64 lines
2.2 KiB
Plaintext
64 lines
2.2 KiB
Plaintext
[/==============================================================================
|
|
Copyright (C) 2001-2010 Joel de Guzman
|
|
Copyright (C) 2001-2005 Dan Marsden
|
|
Copyright (C) 2001-2010 Thomas Heller
|
|
|
|
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
|
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
|
|
===============================================================================/]
|
|
|
|
[section Lazy Functions]
|
|
|
|
As you write more lambda functions, you'll notice certain patterns that you wish
|
|
to refactor as reusable functions. When you reach that point, you'll wish that
|
|
ordinary functions can co-exist with phoenix functions. Unfortunately, the
|
|
/immediate/ nature of plain C++ functions make them incompatible.
|
|
|
|
Lazy functions are your friends. The library provides a facility to make lazy
|
|
functions. The code below is a rewrite of the `is_odd` function using the
|
|
facility:
|
|
|
|
struct is_odd_impl
|
|
{
|
|
typedef bool result_type;
|
|
|
|
template <typename Arg>
|
|
bool operator()(Arg arg1) const
|
|
{
|
|
return arg1 % 2 == 1;
|
|
}
|
|
};
|
|
|
|
function<is_odd_impl> is_odd;
|
|
|
|
[heading Things to note:]
|
|
|
|
[/
|
|
* `result` is a nested metafunction that reflects the return type of the
|
|
function (in this case, bool). This makes the function fully polymorphic:
|
|
It can work with arbitrary `Arg` types.
|
|
* There are as many Args in the `result` metafunction as in the actual
|
|
`operator()`.
|
|
]
|
|
* Result type deduction is implemented with the help of the result_of protocol.
|
|
For more information see __boost_result_of__
|
|
* `is_odd_impl` implements the function.
|
|
* `is_odd`, an instance of `function<is_odd_impl>`, is the lazy function.
|
|
|
|
Now, `is_odd` is a truly lazy function that we can use in conjunction with the
|
|
rest of phoenix. Example:
|
|
|
|
std::find_if(c.begin(), c.end(), is_odd(arg1));
|
|
|
|
(See [@../../example/function.cpp function.cpp])
|
|
|
|
[heading Predefined Lazy Functions]
|
|
|
|
The library is chock full of STL savvy, predefined lazy functions covering the
|
|
whole of the STL containers, iterators and algorithms. For example, there are lazy
|
|
versions of container related operations such as assign, at, back, begin,
|
|
pop_back, pop_front, push_back, push_front, etc. (See [link phoenix.modules.stl
|
|
STL]).
|
|
|
|
[endsect]
|