LWG2349有什么效果？ [英] What Effect Would LWG2349 Have?

问题描述

虽然libstdc ++没有，但是libc ++遵循标准，通过 ios_base :: failbit basic_istream ::例外 对格式化输入没有影响。例如这个代码：

While libstdc++ does not, libc++ does follow the standard which states that passing ios_base::failbit to basic_istream::exceptions has no effect on formatted input. For example this code:

istringstream is{"ASD"};    
double foo;

is.exceptions(istream::failbit);

try {
    is >> foo;
    cout << foo << endl;
} catch(ios_base::failure& fail) {
    cout << "ouch\n";
}

将导致：

• 在libstdc ++上ouch


• libc ++上的0

• "ouch" on libstdc++
• "0" on libc++

我的阅读 LWG2349 是因为它会导致 basic_istream 不会抛出任何格式化的输入。

My reading of LWG2349 is that it would cause basic_istream to not throw on any formatted input.

例如LWG2349提出对标准的27.7.2.3 [istream] / 1的修改，该引用参考将使libc ++的行为像libstdc ++ 的错误无效。更改为粗体，如下所示：

For example LWG2349 proposes a change to the standard's 27.7.2.3 [istream]/1 which was cited with reference to the invalidation of a bug that would have made libc++ behave like libstdc++. The change is in bold and strike through below:

如果异常除了从（如果有的话），那么 ios :: badbit 在 *这个的错误状态。 （ basic_ios<> :: clear（）抛出的异常不会被捕获或重新抛出。）如果 ）& badbit）！= 0 然后异常被重新抛出。

If an exception , other than the ones thrown from clear(), if any, is thrown during input then ios::badbit is turned on in *this’s error state. (Exceptions thrown from basic_ios<>::clear() are not caught or rethrown.) If (exceptions()&badbit) != 0 then the exception is rethrown.

我明白 basic_istream :: clear 是什么对错误的格式化输入的反应，所以我误读了LWG2349，或者它实际上是停止 basic_istream 抛出任何错误？

I understand that basic_istream::clear is what throws in reaction to bad formatted input so am I misreading LWG2349 or is it in fact going to stop basic_istream from throwing any errors?

推荐答案

从 throw（） 抛出异常的要确保 clear（） throws，因为输入函数调用 clear（failbit） 和 （exceptions（）& failbit）！= 0 ，那么badbit没有被设置为结果。 clear（）将继续抛出这种情况，它不会设置badbit。

The point of the language excluding exceptions "thrown from clear()" is to ensure that if clear() throws, because an input function has called clear(failbit) and (exceptions() & failbit) != 0, then badbit is not set as a result. clear() will continue to throw in that case, it just will not set badbit.

如对LWG2349的评论，意图是当用户代码抛出异常时设置badbit：

As described in the commentary to LWG2349, the intention is that badbit is set when an exception is thrown from user code:

PJ和Matt都同意意图（badbit + rethrow）是表示用户代码中出现的异常，而不是iostreams包。

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，什么时候可以用户代码抛出异常，但是在iostreams机器中呢？一个例子是由locale getter：

Now, when can an exception be thrown by "user code" but within the iostreams machinery? One example would be by the locale getters:

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： p>

clang outputs:

eof fail

在LWG2349之后，正确的行为是设置badbit并抛出异常：

After LWG2349, the correct behavior is to set badbit and rethrow the exception:

my_get::do_get
eof bad

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