Passing an ifstream object to another function

[csvka]

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

 

[Victor Bazarov]

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

 

[Gianni Mariani]

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.

 

[Buster]

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.

 

[John Harrison]

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

 

[Tom Davis]

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)

 

[John Harrison]

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/rvout.cgi?from=0sb_beginner_s_c__&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