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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="phoenix.modules.core.values"></a><a class="link" href="values.html" title="Values">Values</a> |
| </h4></div></div></div> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">phoenix</span><span class="special">/</span><span class="identifier">core</span><span class="special">/</span><span class="identifier">value</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| </pre> |
| <p> |
| Whenever we see a constant in a partially applied function, an |
| </p> |
| <pre class="programlisting"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> |
| </pre> |
| <p> |
| (where T is the type of the constant) is automatically created for us. |
| For instance: |
| </p> |
| <pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="number">6</span><span class="special">)</span> |
| </pre> |
| <p> |
| Passing a second argument, <code class="computeroutput"><span class="number">6</span></code>, |
| an <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span></code> is implicitly created behind the |
| scenes. This is also equivalent to <code class="computeroutput"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">val</span><span class="special">(</span><span class="number">6</span><span class="special">))</span></code>. |
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| <pre class="programlisting"><span class="identifier">val</span><span class="special">(</span><span class="identifier">v</span><span class="special">)</span> |
| </pre> |
| <p> |
| generates an <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span></code> |
| where <code class="computeroutput"><span class="identifier">T</span></code> is the type of |
| <code class="computeroutput"><span class="identifier">x</span></code>. In most cases, there's |
| no need to explicitly use <code class="computeroutput"><span class="identifier">val</span></code>, |
| but, as we'll see later on, there are situations where this is unavoidable. |
| </p> |
| <h3> |
| <a name="phoenix.modules.core.values.h0"></a> |
| <span class="phrase"><a name="phoenix.modules.core.values.evaluating_a_value"></a></span><a class="link" href="values.html#phoenix.modules.core.values.evaluating_a_value">Evaluating |
| a Value</a> |
| </h3> |
| <p> |
| Like arguments, values are also actors. As such, values can be evaluated. |
| Invoking a value gives the value's identity. Example: |
| </p> |
| <pre class="programlisting"><span class="identifier">cout</span> <span class="special"><<</span> <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">val</span><span class="special">(</span><span class="string">"Hello World"</span><span class="special">)();</span> |
| </pre> |
| <p> |
| prints out "3 Hello World". |
| </p> |
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