The <functional> header contains many additional functors and helper functions, extending section 20.3. They are implemented in the file stl_function.h:
identity_element for addition and multiplication. *
  
The functor identity, whose operator()
      returns the argument unchanged. *
  
Composition functors unary_function and
      binary_function, and their helpers compose1
      and compose2. *
    
select1st and select2nd, to strip pairs. *
  
project1st and project2nd. * 
A set of functors/functions which always return the same result.  They
      are constant_void_fun, constant_binary_fun,
      constant_unary_fun, constant0,
      constant1, and constant2. * 
The class subtractive_rng. * 
mem_fun adaptor helpers mem_fun1 and
      mem_fun1_ref are provided for backwards compatibility. 
20.4.1 can use several different allocators; they are described on the main extensions page.
  20.4.3 is extended with a special version of
  get_temporary_buffer taking a second argument.  The
  argument is a pointer, which is ignored, but can be used to specify
  the template type (instead of using explicit function template
  arguments like the standard version does).  That is, in addition to
get_temporary_buffer<int>(5);
you can also use
get_temporary_buffer(5, (int*)0);
  A class temporary_buffer is given in stl_tempbuf.h. *
  The specialized algorithms of section 20.4.4 are extended with
  uninitialized_copy_n. *