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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="variadic_macro_data.vmd_examples"></a><a class="link" href="vmd_examples.html" title="Examples using VMD functionality">Examples using VMD functionality</a>
</h2></div></div></div>
<p>
Examples of library use are always highly personal. Any given library employing
macro programming can decide what macro facilities are needed based on the
library itself and then decide if functionality in a macro library like VMD
makes macro programming in that library easier. To that end the examples presented
here are highly arbitrary and are just efforts to illustrate possible use of
functionality of VMD features without worrying too much if those examples have
any practical beneficial use in real programming situations. In these examples
I have endeavored, therefore, to present macro programming "snippets"
using VMD functionality rather than complete solutions to a given practical
problem.
</p>
<h4>
<a name="variadic_macro_data.vmd_examples.h0"></a>
<span class="phrase"><a name="variadic_macro_data.vmd_examples.switch_macro"></a></span><a class="link" href="vmd_examples.html#variadic_macro_data.vmd_examples.switch_macro">Switch
macro</a>
</h4>
<p>
In C++ there is a 'switch' statement which we can emulate in macro programming
using VMD. For the macro emulation we will have as parameters to our macro:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
A value, which can be any data type VMD can parse.
</li>
<li class="listitem">
A tuple of calling values. These will be used when calling the matching
macro.
</li>
<li class="listitem">
Variadic parameters, each of which are tuples. Each tuple consists of two
elements, the name of a value to match and the name of a macro to call.
For the 'default' case the tuple is a single element which is the name
of a macro to call. These are our equivalents to the C++ switch 'case'
statements.
</li>
</ol></div>
<p>
The macro looks like:
</p>
<pre class="programlisting"><span class="identifier">BOOST_VMD_SWITCH</span><span class="special">(</span><span class="identifier">value</span><span class="special">,</span><span class="identifier">calling_values</span><span class="special">,...)</span>
</pre>
<p>
We have to be careful not to parse the name of our macro to call in any way
since this is a failing condition for BOOST_VMD_IS_EMPTY and subsequently for
any parsing of input data we might want to do. Instead we will just extract
the calling macro name and just call it, passing the calling values.
</p>
<p>
Our processing is:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
Convert our variadic parameters to a tuple since access to tuple elements
is easier.
</li>
<li class="listitem">
Use a BOOST_PP_WHILE loop to find the matching value and extract the calling
macro from it. We will use BOOST_VMD_EQUAL to find the matching value.
</li>
<li class="listitem">
Call the calling macro with the calling values when we return from our
BOOST_PP_WHILE loop.
</li>
</ol></div>
<p>
Here is our code:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">setup</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#if</span> <span class="identifier">BOOST_PP_VARIADICS</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">cat</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">arithmetic</span><span class="special">/</span><span class="identifier">inc</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">comparison</span><span class="special">/</span><span class="identifier">equal</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">control</span><span class="special">/</span><span class="identifier">expr_iif</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">control</span><span class="special">/</span><span class="identifier">iif</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">control</span><span class="special">/</span><span class="keyword">while</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">/</span><span class="identifier">elem</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">/</span><span class="keyword">enum</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">facilities</span><span class="special">/</span><span class="identifier">expand</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">/</span><span class="identifier">replace</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">/</span><span class="identifier">size</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">variadic</span><span class="special">/</span><span class="identifier">to_tuple</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">variadic</span><span class="special">/</span><span class="identifier">size</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">equal</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">identity</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">is_empty</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="comment">/*
State index into state values
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_ELEM_INDEX</span> <span class="number">2</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_ELEM_DEFAULT</span> <span class="number">4</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_ELEM_RESULT</span> <span class="number">5</span>
<span class="comment">/*
Retrieve the state value, never changes
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_GET_VALUE</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Retrieve the state tuple of values, never changes
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_GET_CHOICES</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Retrieve the state index
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_GET_INDEX</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Retrieve the state tuple of values size, never changes
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_GET_SIZE</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Retrieve the state default tuple
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_GET_DEFAULT</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">4</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Retrieve the state result tuple
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_GET_RESULT</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">5</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Retrieve the current value tuple
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_GET_CURRENT_CHOICE</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_GET_INDEX</span><span class="special">(</span><span class="identifier">state</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_GET_CHOICES</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Expands to the state
value = value to compare against
tuple = choices as a tuple of values
size = size of tuple of values
None of these ever change in the WHILE state
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE_EXPAND</span><span class="special">(</span><span class="identifier">value</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">,</span><span class="identifier">size</span><span class="special">)</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">value</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">,</span><span class="number">0</span><span class="special">,</span><span class="identifier">size</span><span class="special">,(</span><span class="number">0</span><span class="special">,),(,))</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Expands to the WHILE state
The state to our WHILE consists of a tuple of elements:
1: value to compare against
2: tuple of values. Each value is a value/macro pair or if the default just a macro
3: index into the values
4: tuple for default macro. 0 means no default macro, 1 means default macro and then second value is the default macro.
5: tuple of result matched. Emptiness means no result yet specified, 0 means no match, 1 means match and second value is the matching macro.
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_STATE</span><span class="special">(</span><span class="identifier">value</span><span class="special">,...)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_EXPAND</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">value</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_VARIADIC_TO_TUPLE</span><span class="special">(</span><span class="identifier">__VA_ARGS__</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_VARIADIC_SIZE</span><span class="special">(</span><span class="identifier">__VA_ARGS__</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Sets the state upon a successful match.
macro = is the matching macro found
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_SUCCESS</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">macro</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_REPLACE_D</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">state</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_ELEM_RESULT</span><span class="special">,</span> <span class="special">\</span>
<span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">macro</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Sets the state upon final failure to find a match.
def = default tuple macro, ignored
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_FAILURE</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">def</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_REPLACE_D</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">state</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_ELEM_RESULT</span><span class="special">,</span> <span class="special">\</span>
<span class="special">(</span><span class="number">0</span><span class="special">,)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Increments the state index into the tuple values
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_UPDATE_INDEX</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_REPLACE_D</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">state</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_ELEM_INDEX</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_INC</span><span class="special">(</span><span class="identifier">BOOST_VMD_SWITCH_STATE_GET_INDEX</span><span class="special">(</span><span class="identifier">state</span><span class="special">))</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Choose our current value's macro as our successful match
tuple = current tuple to test
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_VALUE_MATCH</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_SUCCESS</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">))</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Update our state index
tuple = current tuple to test, ignored
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_VALUE_UPDATE_INDEX</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_UPDATE_INDEX</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Test our current value against our value to compare against
tuple = current tuple to test
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_VALUE</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_IIF</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_EQUAL_D</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_GET_VALUE</span><span class="special">(</span><span class="identifier">state</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">)</span> <span class="special">\</span>
<span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_VALUE_MATCH</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_VALUE_UPDATE_INDEX</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Set our default macro and update the index in our WHILE state
tuple = current tuple to test
*/</span>
<span class="preprocessor">#if</span> <span class="identifier">BOOST_VMD_MSVC</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_CREATE_DEFAULT_NN</span><span class="special">(</span><span class="identifier">number</span><span class="special">,</span><span class="identifier">name</span><span class="special">)</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">number</span><span class="special">,</span><span class="identifier">name</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_CREATE_DEFAULT</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_UPDATE_INDEX</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_REPLACE_D</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">state</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_ELEM_DEFAULT</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_CREATE_DEFAULT_NN</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">BOOST_PP_TUPLE_ENUM</span><span class="special">(</span><span class="identifier">tuple</span><span class="special">))</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#else</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_CREATE_DEFAULT</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_UPDATE_INDEX</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_REPLACE_D</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">state</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_ELEM_DEFAULT</span><span class="special">,</span> <span class="special">\</span>
<span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">BOOST_PP_TUPLE_ENUM</span><span class="special">(</span><span class="identifier">tuple</span><span class="special">))</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#endif</span>
<span class="comment">/*
If our current value is a default macro, just set the default macro,
else test our current value.
tuple = current tuple to test
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_TUPLE</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_IIF</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_EQUAL_D</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_SIZE</span><span class="special">(</span><span class="identifier">tuple</span><span class="special">),</span> <span class="special">\</span>
<span class="number">1</span> <span class="special">\</span>
<span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_CREATE_DEFAULT</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_VALUE</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">tuple</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Test the current value in our tuple of values
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT_TUPLE</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">state</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_GET_CURRENT_CHOICE</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Choose the default macro as our successful match
def = default tuple consisting of just the default macro name
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_DEFAULT_RET_CHOSEN</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">def</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_SUCCESS</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">state</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">def</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
If the default macro exists, choose it else indicate no macro was found
def = default tuple consisting of just the default macro name
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_DEFAULT_RET</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">def</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_IIF</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">def</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_DEFAULT_RET_CHOSEN</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_FAILURE</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">,</span><span class="identifier">def</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Try to choose the default macro if it exists
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP_DEFAULT</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_DEFAULT_RET</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">state</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_GET_DEFAULT</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
WHILE loop operation
Check for the next value match or try to choose the default if all matches have been checked
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_OP</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_IIF</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_EQUAL_D</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">d</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_GET_INDEX</span><span class="special">(</span><span class="identifier">state</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_GET_SIZE</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_DEFAULT</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP_TEST_CURRENT</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
WHILE loop predicate
Continue the WHILE loop if a result has not yet been specified
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_PRED</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IS_EMPTY</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="number">0</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_GET_RESULT</span><span class="special">(</span><span class="identifier">state</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Invokes the function-like macro
macro = function-like macro name
tparams = tuple of macro parameters
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_PROCESS_INVOKE_MACRO</span><span class="special">(</span><span class="identifier">macro</span><span class="special">,</span><span class="identifier">tparams</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_EXPAND</span><span class="special">(</span><span class="identifier">macro</span> <span class="identifier">tparams</span><span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Processes our WHILE loop result
callp = tuple of parameters for the called macro
result = tuple. The first tuple element is 0
if no macro has been found or 1 if a macro
has been found. If 1 the second element is
the name of a function-like macro
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_PROCESS</span><span class="special">(</span><span class="identifier">callp</span><span class="special">,</span><span class="identifier">result</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_EXPR_IIF</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">0</span><span class="special">,</span><span class="identifier">result</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_PROCESS_INVOKE_MACRO</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_TUPLE_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">result</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">callp</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="comment">/*
Use BOOST_VMD_SWITCH_IDENTITY to pass a fixed value instead
of a function-like macro as the second element of
any tuple of the variadic parameters, or as the default
value, to BOOST_VMD_SWITCH.
*/</span>
<span class="preprocessor">#if</span> <span class="identifier">BOOST_VMD_MSVC</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_IDENTITY</span><span class="special">(</span><span class="identifier">item</span><span class="special">)</span> <span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">BOOST_VMD_IDENTITY</span><span class="special">(</span><span class="identifier">item</span><span class="special">),)</span>
<span class="preprocessor">#else</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_IDENTITY</span> <span class="identifier">BOOST_VMD_IDENTITY</span>
<span class="preprocessor">#endif</span>
<span class="comment">/*
Switch macro
Parameters are:
value = value to compare against. May be any VMD data value.
callp = tuple of parameters for the called macro
variadic parameters = each parameter must be a tuple.
Each tuple consists of a two-element tuple. The first element is
a value, which may be any VMD data value, and the second element
is the name of a function-like macro to be called if the value
is equal to the value to compare against. For a default value
the tuple is a single-element tuple which contains the name of
a function-like macro to be called if no other value matches.
*/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH</span><span class="special">(</span><span class="identifier">value</span><span class="special">,</span><span class="identifier">callp</span><span class="special">,...)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_PROCESS</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">callp</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE_GET_RESULT</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_WHILE</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_PRED</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_OP</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH_STATE</span><span class="special">(</span><span class="identifier">value</span><span class="special">,</span><span class="identifier">__VA_ARGS__</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#endif</span> <span class="comment">/* BOOST_PP_VARIADICS */</span>
</pre>
<p>
The code is fairly involved but it is commented so that it can be understood.
There are a few workarounds for a VC++ default preprocessor problem, which
I discovered, having to do with passing the name of a function-like macro in
a tuple.
</p>
<p>
The BOOST_VMD_SWITCH macro can be used with either macros to call or with fixed
values to return. When specifying macros to call the macro name is the second
element of the corresponding value-macro tuple, or in the 'default' case it
is just the macro name itself. When specifying fixed values to return the macro
'name' is BOOST_VMD_SWITCH_IDENTITY(fixed_value), whether as the second element
of the corresponding value-macro tuple or as the macro 'name' of the 'default'
case. In the variadic parameters the user can mix macro names and fixed values
as he likes.
</p>
<p>
Some simple examples:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_TEST_1</span><span class="special">(</span><span class="identifier">number</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">test1_</span> <span class="special">##</span> <span class="identifier">number</span>
<span class="comment">/**/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_TEST_2</span><span class="special">(</span><span class="identifier">number</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">test2_</span> <span class="special">##</span> <span class="identifier">number</span>
<span class="comment">/**/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_TEST_3</span><span class="special">(</span><span class="identifier">number</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">test3_</span> <span class="special">##</span> <span class="identifier">number</span>
<span class="comment">/**/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_SWITCH_TEST_DEFAULT</span><span class="special">(</span><span class="identifier">number</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">test_default_</span> <span class="special">##</span> <span class="identifier">number</span>
<span class="comment">/**/</span>
</pre>
<p>
We will use these simple macros in our calls to BOOST_VMD_SWITCH.
</p>
<pre class="programlisting"><span class="identifier">BOOST_VMD_SWITCH</span><span class="special">(</span><span class="number">1</span><span class="special">,</span>
<span class="special">(</span><span class="number">7</span><span class="special">),</span>
<span class="special">(</span><span class="identifier">BOOST_VMD_SWITCH_TEST_DEFAULT</span><span class="special">),</span>
<span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_3</span><span class="special">),</span>
<span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_1</span><span class="special">),</span>
<span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_2</span><span class="special">)</span>
<span class="special">)</span>
</pre>
<p>
Here our macro will return 'test1_7'.
</p>
<p>
Notice that 'cases' can be in any order.
</p>
<pre class="programlisting"><span class="identifier">BOOST_VMD_SWITCH</span><span class="special">(</span><span class="number">4</span><span class="special">,</span>
<span class="special">(</span><span class="number">7</span><span class="special">),</span>
<span class="special">(</span><span class="identifier">BOOST_VMD_SWITCH_TEST_DEFAULT</span><span class="special">),</span>
<span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_2</span><span class="special">),</span>
<span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_1</span><span class="special">),</span>
<span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_3</span><span class="special">)</span>
<span class="special">)</span>
</pre>
<p>
Here are macro uses the default case and returns 'test_default_7'.
</p>
<pre class="programlisting"><span class="identifier">BOOST_VMD_SWITCH</span><span class="special">(</span><span class="number">143</span><span class="special">,</span>
<span class="special">(</span><span class="number">7</span><span class="special">),</span>
<span class="special">(</span><span class="identifier">BOOST_VMD_SWITCH_TEST_DEFAULT</span><span class="special">),</span>
<span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_1</span><span class="special">),</span>
<span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_2</span><span class="special">),</span>
<span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_3</span><span class="special">),</span>
<span class="special">(</span><span class="number">143</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_IDENTITY</span><span class="special">(</span><span class="number">55</span><span class="special">))</span>
<span class="special">)</span>
</pre>
<p>
This shows how the matching case can be a fixed_value as the macro 'name'.
</p>
<pre class="programlisting"><span class="identifier">BOOST_VMD_SWITCH</span><span class="special">(</span><span class="number">155</span><span class="special">,</span>
<span class="special">(</span><span class="number">7</span><span class="special">),</span>
<span class="special">(</span><span class="identifier">BOOST_VMD_SWITCH_IDENTITY</span><span class="special">(</span><span class="number">77</span><span class="special">)),</span>
<span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_1</span><span class="special">),</span>
<span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_2</span><span class="special">),</span>
<span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_3</span><span class="special">),</span>
<span class="special">(</span><span class="number">143</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_IDENTITY</span><span class="special">(</span><span class="number">55</span><span class="special">))</span>
<span class="special">)</span>
</pre>
<p>
This shows how the default value can be a fixed_value as the macro 'name'.
</p>
<pre class="programlisting"><span class="identifier">BOOST_VMD_SWITCH</span><span class="special">(</span><span class="identifier">BOOST_VMD_TYPE_TUPLE</span><span class="special">,</span>
<span class="special">(</span><span class="number">7</span><span class="special">),</span>
<span class="special">(</span><span class="identifier">BOOST_VMD_SWITCH_TEST_DEFAULT</span><span class="special">),</span>
<span class="special">(</span><span class="identifier">BOOST_VMD_TYPE_TUPLE</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_1</span><span class="special">),</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="identifier">BOOST_VMD_SWITCH_TEST_3</span><span class="special">),</span>
<span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_TEST_2</span><span class="special">)</span>
<span class="special">)</span>
</pre>
<p>
This shows that the 'value' and each 'case' matching values can be different
data types just as long as the types are one which VMD can parse.
</p>
<p>
There is more that can be done with the BOOST_VMD_SWITCH code but as it is
I believe it could be useful for programmers writing macro code. For instance
there is no checking that more than one 'case' value is the same. We could
generate a BOOST_VMD_ASSERT if that were the situation. There is no concept
of falling through to the next 'case' as their is when 'break' is not used
at the bottom of a particular C++ 'case' statement. Nonetheless the example
gives the macro programmer an idea of what can be done using the BOOST_VMD_EQUAL
macro in treating data types generically, using BOOST_VMD_IS_EMPTY to test
for emptiness and using BOOST_VMD_IDENTITY to generate a fixed value when a
macro call is made.
</p>
<h4>
<a name="variadic_macro_data.vmd_examples.h1"></a>
<span class="phrase"><a name="variadic_macro_data.vmd_examples.tti_inner_template"></a></span><a class="link" href="vmd_examples.html#variadic_macro_data.vmd_examples.tti_inner_template">TTI
inner template</a>
</h4>
<p>
As a more practical example, just to show the possible use of VMD functionality
in current Boost code, I will briefly illustrate a change that could be made
to the TTI library when using VMD functionality.
</p>
<p>
The Boost TTI library, of which the current developer of VMD is also the developer,
specifies a way to introspect an inner class template of a class. The introspection
can occur for an inner class template of specific template parameters.
</p>
<p>
In the library a macro is used to generate the metafunction which allows the
introspection to work. The macro used is called BOOST_TTI_TEMPLATE. The macro
has both a variadic version and a non-variadic version.
</p>
<p>
In the non-variadic version the macro always takes two parameters for introspecting
for specific template parameters. The first parameter is the name of the template
and the second parameter is an array of the specific template parameters (
with or without the parameter names themselves ). So for a class template of
the form:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span> <span class="identifier">Y</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">MyTemplate</span> <span class="special">{</span> <span class="special">...</span> <span class="identifier">code</span> <span class="special">};</span>
</pre>
<p>
the non-variadic macro would be:
</p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">MyTemplate</span><span class="special">,(</span><span class="number">2</span><span class="special">,(</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">int</span><span class="special">)))</span> <span class="comment">// uses array</span>
</pre>
<p>
I chose a Boost PP array rather than a Boost PP seq or a Boost PP list as I
felt the notation for specifying the template parameters was closer with the
array than with the others. Choosing a Boost PP tuple was not an option since
for non-variadic macros there is no way to automatically know the tuple size,
so an array was preferred.
</p>
<p>
For the variadic version variadic parameters are used so the notation would
be:
</p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">MyTemplate</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">int</span><span class="special">)</span> <span class="comment">// uses variadic parameters</span>
</pre>
<p>
since this is the most natural notation.
</p>
<p>
But for compatibility with the non-variadic version the end-user with variadic
macro support could also choose the Boost PP array form above.
</p>
<p>
Using VMD the variadic version could support any of the other Boost PP composite
types for the specific template parameters, even though I feel that the variadic
parameters form is easiest to use. In this scenario a user could specify:
</p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">MyTemplate</span><span class="special">,(</span><span class="keyword">class</span><span class="special">,(</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)))</span> <span class="comment">// use a list</span>
</pre>
<p>
or
</p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">MyTemplate</span><span class="special">,(</span><span class="keyword">class</span><span class="special">)(</span><span class="keyword">int</span><span class="special">))</span> <span class="comment">// use a seq</span>
</pre>
<p>
or
</p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_TEMPLATE</span><span class="special">(</span><span class="identifier">MyTemplate</span><span class="special">,(</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">int</span><span class="special">))</span> <span class="comment">// use a tuple</span>
</pre>
<p>
The only change needed would be in the code which takes the second parameter
and converts it to the final form used internally ( a Boost PP array ). This
occurs in the macro BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS in
the &lt;boost/tti/detail/dtemplate_params.hpp&gt; file. The code has two situations,
one for VC++8 or below and one for all other compilers. For our example we
will concentrate just on the one for all other compilers. You do not need to
know what the code does internally to complete the creation of the appropriate
metafunction to follow this example. The macro code in question looks like
this:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS</span><span class="special">(</span><span class="identifier">trait</span><span class="special">,</span><span class="identifier">name</span><span class="special">,</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_HAS_MEMBER_WITH_TEMPLATE_SFINAE</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="special">(</span> <span class="identifier">BOOST_PP_ADD</span><span class="special">(</span><span class="identifier">BOOST_PP_ARRAY_SIZE</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">),</span><span class="number">4</span><span class="special">),</span> <span class="special">(</span> <span class="identifier">trait</span><span class="special">,</span> <span class="identifier">name</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">false</span><span class="special">,</span> <span class="identifier">BOOST_PP_ARRAY_ENUM</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">)</span> <span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
</pre>
<p>
In this code we are taking the name of the metafunction ( trait ), the name
of the template ( name ), and our specific template parameters ( tpArray )
and passing the information in the form of a Boost PP array to another macro,
which will eventually create the metafunction which the end-user uses to test
if such a class template exists within some enclosing class. Even if tpArray
were a list, seq, or tuple we still want to pass the information internally
to BOOST_TTI_DETAIL_HAS_MEMBER_WITH_TEMPLATE_SFINAE in the form you can see
above, which is a Boost PP array. We don't need or want to change that internal
representation.
</p>
<p>
The current code, used by both the non-variadic and variadic version of the
BOOST_TTI_TEMPLATE template, assumes the 'tpArray' parameter is a Boost PP
array. But if it could be a tuple, seq, or list in the variadic version the
code could become, with the appropriate Boost PP and VMD header files:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">arithmetic</span><span class="special">/</span><span class="identifier">add</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">array</span><span class="special">/</span><span class="keyword">enum</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">array</span><span class="special">/</span><span class="identifier">size</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">control</span><span class="special">/</span><span class="identifier">expr_iif</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">control</span><span class="special">/</span><span class="identifier">iif</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">list</span><span class="special">/</span><span class="keyword">enum</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">list</span><span class="special">/</span><span class="identifier">size</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">seq</span><span class="special">/</span><span class="keyword">enum</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">seq</span><span class="special">/</span><span class="identifier">size</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">/</span><span class="keyword">enum</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">/</span><span class="identifier">size</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">identity</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">is_array</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">is_list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">is_seq</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">is_tuple</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#if</span> <span class="identifier">BOOST_PP_VARIADICS</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS</span><span class="special">(</span><span class="identifier">trait</span><span class="special">,</span><span class="identifier">name</span><span class="special">,</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_HAS_MEMBER_WITH_TEMPLATE_SFINAE</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE_CONCAT</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">trait</span><span class="special">,</span><span class="identifier">name</span><span class="special">,</span><span class="identifier">tpArray</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IDENTITY_RESULT</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_IIF</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IS_ARRAY</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IDENTITY</span><span class="special">(</span><span class="identifier">ARRAY</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE_LIST</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE_LIST</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IDENTITY_RESULT</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_IIF</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IS_LIST</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IDENTITY</span><span class="special">(</span><span class="identifier">LIST</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE_SEQ</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE_SEQ</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IDENTITY_RESULT</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_IIF</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IS_SEQ</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IDENTITY</span><span class="special">(</span><span class="identifier">SEQ</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE_TUPLE</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE_TUPLE</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IDENTITY_RESULT</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_PP_EXPR_IIF</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IS_TUPLE</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">),</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_IDENTITY</span><span class="special">(</span><span class="identifier">TUPLE</span><span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE_CONCAT</span><span class="special">(</span><span class="identifier">trait</span><span class="special">,</span><span class="identifier">name</span><span class="special">,</span><span class="identifier">tpArray</span><span class="special">,</span><span class="identifier">name</span><span class="special">)</span> <span class="special">\</span>
<span class="special">(</span> <span class="identifier">BOOST_PP_ADD</span><span class="special">(</span><span class="identifier">BOOST_PP_</span> <span class="special">##</span> <span class="identifier">name</span> <span class="special">##</span> <span class="identifier">_SIZE</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">),</span><span class="number">4</span><span class="special">),</span> <span class="special">(</span> <span class="identifier">trait</span><span class="special">,</span> <span class="identifier">name</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">false</span><span class="special">,</span> <span class="identifier">BOOST_PP_</span> <span class="special">##</span> <span class="identifier">name</span> <span class="special">##</span> <span class="identifier">_ENUM</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">)</span> <span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#else</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS</span><span class="special">(</span><span class="identifier">trait</span><span class="special">,</span><span class="identifier">name</span><span class="special">,</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_HAS_MEMBER_WITH_TEMPLATE_SFINAE</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="special">(</span> <span class="identifier">BOOST_PP_ADD</span><span class="special">(</span><span class="identifier">BOOST_PP_ARRAY_SIZE</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">),</span><span class="number">4</span><span class="special">),</span> <span class="special">(</span> <span class="identifier">trait</span><span class="special">,</span> <span class="identifier">name</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">false</span><span class="special">,</span> <span class="identifier">BOOST_PP_ARRAY_ENUM</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">)</span> <span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#endif</span>
</pre>
<p>
This of course gets more elaborate, but could be shortened considerably if
we chose to use BOOST_VMD_GET_TYPE and the invented BOOST_VMD_SWITCH of our
first example. We will assume in this second version of the code above that
our BOOST_VMD_SWITCH macro has been #included from somewhere.
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">arithmetic</span><span class="special">/</span><span class="identifier">add</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">array</span><span class="special">/</span><span class="keyword">enum</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">array</span><span class="special">/</span><span class="identifier">size</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">list</span><span class="special">/</span><span class="keyword">enum</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">list</span><span class="special">/</span><span class="identifier">size</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">seq</span><span class="special">/</span><span class="keyword">enum</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">seq</span><span class="special">/</span><span class="identifier">size</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">/</span><span class="keyword">enum</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">tuple</span><span class="special">/</span><span class="identifier">size</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">get_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#if</span> <span class="identifier">BOOST_PP_VARIADICS</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS</span><span class="special">(</span><span class="identifier">trait</span><span class="special">,</span><span class="identifier">name</span><span class="special">,</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_HAS_MEMBER_WITH_TEMPLATE_SFINAE</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE_CONCAT</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">trait</span><span class="special">,</span><span class="identifier">name</span><span class="special">,</span><span class="identifier">tpArray</span><span class="special">,</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_SWITCH</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_VMD_GET_TYPE</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">),</span> <span class="special">\</span>
<span class="special">(</span><span class="number">1</span><span class="special">),</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">BOOST_VMD_TYPE_ARRAY</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_IDENTITY</span><span class="special">(</span><span class="identifier">ARRAY</span><span class="special">)),</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">BOOST_VMD_TYPE_LIST</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_IDENTITY</span><span class="special">(</span><span class="identifier">LIST</span><span class="special">)),</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">BOOST_VMD_TYPE_SEQ</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_IDENTITY</span><span class="special">(</span><span class="identifier">SEQ</span><span class="special">)),</span> <span class="special">\</span>
<span class="special">(</span><span class="identifier">BOOST_VMD_TYPE_TUPLE</span><span class="special">,</span><span class="identifier">BOOST_VMD_SWITCH_IDENTITY</span><span class="special">(</span><span class="identifier">TUPLE</span><span class="special">))</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS_TYPE_CONCAT</span><span class="special">(</span><span class="identifier">trait</span><span class="special">,</span><span class="identifier">name</span><span class="special">,</span><span class="identifier">tpArray</span><span class="special">,</span><span class="identifier">name</span><span class="special">)</span> <span class="special">\</span>
<span class="special">(</span> <span class="identifier">BOOST_PP_ADD</span><span class="special">(</span><span class="identifier">BOOST_PP_</span> <span class="special">##</span> <span class="identifier">name</span> <span class="special">##</span> <span class="identifier">_SIZE</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">),</span><span class="number">4</span><span class="special">),</span> <span class="special">(</span> <span class="identifier">trait</span><span class="special">,</span> <span class="identifier">name</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">false</span><span class="special">,</span> <span class="identifier">BOOST_PP_</span> <span class="special">##</span> <span class="identifier">name</span> <span class="special">##</span> <span class="identifier">_ENUM</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">)</span> <span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#else</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_TTI_DETAIL_TRAIT_CALL_HAS_TEMPLATE_CHECK_PARAMS</span><span class="special">(</span><span class="identifier">trait</span><span class="special">,</span><span class="identifier">name</span><span class="special">,</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_TTI_DETAIL_HAS_MEMBER_WITH_TEMPLATE_SFINAE</span> <span class="special">\</span>
<span class="special">(</span> <span class="special">\</span>
<span class="special">(</span> <span class="identifier">BOOST_PP_ADD</span><span class="special">(</span><span class="identifier">BOOST_PP_ARRAY_SIZE</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">),</span><span class="number">4</span><span class="special">),</span> <span class="special">(</span> <span class="identifier">trait</span><span class="special">,</span> <span class="identifier">name</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">false</span><span class="special">,</span> <span class="identifier">BOOST_PP_ARRAY_ENUM</span><span class="special">(</span><span class="identifier">tpArray</span><span class="special">)</span> <span class="special">)</span> <span class="special">)</span> <span class="special">\</span>
<span class="special">)</span> <span class="special">\</span>
<span class="comment">/**/</span>
<span class="preprocessor">#endif</span>
</pre>
<p>
This is shorter and easier to understand. The '(1)' passed as the calling values
to BOOST_VMD_SWITCH could just as well be '()' but VC8 has trouble with empty
parentheses so I avoid it here.
</p>
<p>
In the case of the TTI, is such a change worth it to give more flexibility
to the end-user ? In reality, because the variadic version of passing the specific
template parameters as variadic data is syntactically easier to use than any
of the Boost PP composite forms, I am actually happy enough with that use not
to pursue the sort of functionality I presented in this example. But the example
nonetheless shows the power of the VMD functionality for creating macros which
add flexibility when the macro programmer feels he needs it for his library.
</p>
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