虽然 libstdc++ 没有,但 libc++ 确实遵循 the standard which states那路过ios_base::failbit
至 basic_istream::exceptions
对格式化输入没有影响。例如这段代码:
istringstream is{"ASD"};
double foo;
is.exceptions(istream::failbit);
try {
is >> foo;
cout << foo << endl;
} catch(ios_base::failure& fail) {
cout << "ouch\n";
}
会导致:
我对 LWG2349 的阅读是它会导致basic_istream
不要抛出任何格式化的输入。
例如,LWG2349 提议对标准的 27.7.2.3 [istream]/1 进行更改,该标准被引用为引用 the invalidation of a bug that would have made libc++ behave like libstdc++ .更改在下面以粗体和删除线表示:
If an exception , other than the ones thrown from
clear()
, if any, is thrown during input thenios::badbit
is turned on in*this
’s error state.(Exceptions thrown fromIfbasic_ios<>::clear()
are not caught or rethrown.)(exceptions()&badbit) != 0
then the exception is rethrown.
I understand that basic_istream::clear
is what throws in reaction to bad formatted input所以我误读了 LWG2349 还是它实际上会停止 basic_istream
抛出任何错误?
最佳答案
排除异常“从clear()
”抛出的语言的要点是确保if clear()
抛出,因为输入函数调用了 clear(failbit)
and (exceptions() & failbit) != 0
,然后是 badbit结果未设置。 clear()
将在这种情况下继续抛出,只是不会设置 badbit。
如 LWG2349 的评论中所述,其目的是在用户代码抛出异常时设置 badbit:
PJ and Matt both agree that the intention (of badbit + rethrow) is "to signify an exception arising in user code, not the iostreams package".
现在,什么时候“用户代码”可以在 iostream 机制中抛出异常?一个例子是语言环境 setter/getter :
struct my_get : std::num_get<char> {
using iter_type = std::istreambuf_iterator<char>;
iter_type do_get(iter_type, iter_type, std::ios_base&, std::ios_base::iostate&, bool&) const override {
throw std::logic_error{"my_get::do_get"};
}
};
int main() {
std::istringstream iss;
iss.imbue({std::locale{}, new my_get});
iss.exceptions(std::ios_base::failbit | std::ios_base::badbit);
try {
bool b;
iss >> b;
} catch (std::exception& ex) {
std::cout << ex.what() << '\n';
}
std::cout
<< ((iss.rdstate() & std::ios_base::eofbit) ? "eof " : "")
<< ((iss.rdstate() & std::ios_base::failbit) ? "fail " : "")
<< ((iss.rdstate() & std::ios_base::badbit) ? "bad " : "")
<< '\n';
}
目前gcc输出:
eof fail
clang 输出:
eof fail
在 LWG2349 之后,正确的行为是设置 badbit 并重新抛出异常:
my_get::do_get
eof bad
关于c++ - LWG2349 会有什么影响?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/35088800/