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Updated for VC++ quirks and overload code example.

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Edward Diener
2016-04-11 14:24:38 -04:00
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doc/topics/variadic_macros.html
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<head>
<meta content="text/html; charset=windows-1252" http-equiv="content-type">
<title>variadic_macros.html</title>
<link rel="stylesheet" type="text/css" href="../styles.css">
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<body>
<h4>Variadic Macros</h4>
<div> Variadic macros are supported by a number of compilers.
They are
macros of the form:
</div>
<div class="code">
<div> Variadic macros are supported by a number of compilers. They are
macros of the form: </div>
<div class="code">
<pre>#define SOME_MACRO(ZeroOrMoreParameters,...) macro expansion possible specifying __VA_ARGS__</pre>
</div>
<div> The '...' in the parameter list represents the variadic
data when the macro is invoked and the __VA_ARGS__ in the expansion
represents the variadic data in the expansion of the macro. Variadic
data is of the form of 1 or more preprocessor tokens separated by
commas.<br>
<div> The '...' in the parameter list represents the variadic data when the
macro is invoked and the __VA_ARGS__ in the expansion represents the
variadic data in the expansion of the macro. Variadic data is of the form
of 1 or more preprocessor tokens separated by commas.<br>
<br>
The '...' must be the last parameter in the macro definition and there
may be 0 or more non-variadic parameters preceding it.<br>
The '...' must be the last parameter in the macro definition and there may
be 0 or more non-variadic parameters preceding it.<br>
<br>
In the expansion of the macro __VA_ARGS__ may be specified 0 or more
times to represent the variadic data. The variadic data in the
expansion is a comma separated list of preprocessor tokens representing
the variadic data which the invoker of the macro enters as the last
arguments to the macro.<br>
In the expansion of the macro __VA_ARGS__ may be specified 0 or more times
to represent the variadic data. The variadic data in the expansion is a
comma separated list of preprocessor tokens representing the variadic data
which the invoker of the macro enters as the last arguments to the macro.<br>
</div>
<h4>Example<u> - Creating and invoking a variadic macro.</u></h4>
<div class="code">
<div class="code">
<pre>#define INITIALIZE_INT_ARRAY(array_name,...) \ <br> static int array_name[] = { __VA_ARGS__ }; \ <br> /**/<br><br> INITIALIZE_INT_ARRAY(myname,45,789,33510,9346,2)</pre>
</div>
<u> <span style="font-weight: bold;">Preprocessor
<u> <span style="font-weight: bold;">Preprocessor
Library Support<br>
</span></u>
<div>The library offers support for variadic macros for those
@ -56,7 +53,15 @@
When BOOST_PP_VARIADICS is 1, the library offers some extended
functionality
by using variadic macros, and also offers extended support for working
with variadic data.<br>
with variadic data.<br><br>
<a name="vmvcquirk"></a>Visual C++ has a
few quirks related to variadic macros which require the end-user to code
slightly differently. When BOOST_PP_VARIADICS is 1 and Visual C++ is the
compiler BOOST_PP_VARIADICS_MSVC is 1, else when BOOST_PP_VARIADICS is 1
and Visual C++ is not the compiler BOOST_PP_VARIADICS_MSVC is 0. When
BOOST_PP_VARIADICS is 0 then BOOST_PP_VARIADICS_MSVC is not defined.
In this way the end-user, when using variadic macros, can test for the
presence of Visual C++ as the compiler and code accordingly.<br>
<br>
Support for working with variadic
data is largely centered on being able to convert variadic data to
@ -64,7 +69,7 @@
functionality for working with those Boost preprocessor library data
types is much greater than that for working with variadic data directly.<br>
</div>
<a name="VNotation"></a>
<a name="VNotation"></a>
<h4>Notation For Variadic Macros<br>
</h4>
<div>In the documentation, headers which have variadic macros,
@ -131,7 +136,7 @@
use this variadic data reliably as arguments to other macros, one needs
variadic macro support.<br>
</div>
<u style="font-weight: bold;"> Using a Tuple Instead of an Array<br>
<u style="font-weight: bold;"> Using a Tuple Instead of an Array<br>
</u>
<div>An array as a preprocessor data type is a two-element tuple where the
first element is the array size and the second element is a tuple which
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type is essentially obsolete for conforming C++ compilers. Only if your
compiler does not support variadic macros is the preprocessor array data
type still useful.</div>
<u style="font-weight: bold;">Using Variadic Data</u>
<u style="font-weight: bold;">Using Variadic Data</u>
<div>Variadic data exists in the
form of comma-separated preprocessor tokens. This is the case whether
the variadic data comes from the __VA_ARGS__ of a variadic macro, from
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variadic data as a whole to variadic macros as the last parameter of
the macro. However: <br>
<br>
<span style="font-weight: bold;">Attempting to pass
<span style="font-weight: bold;">Attempting to pass
variadic data as a
whole directly into a non-variadic macro is not guaranteed to work and
may fail.<br>
@ -178,7 +183,7 @@
of compilers, currently most notably Visual C++. Even passing variadic
data as arguments to a non-variadic macro, when it is not represented
in
the form of&nbsp; __VA_ARGS__, may fail with certain compilers.<br>
the form of<EFBFBD> __VA_ARGS__, may fail with certain compilers.<br>
<br>
What follows are very simple examples, showing how variadic data can be
passed to a non-variadic macro.<br>
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<h4>Example<u> - Passing variadic data as a whole to a
non-variadic
macro. DO NOT DO.</u></h4>
<div class="code">
<div class="code">
<pre>#define MACRO_ARG_2(x,y) BOOST_PP_ADD(x,y)<br>#define VAR_MACRO(...) __VA_ARGS__<br><br>/* The following should not be done and is not guaranteed to work with compilers. */<br><br><span
style="font-weight: bold;"><span style="font-family: monospace;"></span></span>int xx = MACRO_ARG_2(VAR_MACRO(2,3));</pre>
style="font-weight: bold;"><span style="font-family: monospace;"></span></span>int xx = MACRO_ARG_2(VAR_MACRO(2,3));</pre>
</div>
<div> There are two ways to pass variadic data to a non-variadic
macro.
@ -203,7 +208,7 @@ style="font-weight: bold;"><span style="font-family: monospace;"></span></span>i
a
non-variadic macro.<br>
</u></h4>
<div class="code">
<div class="code">
<pre>#define MACRO_ARG_2(x,y) BOOST_PP_ADD(x,y)<br>#define VAR_MACRO(...) __VA_ARGS__<br><br>/* The following will work correctly */<br><br>int xx = MACRO_ARG_2<br> (<br> BOOST_PP_VARIADIC_ELEM(0,VAR_MACRO(2,3)),<br> BOOST_PP_VARIADIC_ELEM(1,VAR_MACRO(2,3))<br> );</pre>
</div>
<div>The second way is to use a macro in the library called
@ -215,7 +220,7 @@ style="font-weight: bold;"><span style="font-family: monospace;"></span></span>i
BOOST_PP_OVERLOAD
and on to a non-variadic macro.<br>
</u></h4>
<div class="code">
<div class="code">
<pre>#define MACRO_ARG_2(x,y) BOOST_PP_ADD(x,y)<br>#define VAR_MACRO(...) __VA_ARGS__<br><br>/* The following will work correctly */<br><br>int xx = BOOST_PP_OVERLOAD(MACRO_ARG_,VAR_MACRO(2,3))(VAR_MACRO(2,3));<br><br>/* For Visual C++ it is necessary to do this */<br><br>int xx = <br>BOOST_PP_CAT(BOOST_PP_OVERLOAD(MACRO_ARG_,VAR_MACRO(2,3))(VAR_MACRO(2,3)),BOOST_PP_EMPTY());</pre>
</div>
<br>
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</div>
<b>See</b> <b>Also</b><br>
<ul>
<li><a href="../ref/variadics.html">BOOST_PP_VARIADICS</a></li>
<li><a href="../headers/tuple.html">Tuple Macros</a><br>
<li><a href="../ref/variadics.html">BOOST_PP_VARIADICS</a></li>
<li><a href="../headers/tuple.html">Tuple Macros</a><br>
</li>
<li><a href="../headers/variadic.html">Variadic
<li><a href="../headers/variadic.html">Variadic
Macros<br>
</a></li>
<li><a href="../ref/array_enum.html">BOOST_PP_ARRAY_ENUM</a></li>
<li><a href="../ref/list_enum_r.html">BOOST_PP_LIST_ENUM</a></li>
<li><a href="../ref/seq_enum.html">BOOST_PP_SEQ_ENUM</a></li>
<li><a href="../ref/tuple_enum.html">BOOST_PP_TUPLE_ENUM</a></li>
<li><a href="../ref/overload.html">BOOST_PP_OVERLOAD</a></li>
<li><a href="../ref/array_enum.html">BOOST_PP_ARRAY_ENUM</a></li>
<li><a href="../ref/list_enum_r.html">BOOST_PP_LIST_ENUM</a></li>
<li><a href="../ref/seq_enum.html">BOOST_PP_SEQ_ENUM</a></li>
<li><a href="../ref/tuple_enum.html">BOOST_PP_TUPLE_ENUM</a></li>
<li><a href="../ref/overload.html">BOOST_PP_OVERLOAD</a></li>
</ul>
<hr size="1">
<div style="margin-left: 0px;"> <i><EFBFBD> Copyright
<hr size="1">
<div style="margin-left: 0px;"> <i><EFBFBD> Copyright
Edward Diener
2011,2013</i> </div>
<div style="margin-left: 0px;">
2011,2013,2016</i> </div>
<div style="margin-left: 0px;">
<p><small>Distributed under the Boost Software License,
Version 1.0.
(See accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a>
or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p>
(See accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a>
or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p>
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