Getting Error Text from ifstream

[Johannes Bauer]

Hello group,

coming from C, I'm used to somthing like this when opening a file

const char *foobar = "this.txt";
FILE *f;
f = fopen(foobar, "r");
if (!f) {
fprintf(stderr, "Couldn't open %s: %s\n", foobar, strerror(errno) );
exit(EXIT_FAILU RE);
}

translating that somewhat to C++ yields me with

const char *foobar = "this.txt";
std::ifstream f(foobar);
if (!f) {
throw GenericExceptio n("Couldn't open file.");
}

So I can detect that opening the file wasn't successful, but I don't
know *why* (i.e. permissions, no such file or directory, etc.). My guess
is that strerror and errno still do what I expect them to do, however I
think that would be a very C-way to solve things.

How can I accomplish strerror(errno) on the std::ifstream with C++ means?

Greetings,
Johannes

--
"Viele der Theorien der Mathematiker sind falsch und klar
Gotteslästerli ch. Ich vermute, dass diese falschen Theorien genau
deshalb so geliebt werden." -- Prophet und Visionär Hans Joss aka
HJP in de.sci.mathemat ik <47************ **********@news .sunrise.ch>

[David Harmon]

On Tue, 20 Nov 2007 14:45:24 +0100 in comp.lang.c++, Johannes Bauer
<df***********@ gmx.dewrote,

>So I can detect that opening the file wasn't successful, but I don't
know *why* (i.e. permissions, no such file or directory, etc.). My guess
is that strerror and errno still do what I expect them to do, however I
think that would be a very C-way to solve things.

Use strerror(), perror(), and errno and consider yourself lucky. The
C++ standard doesn't exactly promise that they will work for iostreams,
but they do for the compilers I use. That's the best you get for now.

[Johannes Bauer]

David Harmon schrieb:

Use strerror(), perror(), and errno and consider yourself lucky. The
C++ standard doesn't exactly promise that they will work for iostreams,
but they do for the compilers I use. That's the best you get for now.

Oh, that's really a shame. Is anything planned about this in future
specifications?

Greetings,
Johannes

--
"Viele der Theorien der Mathematiker sind falsch und klar
Gotteslästerli ch. Ich vermute, dass diese falschen Theorien genau
deshalb so geliebt werden." -- Prophet und Visionär Hans Joss aka
HJP in de.sci.mathemat ik <47************ **********@news .sunrise.ch>

[David Harmon]

On Tue, 20 Nov 2007 18:23:35 +0100 in comp.lang.c++, Johannes Bauer
<df***********@ gmx.dewrote,

>David Harmon schrieb:

>Use strerror(), perror(), and errno and consider yourself lucky. The
C++ standard doesn't exactly promise that they will work for iostreams,
but they do for the compilers I use. That's the best you get for now.

Oh, that's really a shame. Is anything planned about this in future
specifications ?

I have not heard of any. That would be a good question for
comp.std.c++, where the c++ standardization process is discussed.

[James Kanze]

On Nov 20, 2:45 pm, Johannes Bauer <dfnsonfsdu...@ gmx.dewrote:

coming from C, I'm used to somthing like this when opening a file

const char *foobar = "this.txt";
FILE *f;
f = fopen(foobar, "r");
if (!f) {
fprintf(stderr, "Couldn't open %s: %s\n", foobar, strerror(errno) );
exit(EXIT_FAILU RE);
}

translating that somewhat to C++ yields me with

const char *foobar = "this.txt";
std::ifstream f(foobar);
if (!f) {
throw GenericExceptio n("Couldn't open file.");
}

So I can detect that opening the file wasn't successful, but I
don't know *why* (i.e. permissions, no such file or directory,
etc.). My guess is that strerror and errno still do what I
expect them to do, however I think that would be a very C-way
to solve things.

It's not really a C way either, since C doesn't say anything
about the state of errno after a failed fopen.

Pragmatically, it's what I do as well.

How can I accomplish strerror(errno) on the std::ifstream with
C++ means?

You can't associate errno with an std::ifstream anymore than you
can associate it a FILE*.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Johannes Bauer]

James Kanze schrieb:

It's not really a C way either, since C doesn't say anything
about the state of errno after a failed fopen.

Pragmatically, it's what I do as well.

Does it really not? My Linux manpage says its fopen implementation is in
accordance with ANSI C3.159-1989 ("ANSI C") and errno (according to
another manpage) seems to appear in the ISO-C standard - yet the
connection of the two is purely a Linux thing and could break on any
other system?

>How can I accomplish strerror(errno) on the std::ifstream with
C++ means?

You can't associate errno with an std::ifstream anymore than you
can associate it a FILE*.

This is really a lack, as I think the language should provide means for
that. Many are, in fact, operating system independent after all.

Greetings,
Johannes

--
"Viele der Theorien der Mathematiker sind falsch und klar
Gotteslästerli ch. Ich vermute, dass diese falschen Theorien genau
deshalb so geliebt werden." -- Prophet und Visionär Hans Joss aka
HJP in de.sci.mathemat ik <47************ **********@news .sunrise.ch>