c++ - enum 类型是 unscoped 更喜欢 enum 类而不是 enum？

Question

我是 C++ 新手，我想知道是否有人可以帮助我理解原因

enum difficulty { NOVICE, EASY, NORMAL, HARD, UNBEATABLE };
difficulty myDiffiuclty = EASY

和

enum shipCost { FIGHTER_COST = 25, BOMBER_COST, CRUISER_COST = 50 };
shipCost myShipCost = BOMBER_COST;

有绿色下划线吗？它说它更喜欢枚举类，但是当我将其更改为枚举类时

enum class difficulty { NOVICE, EASY, NORMAL, HARD, UNBEATABLE };
difficulty myDiffiuclty = EASY;

enum class shipCost { FIGHTER_COST = 25, BOMBER_COST, CRUISER_COST = 50 };
shipCost myShipCost = BOMBER_COST;

EASY 变为红色下划线，并且myShipCost 用绿色下划线，BOMBER_COST 用红色下划线，CRUISER_COST 用红色下划线

const int ALIEN_POINTS = 150;
int aliensKilled = 10;
int score = aliensKilled * ALIEN_POINTS;
cout << "score: " << score << endl;

enum difficulty { NOVICE, EASY, NORMAL, HARD, UNBEATABLE };
difficulty myDifficulty = EASY;

enum shipCost { FIGHTER_COST = 25, BOMBER_COST, CRUISER_COST = 50 };
shipCost myShipCost = BOMBER_COST;
cout << "\nTo upgrade my ship to a cruiser will cost "
    << (CRUISER_COST - myShipCost) << " Resource Points.\n";

system("pause");
return 0;

Answer 1

enum class difficulty { NOVICE, EASY, NORMAL, HARD, UNBEATABLE };
difficulty myDiffiuclty = difficulty::EASY;

enum class shipCost { FIGHTER_COST = 25, BOMBER_COST, CRUISER_COST = 50 };
shipCost myShipCost = shipCost::BOMBER_COST;

使用 enum 类，与整数的隐式转换被删除，并且你必须在其中限定常量。

这被认为是一种改进。将其显式转换回整数仍然有效，但这不是偶然发生的。

std::cout << "\nTo upgrade my ship to a cruiser will cost "
    << (static_cast<int>(shipCost::CRUISER_COST) - static_cast<int>(myShipCost)) <<
    " Resource Points.\n";

system("pause");
return 0;

一旦你使用了enum class，你应该重命名它们:

enum class difficulty { novice, easy, normal, hard, unbeatable };
difficulty myDiffiuclty = difficulty::easy;

enum class shipCost { fighter =25, bomber=30, cruiser = 50 };
shipCost myShipCost = shipCost::bomber;

由于名称的 ship 部分现在在 enum 中，因此无需在常量中重复。同样，由于名称是有范围的，您不必SHOUT它们。

但是请注意，如果您只是使用它来创建常量而不打算创建类型，请考虑使用类似的东西:

namespace ship {
  namespace cost {
    constexpr int fighter = 25;
    constexpr int bomber = 30;
    constexpr int cruiser = 50;
  }
}

现在我们将 ship::cost::fighter 作为编译时计算的 int，而不是作为 enum 类 .