How can I read from an std::istream (using operator>>)?

Question

How can I read from an std::istream using operator>>?

I tried the following:

void foo(const std::istream& in) {
  std::string tmp;
  while(in >> tmp) {
     std::cout << tmp;
  }
}

But it gives an error:

error: no match for 'operator>>' in 'in >> tmp'

the istream shouldn't be const for a start. the istream object's state will need to change as a consequence of you pulling data out of it. — jon hanson, Aug 19 '09 at 22:39

score 10 · Accepted Answer · answered Aug 19 '09 at 22:38

10

Operator >> modifies stream, so don't pass by const, just a reference.

answered Aug 19 '09 at 22:38

Eugene

Thanks! Surprising that reading from the stream should modify it, but presumably a position pointer gets advanced by reading. – Frank Aug 19 '09 at 22:43
2

It shouldn't be surprising. Reading from an instream now changes what you will read from that same stream later. This is an observable external effect, and therefore should be considered to modify the object, regardless of the internal implementation detail of the position pointer. – Tyler McHenry Aug 19 '09 at 22:50

score 4 · Answer 2 · answered Aug 19 '09 at 22:40

4

Use a non-const reference:

void foo(std::istream& in) {
  std::string tmp;
  while(in >> tmp) {
     std::cout << tmp;
  }
}

answered Aug 19 '09 at 22:40

John Millikin

Tyler McHenry · Answer 3 · 2009-08-19T22:40:35.580

1

You're doing that the right way. Are you sure you included all the headers you need? (<string> and <iostream>)?

edited Aug 19 '09 at 22:40

answered Aug 19 '09 at 22:33

Tyler McHenry

Yes, and are included. – Frank Aug 19 '09 at 22:36
Eugene is right. I didn't notice the const reference. The reason it complains with this particular error is because there's no version of operator>> that takes a const stream. – Tyler McHenry Aug 19 '09 at 22:40

3 Answers3