Thread: Can you have an ifstream and ofstream open for one file at once?

  1. #1
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    Can you have an ifstream and ofstream open for one file at once + no overwriting?

    This is my first post, so please go easy on me .

    I intend to both read from and write to a file. Can I have an ifstream and ofstream open for the same file at the same time? Or do I have to close one before I open the other?

    Also, will the following code work to ensure the file I am writing to does not already exist, thus preventing overwriting?

    Code:
        ofstream target_out; // ofstream to write to target file
        bool done = false;
        char targetname[80];
    
        while( !done ) {
            cout << "Enter data target file: ";
            cin >> targetname;
            target_out.open( targetname, ios::nocreate );
    
            if( target_out.is_open() ) {
                cout << "The file you entered already exists." << endl;
                target_out.close();
            }
            else
                done = true;
    
        }
    
        target_out.open( targetname, ios::noreplace );
    Thanks!
    Last edited by mosdef; 06-06-2002 at 11:55 AM.

  2. #2
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    Code:
    fstream file; //fstream object
    file.open("myfile.txt", ios::in|ios:: out); //open for input and output
    Hope that helps.

  3. #3
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    Great, that helps a lot. My only question then is how do I ensure that I am not opening a pre-existing file for output; in other words, I wish to prevent any sort of overwriting. Thanks again!

  4. #4
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    Code:
    fstream file; //fstream object
    file.open("myfile.txt", ios::in|ios:: out|ios:: app); /*open for input and output Appending to file only*/
    I believe that's it.

  5. #5
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    Sorry, what I meant was basically "do not open the file if it already exists." I don't want to be able to write to a file if it already exists.

  6. #6
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    Well try replacing ios:: app with ios:: nocreate

  7. #7
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    Allright, forget that, I can't read today, probably need to use an if statement like so:

    Code:
    if(!(file.open("myfile.txt", ios:: in|ios:: out|ios::nocreate))){
                // do reading and writing
    }
    else{
                 cout<<"File aready exists";
    }

  8. #8
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    wouldnt it be more like this:

    Code:
    if(file.open("file.txt"), ios::in  | ios::out | ios::nocreate) 
    {
         cout << "File already exists";
    } else {
         // do reading and writing
    }
    [EDIT]Stupid Smilies[/EDIT]
    "There are three kinds of people in the world...
    Those that can count and those that can't."

  9. #9
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    If you look again you'll see the '!' operator(not), essentially both are codes evaluate the same. I hate those smilies too.

  10. #10
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    oooo sorry didnt see the ! there

    [EDIT]Sureee smilies dont apear when i want them to[/EDIT]
    "There are three kinds of people in the world...
    Those that can count and those that can't."

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