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- <div class="titlepage"><div><div><h3 class="title">
- <a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_splitting"></a><a class="link" href="vmd_modifiers_splitting.html" title="Splitting modifiers">Splitting
- modifiers</a>
- </h3></div></div></div>
- <p>
- The BOOST_VMD_ELEM macro, which by default just returns an element of a sequence,
- has a usage where you can have it return both the element and the remaining
- part of the sequence after the element, or even just the remaining part of
- the sequence after the element by itself. This offers a form of splitting
- the sequence on a particular element. When used to return the remaining part
- of a sequence the remaining data may subsequently be treated as a VMD sequence
- again.
- </p>
- <p>
- To do this another set of optional modifiers are used which will be called
- 'splitting modifers'. These modifiers are:
- </p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- BOOST_VMD_RETURN_AFTER, which returns both the element information and
- the rest of the sequence after the element as a two-element tuple
- </li>
- <li class="listitem">
- BOOST_VMD_RETURN_ONLY_AFTER, which returns only the rest of the sequence
- after the element specified
- </li>
- <li class="listitem">
- BOOST_VMD_RETURN_NO_AFTER, this is the internal default which only returns
- the element itself. It need never be specified but may be used to override
- a previous splitting modifier specified as an optional parameter.
- </li>
- </ul></div>
- <p>
- If more than one of the splitting modifiers are specified as optional parameters
- to BOOST_VMD_ELEM the last one specified is in effect.
- </p>
- <p>
- The splitting modifiers BOOST_VMD_RETURN_NO_AFTER and BOOST_VMD_RETURN_AFTER
- work with either return type modifiers or filtering modifiers if they are
- used. The splitting modifier BOOST_VMD_RETURN_ONLY_AFTER works with filtering
- modifiers if it is used and any return type modifiers will be ignored. Optional
- modifiers may occur in any order after the required parameters to BOOST_VMD_ELEM.
- </p>
- <p>
- If BOOST_VMD_RETURN_AFTER is in effect and an element is not found, either
- because the element number is out of range for the sequence or because filtering
- does not match the element type, a tuple will still be returned but both
- its elements will be empty.
- </p>
- <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">elem</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
- <span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_ANAME</span> <span class="special">(</span><span class="identifier">ANAME</span><span class="special">)</span> <span class="comment">// an identifier must always be registered to be found by VMD</span>
- <span class="preprocessor">#define</span> <span class="identifier">A_SEQUENCE</span> <span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">)</span> <span class="number">46</span> <span class="special">(</span><span class="identifier">list_data1</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span> <span class="identifier">BOOST_VMD_TYPE_SEQ</span> <span class="identifier">ANAME</span>
- <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(list_data1,BOOST_PP_NIL)'</span>
- <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_NO_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(list_data1,BOOST_PP_NIL)'</span>
- <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'((list_data1,BOOST_PP_NIL),BOOST_VMD_TYPE_SEQ ANAME)'</span>
- <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_ONLY_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'BOOST_VMD_TYPE_SEQ ANAME'</span>
- <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">5</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">emptiness</span>
- <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">5</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_NO_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">emptiness</span>
- <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">5</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(,)'</span>
- <span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">5</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_ONLY_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">emptiness</span>
- </pre>
- <p>
- Combining splitting modifiers with return type modifiers:
- </p>
- <pre class="programlisting"><span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_TYPE</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'((BOOST_VMD_TYPE_LIST,(list_data1,BOOST_PP_NIL)),BOOST_VMD_TYPE_SEQ ANAME)'</span>
- </pre>
- <p>
- Combining splitting modifiers with filtering modifiers:
- </p>
- <pre class="programlisting"><span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_LIST</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'((list_data1,BOOST_PP_NIL),BOOST_VMD_TYPE_SEQ ANAME)'</span>
- </pre>
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