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<title>Lazy Operators</title>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="phoenix.starter_kit.lazy_operators"></a><a class="link" href="lazy_operators.html" title="Lazy Operators">Lazy Operators</a>
</h3></div></div></div>
<p>
You can use the usual set of operators to form expressions. 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 expression.
</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 expression.
</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 expression)</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 expression)</span>
</pre>
<p>
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
<code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>
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
as its argument). This will most likely not be the case.
</p>
<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>
<h5>
<a name="phoenix.starter_kit.lazy_operators.h0"></a>
<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
Practical Example</a>
</h5>
<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>
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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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