Re: ifstream::get() surprise
From: Mike Wahler (mkwahler_at_mkwahler.net)
Date: 08/26/04
- Next message: John Black: "is there any known issue about gcc3.2 handling exception?"
- Previous message: Alf P. Steinbach: "Re: A religious question: int* i; /*or*/ int *i;?"
- In reply to: Jacek Dziedzic: "ifstream::get() surprise"
- Next in thread: Jacek Dziedzic: "Re: ifstream::get() surprise"
- Reply: Jacek Dziedzic: "Re: ifstream::get() surprise"
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Date: Thu, 26 Aug 2004 16:55:05 GMT
"Jacek Dziedzic" <jacek__NOSPAM__@janowo.net> wrote in message
news:cgl2kt$grf$1@korweta.task.gda.pl...
> Hi!
>
> Consider the following program
>
> #include <fstream>
> #include <iostream>
> using namespace std;
>
> int main() {
> ifstream in("test.txt");
You need to check here whether the file was opened
successfully or not, and not proceed if it wasn't.
Also note that 'get()' is an unformatted input function.
Using it with a text-mode stream (the default) could have
unexpected results. If you want to read unformatted,
open with 'std::ios::binary'. But I think you're probably
just using the wrong function. See below.
> char buf[40];
> in.get(buf,40);
> cerr << "Read: *" << buf << "*, trouble: " << !in << endl;
> }
>
> and a file, test.txt, starting with an empty line, ie. a lone EOL
> character on the first line.
>
> I was quite surprised to find out, that under these circumstances
> the aforementioned program produced
> "Read **, trouble: 1".
It's not surprising when you read the specification of 'std::istream::get()'
>
> Why does 'in' go to a fail state?
By design.
>I thought 'get' reads up to
> the terminator, stores all characters into 'buf' and leaves the
> terminator inside the stream. That would mean 'buf' containing
> just a \0 char (no chars read), the EOL still in the stream, but
> why a failed state? There are more lines in the file, so we're
> not eof(), and my understanding of this situation is a
> "successful read of zero characters" rather than "read error".
No such thing as 'successful read of zero characters.' If characters
were requested and none were extracted, that's a 'failure'.
============== begin quote ===========================
ISO/IEC 14882:1998(E)
27.6.1.3 Unformatted input functions
basic_istream<charT,traits>& get(char_type* s, streamsize n,
char_type delim );
7 Effects: Extracts characters and stores them into successive locations
of an array whose first element is designated by s. (286) Characters
are extracted and stored until any of the following occurs:
-- n 1 characters are stored;
-- endoffile occurs on the input sequence (in which case the function
calls setstate(eofbit));
-- c == delim for the next available input character c(in which case c
is not extracted).
8 If the function stores no characters, it calls setstate(failbit) (which
may throw ios_base::failure (27.4.4.3)). In any case, it then stores a
null character into the next successive location of the array.
9 Returns: *this.
basic_istream<charT,traits>& get(char_type* s, streamsize n)
10 Effects: Calls get(s,n,widen('\n'))
11 Returns: Value returned by the call.
============== end quote ===========================
>
> How then can I distinguish a successfull reading of an empty
> line from an I/O error during reading a line? Of course I have
> no a priori knowledge if these empty lines exist in my parsed
> file or not.
I recommend you eschew the array and use std::strings and
std::getline to parse your file.
std::string s;
while(std::getline(in, s))
cout << s << '\n';
if(!in.eof())
cerr << "Error reading\n";
Now you don't have to worry if your array is big enough,
and you can get at individual characters the same way
as from an array, e.g.
char c = s[0];
HTH,
-Mike
- Next message: John Black: "is there any known issue about gcc3.2 handling exception?"
- Previous message: Alf P. Steinbach: "Re: A religious question: int* i; /*or*/ int *i;?"
- In reply to: Jacek Dziedzic: "ifstream::get() surprise"
- Next in thread: Jacek Dziedzic: "Re: ifstream::get() surprise"
- Reply: Jacek Dziedzic: "Re: ifstream::get() surprise"
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Relevant Pages
|
