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trouble writing a text file

This is a discussion on trouble writing a text file within the C++ Programming forums, part of the General Programming Boards category; I am writing a text file which has the number of records in it at the start and then each ...

  1. #1
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    trouble writing a text file

    I am writing a text file which has the number of records in it at the start and then each record is stored at a 400 boundary - or rather should be!

    In the following snippet the number of records is being updated and recorded at the start of the file OK but the string I am writing should be at 400, 800, 1200 etc but this part is not working, when I inspect the file there is only the one string and it's position varies only a little.

    I am sure it must be an incorrect setting but I have tried all the ios::beg etc but none seem to make a difference

    Code:
            jcin>>numrecords;
            jcin.close ();
    
            cout<<endl<<endl<<endl<<"There are currently "<<numrecords<<" records in this file.";
    
            ofstream jcout;
            jcout.open(filename, ios::out);
    
            long int pos;
            pos=(numrecords+1)*400;
    
            jcout.seekp (pos);
            jcout<<"hello sailor";//will be replace by a record
            cout<<endl<<"position is ; "<<pos<<endl;
    
            jcout.seekp (0);
            jcout<<numrecords++;//this is updating correctly
    
            jcout.close ();

  2. #2
    - - - - - - - - oogabooga's Avatar
    Join Date
    Jan 2008
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    2,808
    I believe that this
    Code:
    ofstream jcout;
    jcout.open(filename, ios::out);
    will essentially truncate the file. Try an fstream instead of an ofstream. I believe it defaults to in/out mode, so it won't truncate.
    Code:
        fstream jcout(filename);
    The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss

  3. #3
    Registered User whiteflags's Avatar
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    I gave up long ago trying to understand what the default behavior of the objects were. You can just mask together features you want with the pipe operator. A list of features is here: ios_base::openmode - C++ Reference.
    Code:
    ofstream jcout;
    jcout.open (filename, ios::out | ios::app);
    I still think it's good to use ofstream and ifstream though, unless you want to read and write from the same stream.
    laserlight likes this.

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