問題描述

如何在 C++11 中輸出 enum class 的值?在 C++03 中是這樣的:

How can I output the value of an enum class in C++11? In C++03 it's like this:

#include <iostream>

using namespace std;

enum A {
  a = 1,
  b = 69,
  c= 666
};

int main () {
  A a = A::c;
  cout << a << endl;
}

在 c++0x 中，此代碼無法編譯

in c++0x this code doesn't compile

#include <iostream>

using namespace std;

enum class A {
  a = 1,
  b = 69,
  c= 666
};

int main () {
  A a = A::c;
  cout << a << endl;
}


prog.cpp:13:11: error: cannot bind 'std::ostream' lvalue to 'std::basic_ostream<char>&&'
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/ostream:579:5: error:   initializing argument 1 of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char, _Traits = std::char_traits<char>, _Tp = A]'

編譯于 Ideone.com

推薦答案

與無作用域枚舉不同，有作用域枚舉不能隱式轉換為其整數值.您需要顯式使用強制轉換將其轉換為整數:

Unlike an unscoped enumeration, a scoped enumeration is not implicitly convertible to its integer value. You need to explicitly convert it to an integer using a cast:

std::cout << static_cast<std::underlying_type<A>::type>(a) << std::endl;

您可能希望將邏輯封裝到函數模板中:

You may want to encapsulate the logic into a function template:

template <typename Enumeration>
auto as_integer(Enumeration const value)
    -> typename std::underlying_type<Enumeration>::type
{
    return static_cast<typename std::underlying_type<Enumeration>::type>(value);
}

用作:

std::cout << as_integer(a) << std::endl;

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