return value for global getline method

This is a discussion on return value for global getline method within the C++ Programming forums, part of the General Programming Boards category; I am doing line-oriented file input in C++ using the global getline function defined in the <string> header. Various books ...

  1. #1
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    return value for global getline method

    I am doing line-oriented file input in C++ using the global getline function defined in the <string> header. Various books demonstrate getline's idiomatic use:

    Code:
    #include <fstream>
    #include <iostream>
    #include <string>
          .
          .
       std::ifstream infile("foo.txt");
          .
          .
       std::string aline;
       while (std::getline(infile, aline)) {
          // process line
          .
          .
       }
    The function prototype for getline is:
    istream& getline(istream& is, string& s, char delimiter = '\n');

    So getline returns a reference to istream. My question is, how can it be used in a while expression, which expects a boolean? I'm guessing some implicit conversion takes place, but it's not apparent to me from the standard library API docs.

    Thanks for your help.

  2. #2
    Cat without Hat CornedBee's Avatar
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    There's a conversion operator defined that converts an istream to a void*, which is further converted to a bool by using a simple != NULL rule. The void* converter will be a non-NULL value if the stream is ready and NULL if an error occurs.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

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