C++ how to detect eof using getline()

Question

There are some variations of this question but none exactly matches what I'm looking for.

Giving the following code:

string command="";

while (command.compare("quit")!=0)
{
    os << prompt;
    getline(is,command);
}

How may I detect if getline reached eof (end of file)

You should avoid the attempt to use eof() in the loop because of this: [https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-i-e-while-stream-eof-cons](https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-i-e-while-stream-eof-cons) — drescherjm, Aug 03 '20 at 17:30
@drescherjm what do you mean? I want to exit the loop when I reach the end of file — , Aug 03 '20 at 17:30

NathanOliver · Answer 1 · 2020-08-03T17:37:46.063

5

getline returns a reference to the stream you pass to it, and that stream will evaluate to false if it hits a failure state. Knowing that, you can leverage that to move getline into the condition for the while loop so that if it fails, then the condition will be false and the loop stops. You can combine that will your check for quit as well like

while (getline(is,command) && command != "quit")
{
    // stuff
}

You can also add the prompt to the loop like

while (os << prompt && getline(is,command) && command != "quit")

edited Aug 03 '20 at 17:37

answered Aug 03 '20 at 17:31

NathanOliver

171,901
28
288
402

to be compatible to the OP code : `while (os << prompt, getline(is,command) && command != "quit") {}` ;-) – bruno Aug 03 '20 at 17:35
@bruno Good idea. I used `&&` instead of `,` just to keep it consistent. – NathanOliver Aug 03 '20 at 17:38

score 2 · Answer 2 · answered Aug 03 '20 at 17:30

2

while (command.compare("quit")!=0)
{
    os << prompt;
    getline(is,command);
    if (is.eof())
         do something at end of file
}

But note that is reaching end of file does not mean that there isn't something in command. You can read data and reach the end of file at the same time.

What you might be looking for instead is this

os << prompt;
while (getline(is,command) && command != "quit")
{
    do something with command
    os << prompt;
}

That code will quit the loop if you reach end of file and nothing has been input, or if 'quit' is input.

answered Aug 03 '20 at 17:30

john

85,011
4
57
81

How may I save what getline returns in a variable so I can pass it to another function? – Aug 03 '20 at 17:35
@daniel *getline* returns *is* so you already have it – bruno Aug 03 '20 at 17:36
@daniel getline always returns a reference to the stream that was passed to it (`is` in this case). – john Aug 03 '20 at 17:37
`os << prompt`must be made each turn, so `while (os << prompt, getline(is,command) && command != "quit") ...` – bruno Aug 03 '20 at 17:38
I mean what is the variable type int string etc? – Aug 03 '20 at 17:38
The type is `std::istream&`. – john Aug 03 '20 at 17:38

C++ how to detect eof using getline()

2 Answers2