return behavior of getline()

This is a discussion on return behavior of getline() within the C++ Programming forums, part of the General Programming Boards category; Hello. I've been using the different forms of std::getline() in a program recently, and I can't help but notice one ...

  1. #1
    Kiss the monkey. CodeMonkey's Avatar
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    return behavior of getline()

    Hello. I've been using the different forms of std::getline() in a program recently, and I can't help but notice one strange thing about how it sets error flags. I'm reading in some data using
    Code:
    istream& istream::getline(char*buf, int num, char delim = '\n')
    And it seems that if the stream reaches num chars read before a delim, it sets the fail bit. Here's a little test program using ifstream.
    Code:
    #include <iostream>
    #include <cstdlib>
    #include <fstream>
    
    using namespace std;
    
    int main(int argc, char *argv[])
    {
    ifstream group("c:\\windows\\desktop\\input.txt");
    if(!group) { cout << "can't open." << endl; return EXIT_FAILURE; }
    
    char *aunt_sally = new char[25];
    
    cout << boolalpha << "before getline: " << group.fail() << endl;
    group.getline(aunt_sally,25,'@');
    cout << "after getline: " << group.fail() << endl;
    cout << aunt_sally << endl;
    
    group.close();
    
    system("PAUSE");
    return EXIT_SUCCESS;
    }
    
    /********input.txt***************
    This is a text-only test just to see if the damn
    thing works. Of course, as I write this, I haven't
    even compiled all of my new functions. It is likely
    that  I will get tons of error and not even use
    this test file for quite a while. Still, I'm not
    discouraged. (quite more than 25 chars)
    *********************************/
    And my perplexing output:
    Code:
    before getline: false
    after getline: true
    This is a text-only test
    Press any key to continue . . .
    Shouldn't it be "after getline: false" ? I even tried it with a '@' right there at char 25, and then the fail bit was not set. Is this normal behavior?
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

  2. #2
    Just Lurking Dave_Sinkula's Avatar
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    http://www.dinkumware.com/manuals/?m...tream::getline
    In order of testing, extraction stops:

    1. at end of file
    2. after the function extracts an element that compares equal to delim, in which case the element is neither put back nor appended to the controlled sequence
    3. after the function extracts count - 1 elements

    If the function extracts no elements or count - 1 elements, it calls setstate(failbit). In any case, it returns *this.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  3. #3
    Kiss the monkey. CodeMonkey's Avatar
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    Thank you. I suppose I could have googled that one, in retrospect.
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

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