Can you get a specific error condition when a C++

问题:

Is there any way to get a specific error condition when a C++ stream open fails? That is, whether it failed because the file didn't exist, or permissions were wrong, or etc. Basically I'm looking for functionality equivalent to errno for fopen() in plain C.

GCC seems to set errno properly, but that doesn't seem to be required by the C++ standard and I can't determine whether this is just an artifact of the way they implemented streams, or a deliberate feature (and thus I don't know whether it will persist across versions).

Is there a way to get this info reliably, either in standard C++ or non-accidentally in one or more major compilers?

回答1:

You can look at the ios flags (badbit, eofbit, failbit, goodbit) for general reasons. Testing will is easier using ios::bad(), ios::fail(), ios::eof(), or ios::good(). The stream can also be set to generate exceptions on error, using ios::exceptions().

Detailed I/O Error Reporting may be available in some implementations, as you point out for GCC. You may have to rely on this behavior for the different compilers. If there is a chance for multiple compilers, make sure to test and probably include preprocessor statements to check the current compiler, etc.
As far as I know, the only other place it's discussed is in proposed TR2 additions.

回答2:

In Visual Studio fopen and the like set the last-error code value. It can be retrieved with GetLastError(). The result will be one of these values.