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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="predef.using_the_predefs"></a><a class="link" href="using_the_predefs.html" title="Using the predefs">Using the predefs</a>
</h2></div></div></div>
<p>
To use the automatically defined predefs one needs to only include the single
top-level header:
</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">predef</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span>
</pre>
<p>
This defines <span class="bold"><strong>all</strong></span> the version macros known
to the library. For each macro it will be defined to either a <span class="emphasis"><em>zero</em></span>
valued expression for when the particular item is not detected, and to a <span class="emphasis"><em>positive</em></span>
value if it is detected. The predef macros fall onto five categories each with
macros of a particular prefix:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="computeroutput"><span class="identifier">BOOST_ARCH_</span></code>for system/CPU
architecture one is compiling for.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">BOOST_COMP_</span></code> for the compiler
one is using.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">BOOST_LANG_</span></code> for language
standards one is compiling against.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">BOOST_LIB_C_</span></code> and <code class="computeroutput"><span class="identifier">BOOST_LIB_STD_</span></code> for the C and C++ standard
library in use.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">BOOST_OS_</span></code> for the operating
system we are compiling to.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">BOOST_PLAT_</span></code> for platforms
on top of operating system or compilers.
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">BOOST_ENDIAN_</span></code> for endianness
of the os and architecture combination.
</li>
</ul></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
The detected definitions are for the configuration one is targeting during
the compile. In particular in a cross-compile this means the target system,
and not the host system.
</p></td></tr>
</table></div>
<p>
One uses the individual definitions to compare against specific versions by
comparing against the <code class="computeroutput"><span class="identifier">BOOST_VERSION_NUMBER</span></code>
macro. For example, to make a choice based on the version of the GCC C++ compiler
one would:
</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">predef</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">BOOST_COMP_GNUC</span> <span class="special">&gt;=</span> <span class="identifier">BOOST_VERSION_NUMBER</span><span class="special">(</span><span class="number">4</span><span class="special">,</span><span class="number">0</span><span class="special">,</span><span class="number">0</span><span class="special">))</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"GCC compiler is at least version 4.0.0"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">else</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"GCC compiler is at older than version 4.0.0, or not a GCC compiler"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
As you might notice above the <code class="computeroutput"><span class="keyword">else</span></code>
clause also covers the case where the particular compiler is not detected.
But one can make the test also test for the detection. All predef definitions
are defined as a zero (0) expression when not detected. Hence one could use
the detection with a natural single condition. For example:
</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">predef</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">BOOST_COMP_GNUC</span><span class="special">)</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"This is GNU GCC!"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">else</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Not GNU GCC."</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
And since the predef's are preprocessor definitions the same is possible from
the preprocessor:
</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">predef</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#if</span> <span class="identifier">BOOST_COMP_GNUC</span>
<span class="preprocessor">#if</span> <span class="identifier">BOOST_COMP_GNUC</span> <span class="special">&gt;=</span> <span class="identifier">BOOST_VERSION_NUMBER</span><span class="special">(</span><span class="number">4</span><span class="special">,</span><span class="number">0</span><span class="special">,</span><span class="number">0</span><span class="special">)</span>
<span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span> <span class="identifier">the_compiler</span> <span class="special">=</span> <span class="string">"GNU GCC, of at least version 4."</span>
<span class="preprocessor">#else</span>
<span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span> <span class="identifier">the_compiler</span> <span class="special">=</span> <span class="string">"GNU GCC, less than version 4."</span>
<span class="preprocessor">#endif</span>
<span class="preprocessor">#else</span>
<span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span> <span class="identifier">the_compiler</span> <span class="special">=</span> <span class="string">"Not GNU GCC."</span>
<span class="preprocessor">#endif</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">the_compiler</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
In addition, for each version macro defined there is an <code class="computeroutput"><span class="special">*</span><span class="identifier">_AVAILABLE</span></code> macro defined only when the particular
aspect is detected. I.e. a definition equivalent to:
</p>
<pre class="programlisting"><span class="preprocessor">#if</span> <span class="identifier">BOOST_PREDEF_ABC</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_PREDEF_ABC_AVAILABLE</span>
<span class="preprocessor">#endif</span>
</pre>
<p>
Also for each aspect there is a macro defined with a descriptive name of what
the detection is.
</p>
<h4>
<a name="predef.using_the_predefs.h0"></a>
<span class="phrase"><a name="predef.using_the_predefs.the_emulated_macros"></a></span><a class="link" href="using_the_predefs.html#predef.using_the_predefs.the_emulated_macros">The
<code class="computeroutput"><span class="special">*</span><span class="identifier">_EMULATED</span></code>
macros</a>
</h4>
<p>
Predef definitions are guaranteed to be uniquely detected within one category.
But there are contexts under which multiple underlying detections are possible.
The well known example of this is detection of GCC and MSVC compilers which
are commonly emulated by other compilers by defining the same base macros.
To account for this detection headers are allowed to define <code class="computeroutput"><span class="special">*</span><span class="identifier">_EMULATED</span></code> predefs when this situation is
detected. The emulated predefs will be set to the version number of the detection
instead of the regular predef macro for that detection. For example MSVC will
set <code class="computeroutput"><span class="identifier">BOOST_COMP_MSVC_EMULATED</span></code>
but not set <code class="computeroutput"><span class="identifier">BOOST_COM_MSVC</span></code>,
and it will also set <code class="computeroutput"><span class="identifier">BOOST_COMP_MSVC_AVAILABLE</span></code>.
</p>
<h4>
<a name="predef.using_the_predefs.h1"></a>
<span class="phrase"><a name="predef.using_the_predefs.using_the_boost_version_number_m"></a></span><a class="link" href="using_the_predefs.html#predef.using_the_predefs.using_the_boost_version_number_m">Using the
<code class="computeroutput"><span class="identifier">BOOST_VERSION_NUMBER</span></code> macro</a>
</h4>
<p>
All the predefs are defined to be a use of the <code class="computeroutput"><span class="identifier">BOOST_VERSION_NUMBER</span></code>
macro. The macro takes individual major, minor, and patch value expressions:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_VERSION_NUMBER</span><span class="special">(</span> <span class="identifier">major</span><span class="special">,</span> <span class="identifier">minor</span><span class="special">,</span> <span class="identifier">patch</span> <span class="special">)</span> <span class="special">...</span>
</pre>
<p>
The arguments are:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
Major version number, as a constant value expression in the range [0,99].
</li>
<li class="listitem">
Minor version number, as a constant value expression in the range [0,99].
</li>
<li class="listitem">
Patch-level version number, as a constant value expression in the range
[0,99999].
</li>
</ol></div>
<p>
The ranges for each are "enforced" by the use of a modulo ("%"),
i.e. truncation, as opposed to a clamp. And hence this means that the limits
are enforced only enough to keep from having out-of-range problems. But not
enough to prevent other kinds of problems. Like exceeding the range and getting
false detections, or non-detections. It is up to the individual predefs to
ensure correct usage beyond the range guarantee.
</p>
<p>
The values for the arguments can be any preprocessor valid constant value expression.
Only constant value arithmetic is used in the definition of the <code class="computeroutput"><span class="identifier">BOOST_VERSION_NUMBER</span></code> macro and in any of
the other predef macros. This means that any allowed base is possible, i.e.
binary, octal, decimal, and hexadecimal. For example:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">MY_APPLICATION_VERSION_NUMBER</span> <span class="identifier">BOOST_VERSION_NUMBER</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="number">0xA</span><span class="special">,</span><span class="number">015</span><span class="special">)</span>
</pre>
<p>
Is equivalent to:
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">MY_APPLICATION_VERSION_NUMBER</span> <span class="identifier">BOOST_VERSION_NUMBER</span><span class="special">(</span><span class="number">2</span><span class="special">,</span><span class="number">10</span><span class="special">,</span><span class="number">13</span><span class="special">)</span>
</pre>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2005, 2008-2014 Rene Rivera<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
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