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24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="phoenix.starter_kit.lazy_operators"></a><a class="link" href="lazy_operators.html" title="Lazy Operators">Lazy Operators</a> | |
28 | </h3></div></div></div> | |
29 | <p> | |
30 | You can use the usual set of operators to form expressions. Examples: | |
31 | </p> | |
32 | <pre class="programlisting"><span class="identifier">arg1</span> <span class="special">*</span> <span class="identifier">arg1</span> | |
33 | <span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">arg1</span> <span class="special">+</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">z</span><span class="special">)</span> | |
34 | <span class="identifier">arg1</span> <span class="special">=</span> <span class="identifier">arg2</span> <span class="special">+</span> <span class="special">(</span><span class="number">3</span> <span class="special">*</span> <span class="identifier">arg3</span><span class="special">)</span> | |
35 | <span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">arg1</span><span class="special">[</span><span class="identifier">arg2</span><span class="special">]</span> <span class="comment">// assuming arg1 is indexable and arg2 is a valid index</span> | |
36 | </pre> | |
37 | <p> | |
38 | Note the expression: <code class="computeroutput"><span class="number">3</span> <span class="special">*</span> | |
39 | <span class="identifier">arg3</span></code>. This expression is actually | |
40 | a short-hand equivalent to: <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span> | |
41 | <span class="special">*</span> <span class="identifier">arg3</span></code>. | |
42 | In most cases, like above, you can get away with it. But in some cases, you | |
43 | will have to explicitly wrap your values in <code class="computeroutput"><span class="identifier">val</span></code>. | |
44 | Rules of thumb: | |
45 | </p> | |
46 | <div class="itemizedlist"><ul class="itemizedlist" type="disc"> | |
47 | <li class="listitem"> | |
48 | In a binary expression (e.g. <code class="computeroutput"><span class="number">3</span> | |
49 | <span class="special">*</span> <span class="identifier">arg3</span></code>), | |
50 | at least one of the operands must be a phoenix primitive or expression. | |
51 | </li> | |
52 | <li class="listitem"> | |
53 | In a unary expression (e.g. <code class="computeroutput"><span class="identifier">arg1</span><span class="special">++</span></code>), the single operand must be a phoenix | |
54 | primitive or expression. | |
55 | </li> | |
56 | </ul></div> | |
57 | <p> | |
58 | If these basic rules are not followed, the result is either an error, or | |
59 | is immediately evaluated. Some examples: | |
60 | </p> | |
61 | <pre class="programlisting"><span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">=</span> <span class="number">123</span> <span class="comment">// lazy</span> | |
62 | <span class="identifier">x</span> <span class="special">=</span> <span class="number">123</span> <span class="comment">// immediate</span> | |
63 | ||
64 | <span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="comment">// lazy</span> | |
65 | <span class="identifier">x</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="comment">// immediate</span> | |
66 | ||
67 | <span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)[</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">)]</span> <span class="comment">// lazy</span> | |
68 | <span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)[</span><span class="identifier">i</span><span class="special">]</span> <span class="comment">// lazy (equivalent to ref(x)[val(i)])</span> | |
69 | <span class="identifier">x</span><span class="special">[</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">)]</span> <span class="comment">// illegal (x is not a phoenix primitive or expression)</span> | |
70 | <span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">[</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">)])</span> <span class="comment">// illegal (x is not a phoenix primitive or expression)</span> | |
71 | </pre> | |
72 | <p> | |
73 | Why are the last two expression illegal? Although <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code> looks as much like a binary operator as | |
74 | <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code> | |
75 | above it; the difference is that the former must be a member (i.e. <code class="computeroutput"><span class="identifier">x</span></code> must have an <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code> that takes a phoenix primitive or expression | |
76 | as its argument). This will most likely not be the case. | |
77 | </p> | |
78 | <div class="sidebar"> | |
79 | <p class="title"><b></b></p> | |
80 | <p> | |
81 | <span class="inlinemediaobject"><img src="../../images/tip.png"></span> | |
82 | Learn more about operators <a class="link" href="../modules/operator.html" title="Operator">here.</a> | |
83 | </p> | |
84 | </div> | |
85 | <h5> | |
86 | <a name="phoenix.starter_kit.lazy_operators.h0"></a> | |
87 | <span><a name="phoenix.starter_kit.lazy_operators.first_practical_example"></a></span><a class="link" href="lazy_operators.html#phoenix.starter_kit.lazy_operators.first_practical_example">First | |
88 | Practical Example</a> | |
89 | </h5> | |
90 | <p> | |
91 | We've covered enough ground to present a real world example. We want to find | |
92 | the first odd number in an STL container. Normally we use a functor (function | |
93 | object) or a function pointer and pass that in to STL's <code class="computeroutput"><span class="identifier">find_if</span></code> | |
94 | generic function: | |
95 | </p> | |
96 | <p> | |
97 | Write a function: | |
98 | </p> | |
99 | <pre class="programlisting"><span class="keyword">bool</span> | |
100 | <span class="identifier">is_odd</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">arg1</span><span class="special">)</span> | |
101 | <span class="special">{</span> | |
102 | <span class="keyword">return</span> <span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">;</span> | |
103 | <span class="special">}</span> | |
104 | </pre> | |
105 | <p> | |
106 | Pass a pointer to the function to STL's <code class="computeroutput"><span class="identifier">find_if</span></code> | |
107 | algorithm: | |
108 | </p> | |
109 | <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="special">&</span><span class="identifier">is_odd</span><span class="special">)</span> | |
110 | </pre> | |
111 | <p> | |
112 | Using Phoenix, the same can be achieved directly with a one-liner: | |
113 | </p> | |
114 | <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">)</span> | |
115 | </pre> | |
116 | <p> | |
117 | The expression <code class="computeroutput"><span class="identifier">arg1</span> <span class="special">%</span> | |
118 | <span class="number">2</span> <span class="special">==</span> <span class="number">1</span></code> automagically creates a functor with the | |
119 | expected behavior. In FP, this unnamed function is called a lambda function. | |
120 | Unlike the function pointer version, which is monomorphic (expects and works | |
121 | only with a fixed type int argument), the Phoenix version is fully polymorphic | |
122 | and works with any container (of ints, of longs, of bignum, etc.) as long | |
123 | as its elements can handle the <code class="computeroutput"><span class="identifier">arg1</span> | |
124 | <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span></code> expression. | |
125 | </p> | |
126 | <p> | |
127 | (See <a href="../../../../example/find_if.cpp" target="_top">find_if.cpp</a>) | |
128 | </p> | |
129 | <div class="sidebar"> | |
130 | <p class="title"><b></b></p> | |
131 | <p> | |
132 | <span class="inlinemediaobject"><img src="../../images/tip.png"></span> | |
133 | ...<span class="bold"><strong>That's it, we're done</strong></span>. Well if you wish | |
134 | to know a little bit more, read on... | |
135 | </p> | |
136 | </div> | |
137 | </div> | |
138 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
139 | <td align="left"></td> | |
140 | <td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas | |
141 | Heller, John Fletcher<p> | |
142 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
143 | file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) | |
144 | </p> | |
145 | </div></td> | |
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