boost_predef/doc/html/predef/using_the_predefs.html

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<div class="section predef_using_the_predefs">
<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" type="disc">
<li class="listitem">
          <code class="computeroutput"><span class="identifier">BOOST_ARCHITECTURE_</span></code>for
          system/CPU architecture one is compiling for.
        </li>
<li class="listitem">
          <code class="computeroutput"><span class="identifier">BOOST_CXX_</span></code> for the compiler
          one is using.
        </li>
<li class="listitem">
          <code class="computeroutput"><span class="identifier">BOOST_LANGUAGE_</span></code> for language
          standards one is compiling against.
        </li>
<li class="listitem">
          <code class="computeroutput"><span class="identifier">BOOST_LIBC_</span></code> and <code class="computeroutput"><span class="identifier">BOOST_LIBSTD_</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>
</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_CXX_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 specifically 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_CXX_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_CXX_GNUC</span>
  <span class="preprocessor">#if</span> <span class="identifier">BOOST_CXX_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>
<h4>
<a name="predef.using_the_predefs.h0"></a>
      <span><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 Rene Rivera<br>Copyright &#169; 2008-2011 Redshift
      Software Inc<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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