Velocity Reviews - Computer Hardware Reviews

Velocity Reviews > Newsgroups > Programming > C++ > Passing an ifstream object to another function

Reply
Thread Tools

Passing an ifstream object to another function

 
 
csvka
Guest
Posts: n/a
 
      02-15-2004
Hello,

I wonder if I could pick your brains. I'm beginning to learn about C++. I
have opened a file in my program and I want to read lines from it. I would
like this to be done in a separate function called readline() because I
would also like to do some processing on the line each time (ignoring
comments and so on).

I have:

void Fileloader::readline(char *string, ifstream file) {
file.getline(string, sizeof(string), '\n');
}


And in another function I have:

ifstream file;
file.open("myfile.txt", ios::in);
char string[512];
readline(string, file);

To call my readline file.

This comes up with lots of errors like:
/usr/include/c++/3.3.1/bits/ios_base.h:668: error:
`std::ios_base::ios_base(const std::ios_base&)' is private
fileloader.cpp:62: error: within this context


Can anyone see what I'm doing wrong and point me in the right direction?

Thanks very much,
Tom


 
Reply With Quote
 
 
 
 
Victor Bazarov
Guest
Posts: n/a
 
      02-15-2004
"csvka" <(E-Mail Removed)> wrote...
> I wonder if I could pick your brains. I'm beginning to learn about C++.

I
> have opened a file in my program and I want to read lines from it. I

would
> like this to be done in a separate function called readline() because I
> would also like to do some processing on the line each time (ignoring
> comments and so on).
>
> I have:
>
> void Fileloader::readline(char *string, ifstream file) {


Make it

voif Fileloader::readline(string& str, ifstream& file)

> file.getline(string, sizeof(string), '\n');


'sizeof' is not good here (even when your 'string' was a pointer).
It never gives the size of the array behind the pointer.

V


 
Reply With Quote
 
 
 
 
Gianni Mariani
Guest
Posts: n/a
 
      02-15-2004
csvka wrote:
> Hello,
>
> I wonder if I could pick your brains. I'm beginning to learn about C++. I
> have opened a file in my program and I want to read lines from it. I would
> like this to be done in a separate function called readline() because I
> would also like to do some processing on the line each time (ignoring
> comments and so on).
>
> I have:
>
> void Fileloader::readline(char *string, ifstream file) {


Try passing ifstream by reference :

void Fileloader::readline(char *string, ifstream & file) {

> file.getline(string, sizeof(string), '\n');
> }
>
>
> And in another function I have:
>
> ifstream file;
> file.open("myfile.txt", ios::in);
> char string[512];
> readline(string, file);
>
> To call my readline file.
>
> This comes up with lots of errors like:
> /usr/include/c++/3.3.1/bits/ios_base.h:668: error:
> `std::ios_base::ios_base(const std::ios_base&)' is private
> fileloader.cpp:62: error: within this context
>
>
> Can anyone see what I'm doing wrong and point me in the right direction?
>


You can't make copies of ifstream.

The other thing is that you probably don't care about the "f" or file
semantics of a stream.

So this below is better.

void Fileloader::readline(char *string, istream & file)

But wait - there's more - sizeof( string ) will return the sizeof a
char * (which is probably 4 on your machine) which is probably NOT what
you want.

You also don't really want to use char *, try and see if std::string
works for you.





 
Reply With Quote
 
Buster
Guest
Posts: n/a
 
      02-15-2004

"csvka" <(E-Mail Removed)> wrote in message
news:c0ojp0$c8b$(E-Mail Removed)...
> Hello,
>
> I wonder if I could pick your brains. I'm beginning to learn

about C++. I
> have opened a file in my program and I want to read lines from it.

I would
> like this to be done in a separate function called readline()

because I
> would also like to do some processing on the line each time

(ignoring
> comments and so on).
>
> I have:
>
> void Fileloader::readline(char *string, ifstream file) {
> file.getline(string, sizeof(string), '\n');
> }


Here you seem to want to provide readline as an alias for getline.
Why not just
use getline? Is there any reason readline must be a member function?
It doesn't use any of the calling object's data.

But more important than that, don't use std::istream::getline (char
*, int, char)!
Instead use std::getline (std::istream &, std::string &), which is
in the standard
<string> header. The version of getline which takes a
character-array as a
buffer is unsafe unless used properly (which you haven't - the
length of the
null-terminated sequence of characters pointed to by "string" is
unlikely to
be a good buffer size for getline) because the buffer can be
overrun. A
corollary of this advice is that when you want a string, use
std::string, not
character-arrays, because it handles the buffer's allocation and
deallocation
automatically, can be safely copied, etc.

Taking this on board, in your program, where previously you had:
object.readline (array, stream); // for 'object' declared as a
Fileloader instance
you will now want:
std::getline (s, stream); // where s is an std::string object.

>
> And in another function I have:
>
> ifstream file;
> file.open("myfile.txt", ios::in);
> char string[512];
> readline(string, file);
>
> To call my readline file.
>
> This comes up with lots of errors like:
> /usr/include/c++/3.3.1/bits/ios_base.h:668: error:
> `std::ios_base::ios_base(const std::ios_base&)' is private
> fileloader.cpp:62: error: within this context


std::ios_base is a base class of std::ifstream, and the copy
constructor
of std::ios_base is private, so std::ifstream objects cannot be
copied.
You would need to pass the stream by reference, as:

void Fileloader::readline(char *string, std::ifstream & file);

But seriously, use std::string and std::getline (std::istream &,
std::string &)
instead.

> Can anyone see what I'm doing wrong and point me in the right

direction?
>
> Thanks very much,
> Tom


Good luck and best regards,
Buster.


 
Reply With Quote
 
John Harrison
Guest
Posts: n/a
 
      02-15-2004

"csvka" <(E-Mail Removed)> wrote in message
news:c0ojp0$c8b$(E-Mail Removed)...
> Hello,
>
> I wonder if I could pick your brains. I'm beginning to learn about C++.

I
> have opened a file in my program and I want to read lines from it. I

would
> like this to be done in a separate function called readline() because I
> would also like to do some processing on the line each time (ignoring
> comments and so on).
>
> I have:
>
> void Fileloader::readline(char *string, ifstream file) {
> file.getline(string, sizeof(string), '\n');
> }


You're just learning so you are making all the classic mistakes.

Pass streams by reference.

Use istream not ifstream in function. Why do you care what sort of stream it
is, as long as you can read from it who cares. If you say ifstream you are
saying I only want to read from a file, if you say istream you are saying I
want to read from anything, so your code is more flexible.

Don't use char*, use string instead. Especially don't use sizeof on char*
because IT WONT WORK!!! (The answer is usually 4, which is the size of a
char* pointer).

Putting all that together we have

void Fileloader::readline(string& str, istream& input) {
getline(input, str, '\n');
}

which is how you should write that function.

>
>
> And in another function I have:
>
> ifstream file;
> file.open("myfile.txt", ios::in);
> char string[512];
> readline(string, file);


Right change this to

ifstream file("myfile.txt", ios::in);
string str;
readline(str, file);

No need to call open, you can declare the variable and open the file in one
go.

But most importantly, you've replaced the char array (which was limited to
512) with a string which is unlimited! You need to include the header
<string> (no .h in that) to get the string class.

You'll make life much easier for yourself learning C++ if you get into good
habits right away. There a lot of different ways to go wrong in C++. Most
importantly you need a good book, which one are you using?

john



 
Reply With Quote
 
Tom Davis
Guest
Posts: n/a
 
      02-15-2004
John Harrison wrote:

[snipped]

>
> No need to call open, you can declare the variable and open the file in
> one go.
>
> But most importantly, you've replaced the char array (which was limited to
> 512) with a string which is unlimited! You need to include the header
> <string> (no .h in that) to get the string class.


Thank you. That was really helpful.

>
> You'll make life much easier for yourself learning C++ if you get into
> good habits right away. There a lot of different ways to go wrong in C++.
> Most importantly you need a good book, which one are you using?



Well, I've done a bit of Java and a bit of C before. I've picked up the
O'Reilly book "Practical C++ Programming". Can you recommend any other
good ones?

Thanks again for the help,

Tom
(csvka in the other post)
 
Reply With Quote
 
John Harrison
Guest
Posts: n/a
 
      02-16-2004

"Tom Davis" <(E-Mail Removed)> wrote in message
news:c0opj7$fd1$(E-Mail Removed)...
> John Harrison wrote:
>
> [snipped]
>
> >
> > No need to call open, you can declare the variable and open the file in
> > one go.
> >
> > But most importantly, you've replaced the char array (which was limited

to
> > 512) with a string which is unlimited! You need to include the header
> > <string> (no .h in that) to get the string class.

>
> Thank you. That was really helpful.
>
> >
> > You'll make life much easier for yourself learning C++ if you get into
> > good habits right away. There a lot of different ways to go wrong in

C++.
> > Most importantly you need a good book, which one are you using?

>
>
> Well, I've done a bit of Java and a bit of C before. I've picked up the
> O'Reilly book "Practical C++ Programming". Can you recommend any other
> good ones?
>
> Thanks again for the help,
>
> Tom
> (csvka in the other post)


If you have previously programmed before then the one that is normally
recommended by this group is Accelerated C++ by Koenig and Moo.

The one you are reading has a poor review here

http://www.accu.org/cgi-bin/accu/rvo...&file=p001010a

The author is accused of simply translating a C style into C++, since you
are used to C that would be a problem for you. Also it cannot be denied that
the book is fairly old. C++ has moved on a lot in recent years.

john


 
Reply With Quote
 
 
 
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Passing an Object to Another Object Hal Vaughan C++ 12 03-28-2008 05:53 AM
Passing Bound Object value to another Bound Object Phillip Vong ASP .Net 0 07-27-2006 10:54 PM
Passing a C++ object's member function to a C function expecing a function pointer! James Vanns C++ 7 01-21-2004 02:39 AM
Passing a pointer to member function as a parameter to another member function Newsgroup - Ann C++ 5 07-30-2003 02:54 AM
error passing ifstream. why? Giulio C++ 2 06-24-2003 09:19 PM



Advertisments