C++ 标准没有说明枚举类的 operator!=() 和 operator==()

Question

Section 7.2 Enumeration declarations 没有说明任何关于 operator!=() 和作用域的 operator==()枚举。但是下面的代码可以编译。

#include <iostream>

enum class Month{jan = 1, feb, mar, apr, may, jun, jul, aug, sep, oct, nov , dec};

int main()
{
    Month a = Month::feb;
    Month b = Month::jul;
    Month c = a;

    if( a != b ) std::cout << "a |= b" << '\n';
    if( a == c ) std::cout << "a == c" << '\n';
}

Answer 1

内置运算符在 5.10 中指定:

The == (equal to) and the != (not equal to) operators have the same semantic restrictions, conversions, and result type as the relational operators except for their lower precedence and truth-value result.

这将规范推迟到 5.9 中的关系运算符；对于 5.9/5 指定的枚举:

If both operands (after conversions) are of arithmetic or enumeration type, each of the operators shall yield true if the specified relationship is true and false if it is false.

因此，正如人们可能预料的那样，比较运算符适用于枚举，比较数值。