c++ - libstdc++ 和 libc++ : operator>> on bitset 行为差异

Question

考虑以下代码:

#include <bitset>
#include <sstream>
#include <iostream>

int main(int argc, char* argv[])
{
    std::stringstream stream;
    std::bitset<1> bitset(1);
    std::cout<<"before = "<<bitset[0]<<std::endl;
    stream<<"4";
    stream>>bitset;
    std::cout<<"after = "<<bitset[0]<<std::endl;
    return 0;
}

在g++下用libstdc++编译，结果为:

> g++ bitset_empty.cpp -o bitset_empty
> ./bitset_empty 
before = 1
after = 1

在clang++下用libc++编译，结果为:

> clang++ -stdlib=libc++ bitset_empty.cpp -o bitset_empty
> ./bitset_empty 
before = 1
after = 0

哪个是对的？两者(因为未定义的行为)？海湾合作委员会？ clang ？

Answer 1

据我了解，libc++ 就在这里，但它不是唯一正确的行为。

N4140 §20.6.4 [bitset.operators]

Effects: Extracts up to N characters from is. Stores these characters in a temporary object str of type basic_string<charT, traits>, then evaluates the expression x = bitset<N>(str). Characters are extracted and stored until any of the following occurs:

• N characters have been extracted and stored;
• end-of-file occurs on the input sequence;
• the next input character is neither is.widen(’0’) nor is.widen(’1’) (in which case the input character is not extracted).

If no characters are stored in str, calls is.setstate(ios_base::failbit) (which may throw ios_base::failure (27.5.5.4))

重要的是要注意 x = bitset<N>(str)不是有条件的。如果ios_base::failure没有抛出，那么就是执行的表达式。和 bitset<N>(""s) (即空字符串)是 0 .

因此，据我了解，您的 bitset应该归零或者应该抛出上述异常。

如果没有抛出异常，您可能想要测试您的操作是否成功(通过测试返回的流)。