`std::stringstream::fail()` return 在读取和写入之后应该有什么值？（海湾合作委员会与铿锵声）

Question

考虑以下代码片段：

#include <iostream>
#include <sstream>

int main()
{
    std::stringstream ss;
    ss << "12345";
    unsigned short s;
    ss >> s;
    ss << "foo";

    std::cout << std::boolalpha
              << "\nss.eof()  = " << ss.eof()
              << "\nss.good() = " << ss.good()
              << "\nss.bad()  = " << ss.bad()
              << "\nss.fail() = " << ss.fail()
              << "\nss.str()  = " << ss.str();
}

clang++ trunk 打印如下结果：

ss.eof()  = true
ss.good() = false
ss.bad()  = false
ss.fail() = false
ss.str()  = 12345

on wandbox

g++ trunk 打印如下结果：

ss.eof()  = true
ss.good() = false
ss.bad()  = false
ss.fail() = true
ss.str()  = 12345

on wandbox

如您所见，ss.fail() 的值在两个编译器之间是不同的。在这种情况下，标准对 std::stringstream 的行为有何规定？写入已消耗的流时设置 failbit/badbit 是否是实现定义的？

Answer 1

Gcc 是正确的。 std::stringstream 继承自 std::basic_ostream，并且根据 operator<<(std::basic_ostream) 的行为（从 ss << "foo"; 调用），

Effects: Behaves like a formatted inserter (as described in [ostream.formatted.reqmts]) of out.

来自 §30.7.5.2.1/1 Common requirements [ostream.formatted.reqmts]:

（强调我的）

Each formatted output function begins execution by constructing an object of class sentry. If this object returns true when converted to a value of type bool, the function endeavors to generate the requested output. If the generation fails, then the formatted output function does setstate(ios_base::failbit), which might throw an exception.

和§30.7.5.1.3 Class basic_ostream::sentry [ostream::sentry]：

If os.good() is nonzero, prepares for formatted or unformatted output. If os.tie() is not a null pointer, calls os.tie()->flush().

If, after any preparation is completed, os.good() is true, ok_ == true otherwise, ok_ == false. During preparation, the constructor may call setstate(failbit) (which may throw ios_base::failure ([iostate.flags]))

和§30.5.5.4 basic_ios flags functions [iostate.flags]：

iostate rdstate() const;
Returns: The error state of the stream buffer.

bool good() const;
Returns: rdstate() == 0

bool eof() const;
Returns: true if eofbit is set in rdstate().

本例设置了

eofbit，则std::basic_ios::good returns非零，导致写入失败（结果显示），则应设置failbit。

`std::stringstream::fail()` return 在读取和写入之后应该有什么值？（海湾合作委员会与铿锵声）

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`std::stringstream::fail()` return 在读取和写入之后应该有什么值？ （海湾合作委员会与铿锵声）

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