CODEWITHSUNDEEP
c++fileloopsinput
Piero Borrelli
Piero Borrelli

Reputation: 1221

An input loop for reading a file

I'm a beginner in programming and I need help with this snippet of code presented in my book. 

for (My_type var; ist >> var;) { // read until end of file 
// maybe check that var is valid 
// do something with var

}

if (ist.fail()) {
ist.clear(); 
char ch; 
// the error function is created into the book : 
if (!(ist >> ch && ch == '|')) error("Bad termination of input\n"); 

}
// carry on : we found end of file or terminator

This example is about reading values from a file. I've tried to use this but I'm having some troubles in understanding how it works :

  1. If I try to test the stream state after the loop I get both eof and fail state, how is that possibile ? How to avoid triggering both fail and eof ?

  2. When is EOF triggered exactly ? From my test it seems to be triggered when I reach the last value of a sequence, is this definition correct ?

Thank you.

Upvotes: 0

Views: 58

Answers (1)

Rakete1111
Rakete1111

Reputation: 48928

If I try to test the stream state after the loop I get both eof and fail state, how is that possibile ? How to avoid triggering both fail and eof ?

It is possible if there is a character that cannot be converted to My_type, and that character is the last character in the file. Then, failbit and eofbit will be set.

When is EOF triggered exactly ? From my test it seems to be triggered when I reach the last value of a sequence, is this definition correct ?

Yes, eofbit gets set when the last character is read.

Quoting from std::basic_istream:

The extraction stops if one of the following conditions are met:

  • end-of-file occurs on the input sequence;

  • inserting in the output sequence fails (in which case the character to be inserted is not extracted);

  • an exception occurs (in which case the exception is caught, and only rethrown if failbit is enabled in exceptions()).

Upvotes: 4

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