End of file of fstream object

Question

I came across this code which deals with a simple reading of a file and displaying its contents.

#include &ltiostream&gt
#include &ltfstream&gt

int main()
{   
    ...
    fstream file;
    file.open("TEXT.txt", ios::in);
    file.seekg(0);
    while(file) //does file returns 0 when eof is reached?
    {
        file.get(ch);
        cout &lt&lt ch;
    }
    return 0;
}

My question is how does while (file) realizes that the end of the file has been reached.

Alex's Answer below true... For more information I would suggest looking at this tutorial on Stream States http://www.learncpp.com/cpp-tutorial/135-stream-states-and-input-validation/ — silvergasp, Oct 04 '15 at 06:42
Watch out with the above code. `while (file)` enters the body of the loop if the stream is not in an error state, including end of file. This is good, but `file.get(ch);` may still fail and this result is not being tested. A better implementation would be `while(file.get(ch))` because that's going to test after trying to read. — user4581301, Oct 04 '15 at 06:49
@user4581301 The long explanation is here: http://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong — πάντα ῥεῖ, Oct 04 '15 at 06:53
Your loop is backwards, if `file.get` fails it goes on to `cout << ch` anyway — M.M, Oct 04 '15 at 07:04

score 5 · Answer 1 · edited Oct 04 '15 at 07:33

5

The stream implements a boolean cast operator that will return true if the stream is still good or false if there's an error or eof.

edited Oct 04 '15 at 07:33

Darklighter

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answered Oct 04 '15 at 06:37

Alex Fitzpatrick

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Is it equivalent to `while(fin.eof())` ? – Oct 04 '15 at 06:55
1

@NooB No. `while(fin)` catches more error conditions than just EOF. – user4581301 Oct 04 '15 at 06:56
But I should add that, neither one does what you want. Both test for error before operating on the stream and potentially raising an error. Read the the link posted by @πάνταῥεῖ above for the straight dope. – user4581301 Oct 04 '15 at 07:06
@user4581301 great understood, also I know its a bit off topic but 1. Is it necessary to close a file explicitly like `fin.close();` for a small program and 2. Whether all files are automatically closed when a program terminates? – Oct 04 '15 at 07:11
@NooB 1) Depends on when you want the file closed. If you want it closed immediately, call `close`; otherwise, just let the stream go out of scope and the destructor will take care of it for you. 2) two things help you here: The file stream destructor will fire under normal circumstances and close the file. If for some reason you don't call the destructor, called `exit` or `new`ed the stream and never called `delete` most likely, the system should take care of it for you. – user4581301 Oct 04 '15 at 07:29

score 0 · Accepted Answer · answered Oct 04 '15 at 08:02

0

As others have pointed out you should really use something like

char ch;
while (file.get(ch)) {
    ...
}

answered Oct 04 '15 at 08:02

Zaid Khan

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although this helps improve the code, it doesn’t answer the question, both current answers combined would be perfect – Darklighter Oct 04 '15 at 15:00
Yeah it was originally intended to supplement Alex's answer as well as πάντα ῥεῖ's suggestion. – Zaid Khan Oct 06 '15 at 14:25

End of file of fstream object

2 Answers2