c++ - C++ 方法 ostream::exceptions(iostate state) 的行为应该是什么？

Question

我正在编写我自己的输出流媒体库，我正在尝试模仿 std::ostream 行为，它不会与我试图实现的新行为冲突。目前我正在尝试模仿从 ios 继承的这个界面:

std::ostream::exceptions(ios::iostate state)

根据 cplusplus.com :

“此方法为流设置新的异常掩码并清除流的错误状态标志(就像调用成员 clear() 一样)。”

我不清楚这是否意味着所有标志都将被清除，还是仅清除异常掩码中设置的标志，因此我编写了一个测试程序，但得到了意想不到的结果。这是程序:

#include <sstream>
#include <iostream>

using namespace std;
int main(int argc, char** argv)
{
    ostringstream oss;
    try
    {
        cout << "            badbit           = " << ios::badbit        << "\n";
        cout << "            eofbit           = " << ios::eofbit        << "\n";
        cout << "            failbit          = " << ios::failbit       << "\n\n";

        cout << "            oss.rdstate()    = " << oss.rdstate()      << "\n";
        cout << "            oss.exceptions() = " << oss.exceptions()   << "\n\n";

        cout << "executing:  oss.setstate(ios::badbit | ios::failbit);" << "\n";
        oss.setstate(ios::badbit | ios::failbit);
        cout << "            oss.rdstate()    = " << oss.rdstate()      << "\n";
        cout << "            oss.exceptions() = " << oss.exceptions()   << "\n\n";

        cout << "executing:  oss.exceptions(ios::failbit);"             << "\n";
        oss.exceptions(ios::failbit);
        cout << "            oss.rdstate()    = " << oss.rdstate()      << "\n";
        cout << "            oss.exceptions() = " << oss.exceptions()   << "\n";
    }
    catch(const exception& x)
    {
        cout << endl;
        cout << "**** EXCEPTION THROWN ****"                            << "\n";
        cout << argv[0] << ":  " << x.what() << endl;
        cout << "            oss.rdstate()    = " << oss.rdstate()      << "\n";
        cout << "            oss.exceptions() = " << oss.exceptions()   << "\n";
        return 1;
    }
    catch(...)
    {
        cerr << argv[0] << ":  unknown exception." << endl;
        return 1;
    }
    return 0;
}

这是输出:

matt@dworkin:~/dev/ostream/libs/ostream$ g++ --version
g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

matt@dworkin:~/dev/ostream/libs/ostream$ g++ --std=c++17 foo.cpp
matt@dworkin:~/dev/ostream/libs/ostream$ ./a.out

            badbit           = 1
            eofbit           = 2
            failbit          = 4

            oss.rdstate()    = 0
            oss.exceptions() = 0

executing:  oss.setstate(ios::badbit | ios::failbit);
            oss.rdstate()    = 5
            oss.exceptions() = 0

executing:  oss.exceptions(ios::failbit);

**** EXCEPTION THROWN ****
./a.out:  basic_ios::clear: iostream error
            oss.rdstate()    = 5
            oss.exceptions() = 4

因此，根据 cplusplus.com 文档，我根本没想到会抛出异常。从异常处理程序中生成的输出可以看出，没有任何状态标志被清除。这是编译器错误、文档错误，还是我遗漏了什么？

顺便说一句，与记录的行为相比，我更喜欢展示的行为。对我来说，抛出错误的请求会产生消除现有错误的副作用，这对我来说似乎有点奇怪。实际上，我首先以与 g++ 显然相同的方式实现它(假设这是它肯定会工作的方式)，然后才阅读该方法的文档。

Answer 1

cplusplus.com 是错误的。他们是正确的 clear由 exceptions 调用，但它不被调用来清除状态并且状态不被清除。

不管名称如何，clear由 iostreams 在后端使用，including setstate itself , 为流状态的大量设置做繁重的工作，而不仅仅是清除。因此，它是容纳引发异常和其他与标志相关的行为的合乎逻辑的地方。

在exceptions看来clear主要用于测试现有标志以在任何所需标志已设置的情况下引发异常。要在异常后实际清除标志，请调用 clear仍然需要删除错误标志。