Question

I am observing the following behavior under Visual Studio 2013 (Debug/Win32 compilation). Consider the following c++ code:

#include <iostream>
#include <climits>

int main(int argc, char *argv[])
{
  enum { V = (unsigned long long)ULLONG_MAX } E;
  std::cout << sizeof E << std::endl;

  enum : unsigned long long { W = (unsigned long long)ULLONG_MAX } F;
  std::cout << sizeof F << std::endl;

  return 0;
}

After compilation this leads to:

$ ./enum.exe
4
8

If I understand the c++ standard correctly (Standard C++ 7.2/5), this is an invalid c++ behavior. In this case, I should not be required to define the underlying type explicitly, since the value of an enumerator cannot fit in an int or unsigned int.

So:

Is this is a well known limitation of Visual Studio 2013 (maybe other versions are affected) ?
Is there a way to force the compiler to use a proper underlying type for a c++98 style enum ? Or am I required to switch to c++11 notation with fixed-type ?

Update: as suggested I reported a problem at:

https://developercommunity.visualstudio.com/content/problem/524018/underlying-type-of-an-unscoped-enum.html

Answer 1

参考文献中的内容如下（重要部分以粗体显示）：

声明一个非范围枚举类型，其基础类型不是固定的（在这种情况下，基础类型是实现定义的可以代表所有枚举器值的整数类型；这种类型是不得大于int ，除非枚举器的值不能适合 int或unsigned int 。如果枚举器列表为空，则基础类型就像枚举有一个值为0的枚举）。

和

无作用域枚举类型的值可以隐式转换为整数类型。如果基础类型不是固定的，则值为可转换为以下列表中的第一种类型，可以容纳它们的整个值范围：int，unsigned int，long，unsigned long，long 或 unsigned long long 。如果基础类型是固定的，则值可以转换为其提升的基础类型。

总的来说，很明显，这是msvc的一个错误（可能是在某个时候引入的）。更糟糕的是，对此他们保持沉默。

Underlying type of an unscoped enum?

1 个答案: