Thread: return value for global getline method

I am doing line-oriented file input in C++ using the global getline function defined in the <string> header. Various books demonstrate getline's idiomatic use:

Code:

#include <fstream>
#include <iostream>
#include <string>
      .
      .
   std::ifstream infile("foo.txt");
      .
      .
   std::string aline;
   while (std::getline(infile, aline)) {
      // process line
      .
      .
   }

The function prototype for getline is:
istream& getline(istream& is, string& s, char delimiter = '\n');

So getline returns a reference to istream. My question is, how can it be used in a while expression, which expects a boolean? I'm guessing some implicit conversion takes place, but it's not apparent to me from the standard library API docs.

Thanks for your help.