ceph/src/boost/libs/phoenix/doc/starter_kit/function.qbk

   1 [/==============================================================================
   2     Copyright (C) 2001-2010 Joel de Guzman
   3     Copyright (C) 2001-2005 Dan Marsden
   4     Copyright (C) 2001-2010 Thomas Heller
   5
   6     Distributed under the Boost Software License, Version 1.0. (See accompanying
   7     file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   8 ===============================================================================/]
   9
  10 [section Lazy Functions]
  11
  12 As you write more lambda functions, you'll notice certain patterns that you wish
  13 to refactor as reusable functions. When you reach that point, you'll wish that
  14 ordinary functions can co-exist with phoenix functions. Unfortunately, the
  15 /immediate/ nature of plain C++ functions make them incompatible.
  16
  17 Lazy functions are your friends. The library provides a facility to make lazy
  18 functions. The code below is a rewrite of the `is_odd` function using the
  19 facility:
  20
  21     struct is_odd_impl
  22     {
  23         typedef bool result_type;
  24
  25         template <typename Arg>
  26         bool operator()(Arg arg1) const
  27         {
  28             return arg1 % 2 == 1;
  29         }
  30     };
  31
  32     function<is_odd_impl> is_odd;
  33
  34 [heading Things to note:]
  35
  36 [/
  37 * `result` is a nested metafunction that reflects the return type of the
  38   function (in this case, bool). This makes the function fully polymorphic:
  39   It can work with arbitrary `Arg` types.
  40 * There are as many Args in the `result` metafunction as in the actual
  41   `operator()`.
  42 ]
  43 * Result type deduction is implemented with the help of the result_of protocol.
  44   For more information see __boost_result_of__
  45 * `is_odd_impl` implements the function.
  46 * `is_odd`, an instance of `function<is_odd_impl>`, is the lazy function.
  47
  48 Now, `is_odd` is a truly lazy function that we can use in conjunction with the
  49 rest of phoenix. Example:
  50
  51     std::find_if(c.begin(), c.end(), is_odd(arg1));
  52
  53 (See [@../../example/function.cpp function.cpp])
  54
  55 [heading Predefined Lazy Functions]
  56
  57 The library is chock full of STL savvy, predefined lazy functions covering the
  58 whole of the STL containers, iterators and algorithms. For example, there are lazy
  59 versions of container related operations such as assign, at, back, begin,
  60 pop_back, pop_front, push_back, push_front, etc. (See [link phoenix.modules.stl
  61 STL]).
  62
  63 [endsect]