c++ - std::is_signed 不适用于强类型枚举:int

Question

谁能解释一下，为什么

#include <iostream>
#include <type_traits>
using namespace std;
enum E : signed int { a=-1, b = 1,};
int main() {
    std::cout << std::boolalpha;
    cout << "strong typed enum E:int should be signed, but is_signed returns " 
    << is_signed<E>() << "\n";
    return 0;
}

std::is_signed<> 不做，锡上写的是什么？谢谢...

Answer 1

如果我们查看 is_signed 的文档它说:

If T is a signed arithmetic type, provides the member constant value equal true. For any other type, value is false.

并且枚举不是算术类型，因此结果应该为假。来自 C++11 标准草案 3.9.1 基础类型 [basic.fundamental]:

[...]Integral and floating types are collectively called arithmetic types[...]

您可以使用 std::underlying_type 获取枚举的基础类型然后将 std::is_signed 应用于该类型。