C + + 11 strong type enumeration

Enumeration is to define a category and enumerate the individuals under the unified category for code use.

Defects of C + + 98 enumeration:

• Whether namedEnumeration namestillEnumerating members in types, bothGlobal scopeYes, itsThe scope is globalof Appears if the same enumeration member is defined in a different enumerationRedeclarationWrong.

  enum PUBLIC_COLOR { RED, YELLOW, GREEN };
  enum PRIVATE_ COLOR { RED, BLACK, PURPLE };      //  Red redefinition
  enum PUBLIC_ COLOR { WHITE, GRAY }; 				//  PUBLIC_ Color redefinition
  
  int main() {
      int color = RED;
  
      cout << color << endl;
  }

  You may think of using namespaces to encapsulate enumeration types, but if a namespace misses a name, it will become an anonymous space, and all enumeration members in the anonymous space will become members under the global namespace by default. Therefore, this method is not appropriate.

• The members in the enumeration are designed as aliases of constant numbers, so they can be implicitly converted tointType, but this is not expected at some times.

• The space occupied by the enumeration type lock and the symbol are uncertain.

Strong enumeration type syntax format in C + + 11

enum class enum_name: elem_type { elem1, elem2, ... };

Advantages of strong enumeration type in C + + 11

• Strong scope, the name of a strongly typed enumeration member is not output to its parent scope space.

  enum class PUBLIC_COLOR { RED, GREEN, BLACK };  // ok
  enum PRIVATE_COLOR { RED, PURPLE, PINK = 1 };   // ok

• Conversion restrictions, the values of strongly typed enumeration members cannot be implicitly converted to and from each other.

  enum class PUBLIC_COLOR { RED, GREEN, BLACK };
  enum PRIVATE_COLOR { RED, PURPLE, PINK = 1 };
  
  If (public_color:: Green = = 1) {// errror: no acceptable type conversion defined
      cout << "color1 == 1" << endl;
  }
  
  if (PRIVATE_COLOR::PURPLE == 1) {   // ok
      cout << "color2 == 1" << endl;
  }

• You can specify the underlying type. The default underlying type of strongly typed enumeration isintThe underlying type can also be specified explicitly.

  enum class PUBLIC_COLOR: char { RED, GREEN, BLACK };
  enum PRIVATE_COLOR: int { RED, PURPLE, PINK };
  
  cout << sizeof(PUBLIC_COLOR::GREEN) << endl;		// 1
  cout << sizeof(PRIVATE_COLOR::PURPLE) << endl;		// 4

• It is not possible to compare enumeration values of different enumeration types. However, if the specified values are the same between the same enumeration values, they can be compared.

  enum class PUBLIC_COLOR { RED, GREEN, BLACK };
  enum PRIVATE_COLOR { RED, PURPLE, PINK = 1 };
  
  if (PUBLIC_COLOR::GREEN == PRIVATE_COLOR::PURPLE) {     // errror
      cout << "PUBLIC_COLOR::GREEN == PRIVATE_COLOR::PURPLE" << endl;
  }
  
  if (PRIVATE_COLOR::PURPLE == PRIVATE_COLOR::PINK) {     // ok
      cout << "PRIVATE_COLOR::PURPLE == PRIVATE_COLOR::PINK" << endl;
  }

• If you want to output enumerated values, you can cast them

  enum class PUBLIC_COLOR: char { RED, GREEN, BLACK };
  enum PRIVATE_COLOR: int { RED, PURPLE, PINK };
  
  cout << static_cast(PUBLIC_COLOR::RED) << endl;		// 0
  cout << static_cast(PRIVATE_COLOR::PURPLE) << endl;	// 1