哪些运算符是自动为作用域枚举定义的？

Question

对于无范围枚举，答案是“大部分”，因为隐式转换为基础整数类型。但是，作用域枚举没有这种隐式转换。相反，为它们定义了一些但不是所有可用于无作用域枚举的运算符。

#include <iostream>

enum class Color{
    Red,
    Green,
    Blue
};

int main()
{
    std::cout << (Color::Red < Color::Green) << '\n';
    // Fine, operator< is defined for Color
    std::cout << (Color::Red + Color::Green == Color::Green) << '\n';
    // no match for 'operator+'
}

我的猜测是关系运算符已定义但算术运算符未定义，但我在 the cppreference page 上没有看到任何明确说明，也没有进行实际的标准潜水以了解什么C++ proper 不得不说这件事。

Answer 1

通过 [expr.rel]

选择了关系运算符

The usual arithmetic conversions are performed on operands of arithmetic or enumeration type. If both operands are pointers, pointer conversions and qualification conversions are performed to bring them to their composite pointer type. After conversions, the operands shall have the same type.

算术运算符被 ommision 选择退出。例如：[expr.add]

the left operand is a pointer to a completely-defined object type and the right operand has integral or unscoped enumeration type.

哪些运算符是自动为作用域枚举定义的？

Which operators are automatically defined for scoped enumerations?

c++

enums

operators