other languages

avoid unscoped enumerations

Underlying Type Of Enumerations

must be an integer type (char, short, long, …)
int is the default

// 7 values ⇒ char should be enough
enum class day : char {
  mon, tue, wed, thu, fri, sat, sun
};
// less than 10,000 ⇒ short should be enough
enum class language_ISO639 : short {
  abk, aar, afr, aka, amh, ara, arg, …
};

Custom Enumerator Mapping Enumerator Mapping Enumerators

enumerator values can be set explicitly
need not start with 0
some values can be left out
can be partial (only some enumerators with expl. value)

if you set enumerator values explicitly, do it for all enumerators

enum class month {
  jan = 1, feb = 2, mar = 3, apr = 4,  may = 5,  jun = 6,  
  jul = 7, aug = 8, sep = 9, oct = 10, nov = 11, dec = 12 
};
enum class flag {
  A = 2, B = 8, C = 5, D, E, F = 25
};enum class month {
  jan = 1, feb = 2,  mar = 3,  apr = 4,  
  may = 5, jun = 6,  jul = 7,  aug = 8,  
  sep = 9, oct = 10, nov = 11, dec = 12 
};
enum class flag {
  a = 2, b = 8, c = 5
};

Conversions To/From Underlying Type Enumerator ↔ Integer Conversion

enum class month {
  jan = 1, feb = 2,  mar = 3,  apr = 4,  
  may = 5, jun = 6,  jul = 7,  aug = 8,  
  sep = 9, oct = 10, nov = 11, dec = 12 
};

enum class month {
  jan = 1, feb = 2, mar = 3, apr = 4,  may = 5,  jun = 6,  
  jul = 7, aug = 8, sep = 9, oct = 10, nov = 11, dec = 12 
};

Enum → Integer

int i = static_cast<int>(month::mar);  
// i: 3

Integer → Enum

int i = 0;
cin >> i;
// make sure i ≥ 1 and ≤ 12 …
month m1 = static_cast<month>(i);

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Last updated: 2022-03-07

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