file stream open failure reason

[Alexandros Frantzis]

Hello!
Is there a standard way to find the reason a file stream open failed (eg
file doesn't exist, access rights)?
Thanks.

 

[Jonne Lehtinen]

Hello!
Is there a standard way to find the reason a file stream open failed (eg
file doesn't exist, access rights)?
Thanks.

If using <fstream>:

#include <iostream>
#include <fstream>
int main() {

std::fstream file("non-existant-file");
if( !file ) {
std::cout << "could not open file." << std::endl;
}

return 0;

}

- Jonne Lehtinen

 

[Mike Wahler]

Hello!
Is there a standard way to find the reason a file stream open failed (eg
file doesn't exist, access rights)?

No there is not. All you can tell is if it succeeded or
failed. Your implementation might provide extensions to
the library for determining the specific reason for
failure (many do). Check your documentation.

-Mike

 

[Ron Natalie]

No there is not. All you can tell is if it succeeded or
failed. Your implementation might provide extensions to
the library for determining the specific reason for
failure (many do). Check your documentation.

perror() is standard....whether it's useful or not is implementation defined.

 

[Roger Leigh]

std::fstream file("non-existant-file");
if( !file ) {
std::cout << "could not open file." << std::endl;
}

That gives absolutely no indication of the reason for the failure. If
you use open(2) or std::fopen() then it's very easy to get at the
exact reason for the failure and print an informative error message.
Under Linux, there are 17 possible error codes listed in the open(2)
manual page.

If ISO C++ can't indicate the error status properly (why not?) then
what is needed is an equivalant of fdopen(3) to turn a file descriptor
into a stream. Nicolai Josuttis has written one IIRC (fdstream).

 

[Mike Wahler]

perror() is standard....
Right.

whether it's useful or not is implementation defined.

Agreed. So yes, there is a "standard way", but the
*results* will not be "standard". That's what I was
trying to say, albeit not very well.

-Mike