No exception throw after open a file stream with non-exist files?

Question

I am trying to use

std::ifstream inStream;
inStream.open(file_name);

If file_name does not exists, no exception is thrown. How can I make sure to throw in this case? I am using C++11

[`std::ifstream::exceptions`](http://en.cppreference.com/w/cpp/io/basic_ios/exceptions) — πάντα ῥεῖ, Jan 02 '15 at 09:27
@πάνταῥεῖ You should probably post that as an answer, not as a comment. — , Jan 02 '15 at 09:38
it works, but it cannot show which file does not exist; I can only show the failbits which is not important to me. — Adam Lee, Jan 02 '15 at 09:41
@Adam : Eh, you already have `file_name` in scope... What's the problem? — ildjarn, Jan 02 '15 at 09:52
@ildjarn, when exception throw, I have no means to pass out file_name to exception message. — Adam Lee, Jan 02 '15 at 09:55
@Adam : Of course you do: catch the exception, and throw a different one including the filename. — ildjarn, Jan 02 '15 at 10:07

score 14 · Accepted Answer · edited Jan 02 '15 at 11:37

14

You can do so by setting the streams exception mask, before calling open()

std::ifstream inStream;
inStream.exceptions(std::ifstream::failbit);
try {
    inStream.open(file_name);
}
catch (const std::exception& e) {
    std::ostringstream msg;
    msg << "Opening file '" << file_name 
        << "' failed, it either doesn't exist or is not accessible.";
    throw std::runtime_error(msg.str());
}

By default none of the streams failure conditions leads to exceptions.

edited Jan 02 '15 at 11:37

Adam Lee

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answered Jan 02 '15 at 09:42

πάντα ῥεῖ

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e.what() can only returns "unspecified iostream_category error" when file does not exist. – Adam Lee Jan 02 '15 at 09:54
1

@AdamLee That's the best you can get AFAIK. There aren't that many possibilities besides non existing or non accessible files that the failbit is set by `open()`. – πάντα ῥεῖ Jan 02 '15 at 09:55
I edit one place, I need to use const std::exception& e rather than const & std::exception. – Adam Lee Jan 02 '15 at 11:38
BTW, it seems that I have to std::ostrstream rather than ostringstream, please confirm. – Adam Lee Jan 02 '15 at 11:39
@AdamLee THX, yes that was a typo. No `std::ostrstream` is obsolete. You need to `#include ` – πάντα ῥεῖ Jan 02 '15 at 11:39
I see, thanks. Do you mind to tell why the ostrstream is deprecated? – Adam Lee Jan 02 '15 at 11:58
1

@AdamLee See here please: http://stackoverflow.com/questions/2820221/why-was-stdstrstream-deprecated – πάντα ῥεῖ Jan 02 '15 at 12:02

No exception throw after open a file stream with non-exist files?

1 Answers1