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 12As you write more lambda functions, you'll notice certain patterns that you wish 13to refactor as reusable functions. When you reach that point, you'll wish that 14ordinary functions can co-exist with phoenix functions. Unfortunately, the 15/immediate/ nature of plain C++ functions make them incompatible. 16 17Lazy functions are your friends. The library provides a facility to make lazy 18functions. The code below is a rewrite of the `is_odd` function using the 19facility: 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 48Now, `is_odd` is a truly lazy function that we can use in conjunction with the 49rest 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 57The library is chock full of STL savvy, predefined lazy functions covering the 58whole of the STL containers, iterators and algorithms. For example, there are lazy 59versions of container related operations such as assign, at, back, begin, 60pop_back, pop_front, push_back, push_front, etc. (See [link phoenix.modules.stl 61STL]). 62 63[endsect] 64