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

// std::back_inserter usage ( std::inserter for map )
std::copy( source.begin(), source.end(), std::back_inserter( container ) );  

-

// mem_fun and bind usage (but boost better)
std::some_algorithm(..., std::mem_fun( func ) );  

not so useful, but powerful:

check is container sorted

std::adjacent_find( container.begin(), container.end(), greater() ) == container.end()

also examples mentioned by you and dirkgently.