ceph/src/boost/libs/spirit/classic/phoenix/doc/statements.html

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  15       <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Statements</b></font>
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  28 <a name="lazy_statements"></a><h2>Lazy statements</h2><p>
  29 The primitives and composite building blocks presented before are sufficiently powerful to construct quite elaborate structures and facilities. We have presented lazy-functions and lazy-operators. How about lazy-statements? First, an appetizer:</p>
  30 <p>
  31 Print all odd-numbered contents of an STL container using std::for_each (sample4.cpp):</p>
  32 <code><pre>
  33     <span class=identifier>for_each</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>(),
  34         </span><span class=identifier>if_</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>)
  35         [
  36             </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>arg1 </span><span class=special>&lt;&lt; </span><span class=literal>' '
  37         </span><span class=special>]
  38     );
  39 </span></pre></code>
  40 <p>
  41 Huh? Is that valid C++? Read on...</p>
  42 <p>
  43 Yes, it is valid C++. The sample code above is as close as you can get to the syntax of C++. This stylized C++ syntax differs from actual C++ code. First, the if has a trailing underscore. Second, the block uses square brackets  instead of the familiar curly braces {}.</p>
  44 <p>
  45 Here are more examples with annotations. The code almost speaks for itself.</p>
  46 <p>
  47 <b>1) block statement:</b></p>
  48 <code><pre>
  49     <span class=identifier>statement</span><span class=special>,
  50     </span><span class=identifier>statement</span><span class=special>,
  51     ....
  52     </span><span class=identifier>statement
  53 </span></pre></code>
  54 <p>
  55 Basically, these are comma separated statements. Take note that unlike the C/C++ semicolon, the comma is a separator put *in-between* statements. This is like Pascal's semicolon separator, rather than C/C++'s semicolon terminator. For example:</p>
  56 <code><pre>
  57     <span class=identifier>statement</span><span class=special>,
  58     </span><span class=identifier>statement</span><span class=special>,
  59     </span><span class=identifier>statement</span><span class=special>,     //  </span><span class=identifier>ERROR</span><span class=special>!
  60 </span></pre></code>
  61 <p>
  62 Is an error. The last statement should not have a comma. Block statements can be grouped using the parentheses. Again, the last statement in a group should not have a trailing comma.</p>
  63 <code><pre>
  64     <span class=identifier>statement</span><span class=special>,
  65     </span><span class=identifier>statement</span><span class=special>,
  66     (
  67         </span><span class=identifier>statement</span><span class=special>,
  68         </span><span class=identifier>statement
  69     </span><span class=special>),
  70     </span><span class=identifier>statement
  71 </span></pre></code>
  72 <p>
  73 Outside the square brackets, block statements should be grouped. For example:</p>
  74 <code><pre>
  75     <span class=identifier>for_each</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>(),
  76         (
  77             </span><span class=identifier>do_this</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>),
  78             </span><span class=identifier>do_that</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>)
  79         )
  80     );
  81 </span></pre></code>
  82 <p>
  83 <b>2) if_ statement:</b></p>
  84 <p>
  85 We have seen the if_ statement. The syntax is:</p>
  86 <code><pre>
  87     <span class=identifier>if_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>)
  88     [
  89         </span><span class=identifier>sequenced_statements
  90     </span><span class=special>]
  91 </span></pre></code>
  92 <p>
  93 <b>3) if_ else_ statement:</b></p>
  94 <p>
  95 The syntax is</p>
  96 <code><pre>
  97     <span class=identifier>if_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>)
  98     [
  99         </span><span class=identifier>sequenced_statements
 100     </span><span class=special>]
 101     .</span><span class=identifier>else_
 102     </span><span class=special>[
 103         </span><span class=identifier>sequenced_statements
 104     </span><span class=special>]
 105 </span></pre></code>
 106 <p>
 107 Take note that else has a prefix dot and a trailing underscore: .else_</p>
 108 <p>
 109 Example: This code prints out all the elements and appends &quot; &gt; 5&quot;, &quot; == 5&quot; or &quot; &lt; 5&quot; depending on the element's actual value:</p>
 110 <code><pre>
 111     <span class=identifier>for_each</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>(),
 112         </span><span class=identifier>if_</span><span class=special>(</span><span class=identifier>arg1 </span><span class=special>&gt; </span><span class=number>5</span><span class=special>)
 113         [
 114             </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>arg1 </span><span class=special>&lt;&lt; </span><span class=string>&quot; &gt; 5\n&quot;
 115         </span><span class=special>]
 116         .</span><span class=identifier>else_
 117         </span><span class=special>[
 118             </span><span class=identifier>if_</span><span class=special>(</span><span class=identifier>arg1 </span><span class=special>== </span><span class=number>5</span><span class=special>)
 119             [
 120                 </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>arg1 </span><span class=special>&lt;&lt; </span><span class=string>&quot; == 5\n&quot;
 121             </span><span class=special>]
 122             .</span><span class=identifier>else_
 123             </span><span class=special>[
 124                 </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>arg1 </span><span class=special>&lt;&lt; </span><span class=string>&quot; &lt; 5\n&quot;
 125             </span><span class=special>]
 126         ]
 127     );
 128 </span></pre></code>
 129 <p>
 130 Notice how the if_ else_ statement is nested.</p>
 131 <p>
 132 <b>4) while_ statement:</b></p>
 133 <p>
 134 The syntax is:</p>
 135 <code><pre>
 136     <span class=identifier>while_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>)
 137     [
 138         </span><span class=identifier>sequenced_statements
 139     </span><span class=special>]
 140 </span></pre></code>
 141 <p>
 142 Example: This code decrements each element until it reaches zero and prints out the number at each step. A newline terminates the printout of each value.</p>
 143 <code><pre>
 144     <span class=identifier>for_each</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>(),
 145         (
 146             </span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>--)
 147             [
 148                 </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>arg1 </span><span class=special>&lt;&lt; </span><span class=string>&quot;, &quot;
 149             </span><span class=special>],
 150             </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>val</span><span class=special>(</span><span class=string>&quot;\n&quot;</span><span class=special>)
 151         )
 152     );
 153 </span></pre></code>
 154 <p>
 155 <b>5) do_ while_ statement:</b></p>
 156 <p>
 157 The syntax is:</p>
 158 <code><pre>
 159     <span class=identifier>do_
 160     </span><span class=special>[
 161         </span><span class=identifier>sequenced_statements
 162     </span><span class=special>]
 163     .</span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>conditional_expression</span><span class=special>)
 164 </span></pre></code>
 165 <p>
 166 Again, take note that while has a prefix dot and a trailing underscore: .while_</p>
 167 <p>
 168 Example: This code is almost the same as the previous example above with a slight twist in logic.</p>
 169 <code><pre>
 170     <span class=identifier>for_each</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>(),
 171         (
 172             </span><span class=identifier>do_
 173             </span><span class=special>[
 174                 </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>arg1 </span><span class=special>&lt;&lt; </span><span class=string>&quot;, &quot;
 175             </span><span class=special>]
 176             .</span><span class=identifier>while_</span><span class=special>(</span><span class=identifier>arg1</span><span class=special>--),
 177             </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>val</span><span class=special>(</span><span class=string>&quot;\n&quot;</span><span class=special>)
 178         )
 179     );
 180 </span></pre></code>
 181 <p>
 182 <b>6) for_ statement:</b></p>
 183 <p>
 184 The syntax is:</p>
 185 <code><pre>
 186     <span class=identifier>for_</span><span class=special>(</span><span class=identifier>init_statement</span><span class=special>, </span><span class=identifier>conditional_expression</span><span class=special>, </span><span class=identifier>step_statement</span><span class=special>)
 187     [
 188         </span><span class=identifier>sequenced_statements
 189     </span><span class=special>]
 190 </span></pre></code>
 191 <p>
 192 It is again almost similar to C++ for statement. Take note that the init_statement, conditional_expression and step_statement are separated by the comma instead of the semi- colon and each must be present (i.e. for_(,,) is invalid).</p>
 193 <p>
 194 Example: This code prints each element N times where N is the element's value. A newline terminates the printout of each value.</p>
 195 <code><pre>
 196     <span class=keyword>int </span><span class=identifier>iii</span><span class=special>;
 197     </span><span class=identifier>for_each</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>(),
 198         (
 199             </span><span class=identifier>for_</span><span class=special>(</span><span class=identifier>var</span><span class=special>(</span><span class=identifier>iii</span><span class=special>) = </span><span class=number>0</span><span class=special>, </span><span class=identifier>var</span><span class=special>(</span><span class=identifier>iii</span><span class=special>) &lt; </span><span class=identifier>arg1</span><span class=special>, ++</span><span class=identifier>var</span><span class=special>(</span><span class=identifier>iii</span><span class=special>))
 200             [
 201                 </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>arg1 </span><span class=special>&lt;&lt; </span><span class=string>&quot;, &quot;
 202             </span><span class=special>],
 203             </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>val</span><span class=special>(</span><span class=string>&quot;\n&quot;</span><span class=special>)
 204         )
 205     );
 206 </span></pre></code>
 207 <p>
 208 As before, all these are lazily evaluated. The result of such statements are in fact composites that are passed on to STL's for_each function. In the viewpoint of for_each, what was passed is just a functor, no more, no less.</p>
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 211     <td class="note_box">
 212 <img src="theme/note.gif"></img> Unlike lazy functions and lazy operators, lazy statements always return void.    </td>
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 224 <p class="copyright">Copyright &copy; 2001-2002 Joel de Guzman<br>
 225   <br>
 226 <font size="2">Use, modification and distribution is subject to the Boost Software
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