英文:
auto open_flags = std::ios::binary; seems to produce the wrong type in MSVC. Is it a bug?
问题
这段C++代码在gcc、icc和clang下编译正常，但在MSVC下失败：
#include <ios>
int main()
{
auto open_flags = std::ios::binary;
open_flags |= std::ios::app;
return 0;
}
错误信息为：<source>(6): error C2678: binary '|=': no operator found which takes a left-hand operand of type 'std::_Iosb<int>::_Openmode' (or there is no acceptable conversion)
将代码更改为以下形式会得到更有帮助的错误信息：
#include <ios>
int main()
{
auto open_flags = std::ios::binary;
open_flags = open_flags | std::ios::app;
return 0;
}
错误信息为：<source>(6): error C2440: '=': cannot convert from 'int' to 'std::_Iosb<int>::_Openmode'
而下面的代码编译正常：
#include <ios>
int main()
{
auto open_flags = std::ios::binary | std::ios::out;
open_flags = open_flags | std::ios::app;
return 0;
}
这看起来像是不正确的行为。似乎MSVC已将|运算符的返回类型实现为int，而不是ios::openmode。
值得注意的是，如果我使用std::ios::openmode而不是auto，原始代码会编译通过，可能是通过隐式转换实现的。
这是否是MSVC的错误，还是我漏掉了什么？欢迎参考标准文档！
英文:
This c++ code compiles fine with gcc, icc, and clang, but fails with MSVC:
#include <ios>
int main()
{
auto open_flags = std::ios::binary;
open_flags |= std::ios::app;
return 0;
}
> <source>(6): error C2678: binary '|=': no operator found which takes a left-hand operand of type 'std::_Iosb<int>::_Openmode' (or there is no acceptable conversion)
https://godbolt.org/z/999fffPEx
Changing the code to this gives a more helpful error message:
#include <ios>
int main()
{
auto open_flags = std::ios::binary;
open_flags = open_flags | std::ios::app;
return 0;
}
> <source>(6): error C2440: '=': cannot convert from 'int' to 'std::_Iosb<int>::_Openmode'
And this compiles fine:
#include <ios>
int main()
{
auto open_flags = std::ios::binary | std::ios::out;
open_flags = open_flags | std::ios::app;
return 0;
}
This looks like incorrect behaviour to me. Like MSVC has implemented the | operator with return type int instead of ios::openmode.
It's also worth noting that the original code compiles if I use std::ios::openmode instead of auto, presumably through implicit conversion.
Is this an MSVC bug, or am I missing something? Standards references welcome!
答案1
得分: 4
这是一个 MSVC 的错误。
根据标准：
binary具有类型std::ios_base::openmode（ios.base.general）;std::ios_base::openmode是一个位掩码类型（ios.openmode）。
但在 MSVC 上：
binary具有枚举类型（内部类型_Openmode)，但std::ios_base::openmode是int。_Openmode不是位掩码类型（因此只能使用内置运算符|,&,^,~）。
请在 https://github.com/microsoft/STL 提交一个错误报告（或两个）。
英文:
It's a MSVC bug.
According to the standard:
binaryhas typestd::ios_base::openmode([ios.base.general]);std::ios_base::openmodeis a bitmask type ([ios.openmode]).
But on MSVC:
binaryhas an enum type (the internal type_Openmode), butstd::ios_base::openmodeisint._Openmodeis not a bitmask type (and thus only built-in operators|,&,^,~are applicable).
Please file a bug report (or two) at https://github.com/microsoft/STL.
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