How to read

[zenniz]

Still a beginner here.
I have a txt file with all the values (Values are not fixed)

//Information inside values.txt
1234
12345
123
12345678

How to for example overwrite the first sentence (1234) to 1234567 so that the end result will be

//Information inside values.txt
1234567
12345
123
12345678

[kbw]

You need to rewrite the file. You can't insert into a sequential stream without changing the whole structure.

[keskiverto]

http://www.cplusplus.com/doc/tutorial/files/

[kbw]

That example isn't quite right. You shouldn't call open/close on a stream. You should specify the file name in the constructor and not call close.

[Aceix]

kbw wrote:

You shouldn't call open/close on a stream.

whatabout if you want to deal with multiple files with a stream object?

Aceix.

[kbw]

Create an object for each stream.

Have you considered RAII?

[Aceix]

what is its advantage over opening and closing? and also i think opening and closing helps sace memory?

I read about RAII, which is actually a very good practice. I was worried about memory consumption(even though it is small amounts).
I also have one question. If one uses new with RAII, is it still necessary to call delete?

Aceix.

[kbw]

There are many arguments why that is less than ideal.

When you use an fstream, you shouldn't thinking file. You ought to be thinking stream. You don't open/close streams. You just read/write them.

Like I said before, RAII. If you have an fstream, it should be in a usable and functional state from the start. There are a number of interlocking concepts that support robust design, such as Adam's exception guarantees. Being careless about state doesn't help.

I think it's just plain wrong to have a beginner's tutorial on fstreams that calls open/close on them. You'll be checking for EOF next It's not called the I/O stream library for nothing.

[JLBorges]

> whatabout if you want to deal with multiple files with a stream object?

C++ streams are moveable.
If the standard library implementation is conforming (for example Microsoft), we can do this:

std::ifstream file( "some_file" ) ;
std::cout << file.rdbuf() << '\n' ;

file = std::ifstream ( "another_file" ) ; // move
std::cout << file.rdbuf() << '\n' ;
        
std::swap( file, std::ifstream ( "a third file" ) ) ; // swap => move
std::cout << file.rdbuf() << '\n' ;

To modify a text file containing white space delimited values:

#include <fstream>
#include <vector>
#include <cstdio>

int main()
{
    const char* const file_name = "values.txt" ;
    const char* const backup_file_name = "values.txt.bak" ;

    // read the values into a vector
    std::vector<int> values ;
    {
       std::ifstream file(file_name) ; // open for input
       int value ;
       while( file >> value ) values.push_back(value) ;
    }

    // modify the values as needed
    values.push_back(234) ;
    values[0] = 1234567 ;
    // etc.

    // just to be safe, rename the old file to create a backup
    std::rename( file_name, backup_file_name ) ;

    // create the file afresh and write the modified values, one value per line
    {
       std::ofstream file(file_name) ; // create, and open for output
       for( int v : values ) file << v << '\n' ;
    }

    // note: in this snippet, error checking has been elided for brevity
    // if we have reached here with no errors, the backup may be deleted.
    // std::remove( backup_file_name ) ;
}

[Aceix]

@kbw
editted before refreshing...

Aceix.

[keskiverto]

kbw wrote:

That example isn't quite right. You shouldn't call open/close on a stream. You should specify the file name in the constructor and not call close.

Good point; the maintainer of the tutorials should be notified about it.