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Brian Silverman

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<title>Integer Masks</title>

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<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>

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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>

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<a name="boost_integer.mask"></a><a class="link" href="mask.html" title="Integer Masks">Integer Masks</a>

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</h2></div></div></div>

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<dt><span class="section"><a href="mask.html#boost_integer.mask.overview">Overview</a></span></dt>

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<dt><span class="section"><a href="mask.html#boost_integer.mask.synopsis">Synopsis</a></span></dt>

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<dt><span class="section"><a href="mask.html#boost_integer.mask.single_bit_mask_class_template">Single

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Bit-Mask Class Template</a></span></dt>

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<dt><span class="section"><a href="mask.html#boost_integer.mask.group_bit_mask_class_template">Group

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Bit-Mask Class Template</a></span></dt>

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<dt><span class="section"><a href="mask.html#boost_integer.mask.implementation_notes">Implementation

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Notes</a></span></dt>

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<dt><span class="section"><a href="mask.html#boost_integer.mask.example">Example</a></span></dt>

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<dt><span class="section"><a href="mask.html#boost_integer.mask.demonstration_program">Demonstration

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Program</a></span></dt>

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<dt><span class="section"><a href="mask.html#boost_integer.mask.rationale">Rationale</a></span></dt>

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<dt><span class="section"><a href="mask.html#boost_integer.mask.credits">Credits</a></span></dt>

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<a name="boost_integer.mask.overview"></a><a class="link" href="mask.html#boost_integer.mask.overview" title="Overview">Overview</a>

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</h3></div></div></div>

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<p>

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The class templates in <a href="../../../../../boost/integer/integer_mask.hpp" target="_top"><boost/integer/integer_mask.hpp></a>

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provide bit masks for a certain bit position or a contiguous-bit pack of

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a certain size. The types of the masking constants come from the <a class="link" href="integer.html" title="Integer Type Selection">integer

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type selection templates</a> header.

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<a name="boost_integer.mask.synopsis"></a><a class="link" href="mask.html#boost_integer.mask.synopsis" title="Synopsis">Synopsis</a>

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</h3></div></div></div>

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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstddef</span><span class="special">></span> <span class="comment">// for std::size_t</span>

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<span class="keyword">namespace</span> <span class="identifier">boost</span>

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<span class="keyword">template</span> <span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">Bit</span><span class="special">></span>

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<span class="keyword">struct</span> <span class="identifier">high_bit_mask_t</span>

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<span class="keyword">typedef</span> <span class="emphasis"><em>implementation-defined-type</em></span> <span class="identifier">least</span><span class="special">;</span>

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<span class="keyword">typedef</span> <span class="emphasis"><em>implementation-defined-type</em></span> <span class="identifier">fast</span><span class="special">;</span>

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<span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">least</span> <span class="identifier">high_bit</span> <span class="special">=</span> <span class="emphasis"><em>implementation-defined</em></span><span class="special">;</span>

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<span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">fast</span> <span class="identifier">high_bit_fast</span> <span class="special">=</span> <span class="emphasis"><em>implementation-defined</em></span><span class="special">;</span>

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<span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bit_position</span> <span class="special">=</span> <span class="identifier">Bit</span><span class="special">;</span>

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<span class="keyword">template</span> <span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">Bits</span><span class="special">></span>

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<span class="keyword">struct</span> <span class="identifier">low_bits_mask_t</span>

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<span class="keyword">typedef</span> <span class="emphasis"><em>implementation-defined-type</em></span> <span class="identifier">least</span><span class="special">;</span>

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<span class="keyword">typedef</span> <span class="emphasis"><em>implementation-defined-type</em></span> <span class="identifier">fast</span><span class="special">;</span>

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<span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">least</span> <span class="identifier">sig_bits</span> <span class="special">=</span> <span class="emphasis"><em>implementation-defined</em></span><span class="special">;</span>

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<span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">fast</span> <span class="identifier">sig_bits_fast</span> <span class="special">=</span> <span class="emphasis"><em>implementation-defined</em></span><span class="special">;</span>

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<span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bit_count</span> <span class="special">=</span> <span class="identifier">Bits</span><span class="special">;</span>

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<span class="comment">// Specializations for low_bits_mask_t exist for certain bit counts.</span>

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<span class="special">}</span> <span class="comment">// namespace boost</span>

</pre>

</div>

<a name="boost_integer.mask.single_bit_mask_class_template"></a><a class="link" href="mask.html#boost_integer.mask.single_bit_mask_class_template" title="Single Bit-Mask Class Template">Single

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Bit-Mask Class Template</a>

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</h3></div></div></div>

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<p>

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The <code class="literal">boost::high_bit_mask_t</code> class template provides constants

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for bit masks representing the bit at a certain position. The masks are equivalent

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to the value 2<sup>Bit</sup>, where <code class="literal">Bit</code> is the template parameter.

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The bit position must be a nonnegative number from zero to <span class="emphasis"><em>Max</em></span>,

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where Max is one less than the number of bits supported by the largest unsigned

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built-in integral type. The following table describes the members of an instantiation

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of <code class="literal">high_bit_mask_t</code>.

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</p>

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<a name="boost_integer.mask.single_bit_mask_class_template.members_of_the__boost__high_bit_mask_t__class_template"></a><p class="title"><b>Table 2. Members of the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">high_bit_mask_t</span></code>

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Class Template</b></p>

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<div class="table-contents"><table class="table" summary="Members of the boost::high_bit_mask_t

Class Template">

<col>

<col>

</colgroup>

<th>

<p>

Member

</p>

</th>

<th>

<p>

Meaning

</p>

</th>

</tr></thead>

<tbody>

<tr>

<td>

<p>

<code class="literal">least</code>

</p>

</td>

<td>

<p>

The smallest, unsigned, built-in type that supports the given bit

position.

</p>

</td>

</tr>

<tr>

<td>

<p>

</p>

</td>

<td>

<p>

The easiest-to-manipulate analog of <code class="literal">least</code>.

</p>

</td>

</tr>

<tr>

<td>

<p>

</p>

</td>

<td>

<p>

A <code class="literal">least</code> constant of the value 2<sup>Bit</sup>.

</p>

</td>

</tr>

<tr>

<td>

<p>

</p>

</td>

<td>

<p>

A <code class="literal">fast</code> analog of <code class="literal">high_bit</code>.

</p>

</td>

</tr>

<tr>

<td>

<p>

<code class="literal">bit_position</code>

</p>

</td>

<td>

<p>

The value of the template parameter, in case its needed from a

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renamed instantiation of the class template.

</p>

</td>

</tr>

</tbody>

</table></div>

</div>

</div>

<a name="boost_integer.mask.group_bit_mask_class_template"></a><a class="link" href="mask.html#boost_integer.mask.group_bit_mask_class_template" title="Group Bit-Mask Class Template">Group

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Bit-Mask Class Template</a>

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</h3></div></div></div>

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<p>

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The <code class="literal">boost::low_bits_mask_t</code> class template provides constants

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for bit masks equivalent to the value (2<sup>Bits</sup> - 1), where <code class="literal">Bits</code>

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is the template parameter. The parameter <code class="literal">Bits</code> must be

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a non-negative integer from zero to <span class="emphasis"><em>Max</em></span>, where Max is

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the number of bits supported by the largest, unsigned, built-in integral

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type. The following table describes the members of <code class="literal">low_bits_mask_t</code>.

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</p>

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<a name="boost_integer.mask.group_bit_mask_class_template.members_of_the___boost__low_bits_mask_t__class_template"></a><p class="title"><b>Table 3. Members of the <code class="literal">boost::low_bits_mask_t</code> Class Template</b></p>

<col>

<col>

</colgroup>

<th>

<p>

Member

</p>

</th>

<th>

<p>

Meaning

</p>

</th>

</tr></thead>

<tbody>

<tr>

<td>

<p>

<code class="literal">least</code>

</p>

</td>

<td>

<p>

The smallest, unsigned built-in type that supports the given bit

count.

</p>

</td>

</tr>

<tr>

<td>

<p>

</p>

</td>

<td>

<p>

The easiest-to-manipulate analog of <code class="literal">least</code>.

</p>

</td>

</tr>

<tr>

<td>

<p>

</p>

</td>

<td>

<p>

A <code class="literal">least</code> constant of the desired bit-masking

value.

</p>

</td>

</tr>

<tr>

<td>

<p>

</p>

</td>

<td>

<p>

A <code class="literal">fast</code> analog of <code class="literal">sig_bits</code>.

</p>

</td>

</tr>

<tr>

<td>

<p>

<code class="literal">bit_count</code>

</p>

</td>

<td>

<p>

The value of the template parameter, in case its needed from a

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renamed instantiation of the class template.

</p>

</td>

</tr>

</tbody>

</table></div>

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<a name="boost_integer.mask.implementation_notes"></a><a class="link" href="mask.html#boost_integer.mask.implementation_notes" title="Implementation Notes">Implementation

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Notes</a>

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</h3></div></div></div>

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<p>

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When <code class="literal">Bits</code> is the exact size of a built-in unsigned type,

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the implementation has to change to prevent undefined behavior. Therefore,

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there are specializations of <code class="literal">low_bits_mask_t</code> at those

bit counts.

</p>

</div>

<a name="boost_integer.mask.example"></a><a class="link" href="mask.html#boost_integer.mask.example" title="Example">Example</a>

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</h3></div></div></div>

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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">integer</span><span class="special">/</span><span class="identifier">integer_mask</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>

<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">high_bit_mask_t</span><span class="special"><</span><span class="number">29</span><span class="special">></span> <span class="identifier">mask1_type</span><span class="special">;</span>

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<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">low_bits_mask_t</span><span class="special"><</span><span class="number">15</span><span class="special">></span> <span class="identifier">mask2_type</span><span class="special">;</span>

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<span class="identifier">mask1_type</span><span class="special">::</span><span class="identifier">least</span> <span class="identifier">my_var1</span><span class="special">;</span>

</pre>

</div>

<a name="boost_integer.mask.demonstration_program"></a><a class="link" href="mask.html#boost_integer.mask.demonstration_program" title="Demonstration Program">Demonstration

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Program</a>

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</h3></div></div></div>

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<p>

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The program <a href="../../../test/integer_mask_test.cpp" target="_top">integer_mask_test.cpp</a>

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is a simplistic demonstration of the results from instantiating various examples

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of the bit mask class templates.