C++ ifstream 打开失败时如何获取错误消息
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/17337602/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How to get error message when ifstream open fails
提问by Alex F
ifstream f;
f.open(fileName);
if ( f.fail() )
{
// I need error message here, like "File not found" etc. -
// the reason of the failure
}
How to get error message as string?
如何以字符串形式获取错误消息？
采纳答案by Matthieu Rouget
Every system call that fails update the errnovalue.
每个失败的系统调用都会更新该errno值。
Thus, you can have more information about what happens when a ifstreamopen fails by using something like :
因此，您可以ifstream通过使用以下内容获得有关打开失败时会发生什么的更多信息：
cerr << "Error: " << strerror(errno);
However, since everysystem call updates the global errnovalue, you may have issues in a multithreaded application, if another system call triggers an error between the execution of the f.openand use of errno.
但是，由于每个系统调用都会更新全局errno值，因此在多线程应用程序中可能会遇到问题，如果另一个系统调用f.open在errno.
On system with POSIX standard:
在具有 POSIX 标准的系统上：
errno is thread-local; setting it in one thread does not affect its value in any other thread.
errno 是线程本地的；在一个线程中设置它不会影响它在任何其他线程中的值。
Edit(thanks to Arne Mertz and other people in the comments):
编辑（感谢 Arne Mertz 和评论中的其他人）：
e.what()seemed at first to be a more C++-idiomatically correct way of implementing this, however the string returned by this function is implementation-dependant and (at least in G++'s libstdc++) this string has no useful information about the reason behind the error...
e.what()起初似乎是一种更符合 C++-惯用正确方式的实现方式，但是该函数返回的字符串是依赖于实现的，并且（至少在 G++ 的 libstdc++ 中）该字符串没有关于错误背后原因的有用信息......
回答by Arne Mertz
You could try letting the stream throw an exception on failure:
您可以尝试让流在失败时抛出异常：
std::ifstream f;
//prepare f to throw if failbit gets set
std::ios_base::iostate exceptionMask = f.exceptions() | std::ios::failbit;
f.exceptions(exceptionMask);
try {
f.open(fileName);
}
catch (std::ios_base::failure& e) {
std::cerr << e.what() << '\n';
}
e.what(), however, does not seem to be very helpful:
e.what()，然而，似乎不是很有帮助：
- I tried it on Win7, Embarcadero RAD Studio 2010 where it gives "ios_base::failbit set" whereas
strerror(errno)gives "No such file or directory." - On Ubuntu 13.04, gcc 4.7.3 the exception says "basic_ios::clear" (thanks to arne)
- 我在 Win7、Embarcadero RAD Studio 2010 上尝试过，它给出了“ios_base::failbit set”，而
strerror(errno)给出了“No such file or directory”。 - 在 Ubuntu 13.04、gcc 4.7.3 上，异常显示为“basic_ios::clear”（感谢arne）
If e.what()does not work for you (I don't know what it will tell you about the error, since that's not standardized), try using std::make_error_condition(C++11 only):
如果e.what()对您不起作用（我不知道它会告诉您什么错误，因为这不是标准化的），请尝试使用std::make_error_condition（仅限 C++11）：
catch (std::ios_base::failure& e) {
if ( e.code() == std::make_error_condition(std::io_errc::stream) )
std::cerr << "Stream error!\n";
else
std::cerr << "Unknown failure opening file.\n";
}
回答by rthur
Following on @Arne Mertz's answer, as of C++11 std::ios_base::failureinherits from system_error(see http://www.cplusplus.com/reference/ios/ios_base/failure/), which contains both the error code and message that strerror(errno)would return.
按照@Arne Mertz 的回答，从 C++11 开始std::ios_base::failure继承自system_error（参见http://www.cplusplus.com/reference/ios/ios_base/failure/），其中包含strerror(errno)将返回的错误代码和消息。
std::ifstream f;
// Set exceptions to be thrown on failure
f.exceptions(std::ifstream::failbit | std::ifstream::badbit);
try {
f.open(fileName);
} catch (std::system_error& e) {
std::cerr << e.code().message() << std::endl;
}
This prints No such file or directory.if fileNamedoesn't exist.
No such file or directory.如果fileName不存在，则打印。
回答by ?eurobur?
You can also throw a std::system_erroras shown in the test code below. This method seems to produce more readable output than f.exception(...).
您也可以抛出 a std::system_error，如下面的测试代码所示。这种方法似乎比f.exception(...).
#include <exception> // <-- requires this
#include <fstream>
#include <iostream>
void process(const std::string& fileName) {
std::ifstream f;
f.open(fileName);
// after open, check f and throw std::system_error with the errno
if (!f)
throw std::system_error(errno, std::system_category(), "failed to open "+fileName);
std::clog << "opened " << fileName << std::endl;
}
int main(int argc, char* argv[]) {
try {
process(argv[1]);
} catch (const std::system_error& e) {
std::clog << e.what() << " (" << e.code() << ")" << std::endl;
}
return 0;
}
Example output (Ubuntu w/clang):
示例输出（Ubuntu w/clang）：
$ ./test /root/.profile
failed to open /root/.profile: Permission denied (system:13)
$ ./test missing.txt
failed to open missing.txt: No such file or directory (system:2)
$ ./test ./test
opened ./test
$ ./test $(printf '%0999x')
failed to open 000...000: File name too long (system:36)
