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| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="phoenix.starter_kit.composites.lazy_operators"></a><a class="link" href="lazy_operators.html" title="Lazy Operators">Lazy |
| Operators</a> |
| </h4></div></div></div> |
| <p> |
| You can use the usual set of operators to form composites. Examples: |
| </p> |
| <pre class="programlisting"><span class="identifier">arg1</span> <span class="special">*</span> <span class="identifier">arg1</span> |
| <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> |
| <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> |
| <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></pre> |
| <p> |
| Note the expression: <code class="computeroutput"><span class="number">3</span> <span class="special">*</span> |
| <span class="identifier">arg3</span></code>. This expression is actually |
| 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> |
| <span class="special">*</span> <span class="identifier">arg3</span></code>. |
| In most cases, like above, you can get away with it. But in some cases, |
| you will have to explicitly wrap your values in <code class="computeroutput"><span class="identifier">val</span></code>. |
| Rules of thumb: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| In a binary expression (e.g. <code class="computeroutput"><span class="number">3</span> |
| <span class="special">*</span> <span class="identifier">arg3</span></code>), |
| at least one of the operands must be a phoenix primitive or composite. |
| </li> |
| <li class="listitem"> |
| 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 |
| primitive or composite. |
| </li> |
| </ul></div> |
| <p> |
| If these basic rules are not followed, the result is either an error, or |
| is immediately evaluated. Some examples: |
| </p> |
| <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><span class="identifier">x</span> <span class="special">=</span> <span class="number">123</span> <span class="comment">// immediate |
| </span> |
| <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><span class="identifier">x</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="comment">// immediate |
| </span> |
| <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><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><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 composite) |
| </span><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 composite) |
| </span></pre> |
| <div class="sidebar"> |
| <div class="titlepage"></div> |
| <p> |
| <span class="inlinemediaobject"><img src="../../../images/tip.png" alt="tip"></span> Learn more about operators <a class="link" href="../../modules/operator.html" title="Operator">here.</a> |
| </p> |
| </div> |
| <a name="phoenix.starter_kit.composites.lazy_operators.first_practical_example"></a><h6> |
| <a name="id682284"></a> |
| <a class="link" href="lazy_operators.html#phoenix.starter_kit.composites.lazy_operators.first_practical_example">First |
| Practical Example</a> |
| </h6> |
| <p> |
| We've covered enough ground to present a real world example. We want to |
| find the first odd number in an STL container. Normally we use a functor |
| (function object) or a function pointer and pass that in to STL's <code class="computeroutput"><span class="identifier">find_if</span></code> generic function: |
| </p> |
| <p> |
| Write a function: |
| </p> |
| <pre class="programlisting"><span class="keyword">bool</span> |
| <span class="identifier">is_odd</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">arg1</span><span class="special">)</span> |
| <span class="special">{</span> |
| <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| Pass a pointer to the function to STL's <code class="computeroutput"><span class="identifier">find_if</span></code> |
| algorithm: |
| </p> |
| <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> |
| </pre> |
| <p> |
| Using Phoenix, the same can be achieved directly with a one-liner: |
| </p> |
| <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> |
| </pre> |
| <p> |
| The expression <code class="computeroutput"><span class="identifier">arg1</span> <span class="special">%</span> |
| <span class="number">2</span> <span class="special">==</span> <span class="number">1</span></code> automagically creates a functor with the |
| expected behavior. In FP, this unnamed function is called a lambda function. |
| Unlike the function pointer version, which is monomorphic (expects and |
| works only with a fixed type int argument), the Phoenix version is fully |
| polymorphic and works with any container (of ints, of longs, of bignum, |
| etc.) as long as its elements can handle the <code class="computeroutput"><span class="identifier">arg1</span> |
| <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span></code> expression. |
| </p> |
| <p> |
| (See <a href="../../../../../example/find_if.cpp" target="_top">find_if.cpp</a>) |
| </p> |
| <div class="sidebar"> |
| <div class="titlepage"></div> |
| <p> |
| <span class="inlinemediaobject"><img src="../../../images/tip.png" alt="tip"></span> ...<span class="bold"><strong>That's it, we're done</strong></span>. |
| Well if you wish to know a little bit more, read on... |
| </p> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
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| <td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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>) |
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