Distance between istream_iterators

Question

I am trying to count the amount of elements read by std::cin by using std::distance for the constructor argument of a vector in advance, like so:

// Gives correct amount, but now can't use to put into vector
int size = std::distance(std::istream_iterator<std::string>(std::cin),
        std::istream_iterator<std::string>());

std::vector v(size);

// Already read from stream
std::copy(...);

Obviously I can complete this in one step with std::copy, but that would require setting a size beforehand. My question isn't about vectors though, it's about getting the size of an arbitrary input using std::istream_iterator without affecting the stream. Any ideas?

Answer 1

std::istream_iterator is an InputIterator, which means it only supports a single pass over the range you're iterating. There's no way to figure out the size, and then go back to the beginning to read the data.

You can read from std::cin (or any other input stream) using std::copy without knowing the size beforehand, just use std::back_inserter to append the data being read to the vector.

std::vector v;
std::copy( std::istream_iterator<std::string>(std::cin), 
           std::istream_iterator<std::string>(),
           std::back_inserter(v) );