Question

I’m trying to initialize string with iterators and something like this works:

ifstream fin("tmp.txt");  
istream_iterator<char> in_i(fin), eos; 
//here eos is 1 over the end  
string s(in_i, eos);

but this doesn’t:

ifstream fin("tmp.txt");
istream_iterator<char> in_i(fin), eos(fin);
/* here eos is at this same position as in_i*/
//moving eos forward 
for (int i = 0; i < 20; ++i)
{
    ++eos; 
}
// trying to initialize string with 
// pair of iterators gives me "" 
// result  
string s(in_i, eos);

Thank you.

Answer 1

I don’t think you can advance the end iterator to a suitable position: to advance the iterator means to read input, also both iterators are referencing the same stream – therefore advancing one iterator means to advance the second. They both end up referencing the same position in the stream.

Unless you are willing to write or find an iterator adaptor (boost?) that does an operation on n items referenced by some iterator, it might not be possible to initialize the string like that. Or you read the value with other methods and set the value of the string later.