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<a name="typeof.tuto"></a> Tutorial</h2></div></div></div>
<p>
To start using typeof include the typeof header:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
To deduce the type of an expression at compile time use the <code class="computeroutput"><span class="identifier">BOOST_TYPEOF</span></code>
macro:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">ex1</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">BOOST_TYPEOF</span><span class="special">(</span><span class="number">1</span> <span class="special">+</span> <span class="number">0.5</span><span class="special">)</span> <span class="identifier">type</span><span class="special">;</span>
<span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">type</span><span class="special">,</span> <span class="keyword">double</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span>
<span class="special">}</span>
</pre>
<p>
In the dependent context use <code class="computeroutput"><span class="identifier">BOOST_TYPEOF_TPL</span></code>
instead of <code class="computeroutput"><span class="identifier">BOOST_TYPEOF</span></code>:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">ex2</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span>
<span class="identifier">BOOST_TYPEOF_TPL</span><span class="special">(</span><span class="identifier">T</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">U</span><span class="special">())</span> <span class="identifier">add</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">t</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">U</span><span class="special">&</span> <span class="identifier">u</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">t</span> <span class="special">+</span> <span class="identifier">u</span><span class="special">;</span>
<span class="special">};</span>
<span class="keyword">typedef</span> <span class="identifier">BOOST_TYPEOF</span><span class="special">(</span><span class="identifier">add</span><span class="special">(</span><span class="char">'a'</span><span class="special">,</span> <span class="number">1.5</span><span class="special">))</span> <span class="identifier">type</span><span class="special">;</span>
<span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">type</span><span class="special">,</span> <span class="keyword">double</span><span class="special">>::</span><span class="identifier">value</span><span class="special">));</span>
<span class="special">}</span>
</pre>
<p>
The above examples are possible because the Typeof Library knows about primitive
types, such as <code class="computeroutput"><span class="keyword">int</span></code>, <code class="computeroutput"><span class="keyword">double</span></code>, <code class="computeroutput"><span class="keyword">char</span></code>,
etc. The Typeof Library also knows about most types and templates defined by
the Standard C++ Library, but the appropriate headers need to be included to
take advantage of this:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">utility</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">namespace</span> <span class="identifier">ex3</span>
<span class="special">{</span>
<span class="identifier">BOOST_AUTO</span><span class="special">(</span><span class="identifier">p</span><span class="special">,</span> <span class="identifier">make_pair</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="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">BOOST_TYPEOF</span><span class="special">(</span><span class="identifier">p</span><span class="special">),</span> <span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special">));</span>
<span class="special">}</span>
</pre>
<p>
Here <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">utility</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
includes <code class="computeroutput"><span class="special"><</span><span class="identifier">utility</span><span class="special">></span></code> and contains knowledge about templates
defined there. This naming convention applies in general, for example to let
the Typeof Library handle <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code>,
include <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>,
etc.
</p>
<p>
To deduce the type of a variable from the expression, this variable is initialized
with, use the <code class="computeroutput"><span class="identifier">BOOST_AUTO</span></code> macro
(or <code class="computeroutput"><span class="identifier">BOOST_AUTO_TPL</span></code> in a dependent
context:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">typeof</span><span class="special">/</span><span class="identifier">std</span><span class="special">/</span><span class="identifier">string</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">namespace</span> <span class="identifier">ex4</span>
<span class="special">{</span>
<span class="identifier">BOOST_AUTO</span><span class="special">(</span><span class="identifier">p</span><span class="special">,</span> <span class="keyword">new</span> <span class="keyword">int</span><span class="special">[</span><span class="number">20</span><span class="special">]);</span>
<span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">BOOST_TYPEOF</span><span class="special">(</span><span class="identifier">p</span><span class="special">),</span> <span class="keyword">int</span><span class="special">*>::</span><span class="identifier">value</span><span class="special">));</span>
<span class="special">}</span>
</pre>
<p>
Both <code class="computeroutput"><span class="identifier">BOOST_TYPEOF</span></code> and <code class="computeroutput"><span class="identifier">BOOST_AUTO</span></code> strip top-level qualifiers. Therefore,
to allocate for example a reference, it has to be specified explicitly:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">ex5</span>
<span class="special">{</span>
<span class="identifier">string</span><span class="special">&</span> <span class="identifier">hello</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="identifier">string</span> <span class="identifier">s</span> <span class="special">=</span> <span class="string">"hello"</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">s</span><span class="special">;</span>
<span class="special">}</span>
<span class="identifier">BOOST_AUTO</span><span class="special">(&</span><span class="identifier">s</span><span class="special">,</span> <span class="identifier">hello</span><span class="special">());</span>
<span class="special">}</span>
</pre>
<p>
To better understand this syntax, note that this gets expanded into:
</p>
<pre class="programlisting"><span class="identifier">BOOST_TYPEOF</span><span class="special">(</span><span class="identifier">hello</span><span class="special">())</span> <span class="special">&</span><span class="identifier">s</span> <span class="special">=</span> <span class="identifier">hello</span><span class="special">();</span>
</pre>
<p>
If your define your own type, the Typeof Library cannot handle it unless you
let it know about this type. You tell the Typeof Library about a type (or template)
by the means of "registering" this type/template.
</p>
<p>
Any source or header file where types/templates are registered has to contain
the following line before any registration is done:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="identifier">BOOST_TYPEOF_INCREMENT_REGISTRATION_GROUP</span><span class="special">()</span>
</pre>
<p>
After this a type can be registered:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">ex6</span>
<span class="special">{</span>
<span class="keyword">struct</span> <span class="identifier">MyType</span>
<span class="special">{};</span>
<span class="special">}</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TYPE</span><span class="special">(</span><span class="identifier">ex6</span><span class="special">::</span><span class="identifier">MyType</span><span class="special">)</span>
</pre>
<p>
The registration must be done from the context of global namespace; fully qualified
type name has to be used.
</p>
<p>
Any number of types can be registered in one file, each on a separate line.
</p>
<p>
Once your type is registered, the Typeof Library can handle it in any context:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">ex6</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">BOOST_TYPEOF</span><span class="special">(</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">MyType</span><span class="special">()))</span> <span class="identifier">type</span><span class="special">;</span>
<span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">MyType</span><span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special">));</span>
<span class="special">}</span>
</pre>
<p>
A template is registered by specifying its fully qualified name, and describing
its parameters. In the simplest case, when all parameters are type parameters,
only their number needs to be specified:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">ex7</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">MyTemplate</span>
<span class="special">{};</span>
<span class="special">}</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span><span class="special">(</span><span class="identifier">ex7</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">namespace</span> <span class="identifier">ex7</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">BOOST_TYPEOF</span><span class="special">(</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">MyTemplate</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">ex6</span><span class="special">::</span><span class="identifier">MyType</span><span class="special">>()))</span> <span class="identifier">type</span><span class="special">;</span>
<span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">type</span><span class="special">,</span>
<span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">MyTemplate</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">ex6</span><span class="special">::</span><span class="identifier">MyType</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>
</pre>
<p>
When a template has integral template parameters, all parameters need to be
described in the preprocessor sequence:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">ex8</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">n</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">MyTemplate</span>
<span class="special">{};</span>
<span class="special">}</span>
<span class="identifier">BOOST_TYPEOF_REGISTER_TEMPLATE</span><span class="special">(</span><span class="identifier">ex8</span><span class="special">::</span><span class="identifier">MyTemplate</span><span class="special">,</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="keyword">namespace</span> <span class="identifier">ex8</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">BOOST_TYPEOF</span><span class="special">(</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="identifier">MyTemplate</span><span class="special"><</span><span class="identifier">ex7</span><span class="special">::</span><span class="identifier">MyTemplate</span><span class="special"><</span><span class="identifier">ex6</span><span class="special">::</span><span class="identifier">MyType</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>,</span> <span class="number">0</span><span class="special">>()))</span> <span class="identifier">type</span><span class="special">;</span>
<span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">((</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">type</span><span class="special">,</span>
<span class="identifier">pair</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">MyTemplate</span><span class="special"><</span><span class="identifier">ex7</span><span class="special">::</span><span class="identifier">MyTemplate</span><span class="special"><</span><span class="identifier">ex6</span><span class="special">::</span><span class="identifier">MyType</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>,</span> <span class="number">0</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>
</pre>
<p>
Please see the reference for more details.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2004, 2005 Arkadiy Vertleyb, Peder Holt<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>
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