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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 Values] | |
11 | ||
12 | Values are functions! Examples: | |
13 | ||
14 | val(3) | |
15 | val("Hello, World") | |
16 | ||
17 | The first evaluates to a nullary function (a function taking no arguments) that | |
18 | returns an `int`, `3`. The second evaluates to a nullary function that returns | |
19 | a `char const(&)[13]`, `"Hello, World"`. | |
20 | ||
21 | [heading Lazy Evaluation] | |
22 | ||
23 | Confused? `val` is a unary function and `val(3)` invokes it, you say? | |
24 | Yes. However, read carefully: /"evaluates to a nullary function"/. | |
25 | `val(3)` evaluates to (returns) a nullary function. Aha! `val(3)` | |
26 | returns a function! So, since `val(3)` returns a function, you can | |
27 | invoke it. Example: | |
28 | ||
29 | std::cout << val(3)() << std::endl; | |
30 | ||
31 | (See [@../../example/values.cpp values.cpp]) | |
32 | ||
33 | [blurb __tip__ Learn more about values [link phoenix.modules.core.values here.]] | |
34 | ||
35 | The second function call (the one with no arguments) calls the nullary function | |
36 | which then returns `3`. The need for a second function call is the reason why | |
37 | the function is said to be [*/Lazily Evaluated/]. The first call doesn't do | |
38 | anything. You need a second call to finally evaluate the thing. The first call | |
39 | lazily evaluates the function; i.e. doesn't do anything and defers the evaluation | |
40 | for later. | |
41 | ||
42 | [heading Callbacks] | |
43 | ||
44 | It may not be immediately apparent how lazy evaluation can be useful by just | |
45 | looking at the example above. Putting the first and second function call in a | |
46 | single line is really not very useful. However, thinking of `val(3)` as a | |
47 | callback function (and in most cases they are actually used that way), will make | |
48 | it clear. Example: | |
49 | ||
50 | template <typename F> | |
51 | void print(F f) | |
52 | { | |
53 | cout << f() << endl; | |
54 | } | |
55 | ||
56 | int | |
57 | main() | |
58 | { | |
59 | print(val(3)); | |
60 | print(val("Hello World")); | |
61 | return 0; | |
62 | } | |
63 | ||
64 | (See [@../../example/callback.cpp callback.cpp]) | |
65 | ||
66 | [endsect] | |
67 |