_Myt& read(_Elem *_Str, streamsize _Count)
{ // read up to _Count characters into buffer
ios_base::iostate _State = ios_base::goodbit;
_Chcount = 0;
const sentry _Ok(*this, true);
if (_Ok)
{ // state okay, use facet to extract
_TRY_IO_BEGIN
const streamsize _Num = _Myios::rdbuf()->sgetn(_Str, _Count);
_Chcount += _Num;
if (_Num != _Count)
_State |= ios_base::eofbit | ios_base::failbit; // short read
_CATCH_IO_END
}
_Myios::setstate(_State);
return (*this);
}
如果_Myios::rdbuf()->sgetn(_Str, _Count)抛出异常，会被_TRY_IO_BEGIN，_CATCH_IO_END catch住(默认行为，定义_HAS_EXCEPTIONS)，然后将state置为badbit，然后跳到语句：_Myios::setstate(_State);但是这里的_State却是goodbit，又把流状态置为正常了。
调用者岂不是用stream.good()也没办法正确判断流状态？
难道我一定要在我的实现(streambuf继承)里将异常捕获返回错误(eof)才能适合这种read方法？
