Most useful or amazing STL short liners

Question

I\'m looking for practical and educational samples of C++ / STL code fitting in few lines. My actual favorites are:

1. Empty a vector freeing its reserved memory:<

Answer 1

My favorite is copying containers to the output: And copying the input stream into a container.

#include 
#include  
#include 
#include 

int main()
{
    std::vector   data;
    std::copy(std::istream_iterator(std::cin),
              std::istream_iterator(),
              std::back_inserter(data)
             );

    std::copy(data.begin(),data.end(),
              std::ostream_iterator(std::cout,"\n")
             );
}