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302 lines
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<title>checked_result<R></title>
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<td valign="top"><img href="index.html" height="164px" src="pre-boost.jpg" alt="Library Documentation Index"></td>
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<td><h2>Safe Numerics</h2></td>
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<div class="titlepage"><div><div><h3 class="title">
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<a name="safenumerics.checked_result"></a>checked_result<R></h3></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="checked_result.html#idm130202581904">Description</a></span></dt>
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<dt><span class="section"><a href="checked_result.html#idm130202564160">Notation</a></span></dt>
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<dt><span class="section"><a href="checked_result.html#idm130202541568">Template Parameters</a></span></dt>
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<dt><span class="section"><a href="checked_result.html#idm130202533440">Model of</a></span></dt>
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<dt><span class="section"><a href="checked_result.html#idm130202531728">Valid Expressions</a></span></dt>
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<dt><span class="section"><a href="checked_result.html#idm130202483424">Example of use</a></span></dt>
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<dt><span class="section"><a href="checked_result.html#idm130202416320">See Also</a></span></dt>
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<dt><span class="section"><a href="checked_result.html#idm130202414608">Header</a></span></dt>
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</dl></div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="idm130202581904"></a>Description</h4></div></div></div>
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<p>checked_result is a special kind of variant class designed to hold
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the result of some operation. It can hold either the result of the
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operation or information on why the operation failed to produce a valid
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result. It is similar to other types proposed for and/or included to the
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C++ standard library or Boost such as<code class="computeroutput"> expected</code>,
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<code class="computeroutput">variant</code>, <code class="computeroutput">optional</code> and <code class="computeroutput">outcome</code>. In
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some circumstances it may be referred to as a "monad".</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem"><p>All instances of <code class="computeroutput">checked_result<R></code> are
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immutable. That is, once constructed, they cannot be altered.</p></li>
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<li class="listitem"><p>There is no default constructor.</p></li>
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<li class="listitem"><p><code class="computeroutput">checked_result<R></code> is never empty.</p></li>
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<li class="listitem"><p>Binary operations supported by type R are guaranteed to be
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supported by checked_result<R>.</p></li>
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<li class="listitem"><p>Binary operations can be invoked on a pair of
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<code class="computeroutput">checked_result<R></code> instances if and only if the
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underlying type (R) is identical for both instances. They will
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return a value of type <code class="computeroutput">checked_result<R></code>.</p></li>
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<li class="listitem"><p>Unary operations can be invoked on
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<code class="computeroutput">checked_result<R></code> instances. They will return a
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value of type <code class="computeroutput">checked_result<R></code>.</p></li>
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<li class="listitem"><p>Comparison operations will return a
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<code class="computeroutput">boost::logic::tribool</code>. Other binary operations will a
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value of the same type as the arguments.</p></li>
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</ul></div>
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<p>Think of <code class="computeroutput">checked<R></code> as an "extended" version of R
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which can hold all the values that R can hold in addition other "special
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values". For example, consider checked<int>.</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="idm130202564160"></a>Notation</h4></div></div></div>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col align="left">
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<col align="left">
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</colgroup>
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<thead><tr>
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<th align="left">Symbol</th>
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<th align="left">Description</th>
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</tr></thead>
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<tbody>
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<tr>
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<td align="left"><code class="computeroutput">R</code></td>
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<td align="left">Underlying type</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">r</code></td>
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<td align="left">An instance of type R</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">c, c1, c2</code></td>
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<td align="left">an instance of checked_result<R></td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">t</code></td>
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<td align="left">an instance of checked_result<T> for some type T not
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necessarily the same as R</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">e</code></td>
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<td align="left">An instance of type <a class="link" href="exception.html#safe_numerics.safe_numerics_error" title="enum class safe_numerics_error"><code class="computeroutput">safe_numerics_error</code></a>
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</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">msg</code></td>
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<td align="left">An instance of type const char *</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">OS</code></td>
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<td align="left">A type convertible to <a class="link" href="exception.html#safe_numerics.safe_numerics_error" title="enum class safe_numerics_error"><code class="computeroutput">std::basic_ostream</code></a>
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</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">os</code></td>
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<td align="left">An instance of type convertible to <a class="link" href="exception.html#safe_numerics.safe_numerics_error" title="enum class safe_numerics_error"><code class="computeroutput">std::basic_ostream</code></a>
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</td>
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</tr>
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</tbody>
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</table></div>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="idm130202541568"></a>Template Parameters</h4></div></div></div>
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<p>R must model the type requirements of <a class="link" href="numeric.html" title="Numeric<T>">Numeric</a></p>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col align="left">
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<col align="left">
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</colgroup>
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<thead><tr>
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<th align="left">Parameter</th>
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<th align="left">Description</th>
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</tr></thead>
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<tbody><tr>
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<td align="left"><code class="computeroutput">R</code></td>
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<td align="left">Underlying type</td>
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</tr></tbody>
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</table></div>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="idm130202533440"></a>Model of</h4></div></div></div>
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<p><a class="link" href="numeric.html" title="Numeric<T>">Numeric</a></p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="idm130202531728"></a>Valid Expressions</h4></div></div></div>
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<p>All expressions are <code class="computeroutput">constexpr</code>.</p>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col align="left">
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<col align="left">
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<col align="left">
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</colgroup>
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<thead><tr>
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<th align="left">Expression</th>
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<th align="left">Return Type</th>
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<th align="left">Semantics</th>
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</tr></thead>
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<tbody>
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<tr>
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<td align="left"><code class="computeroutput">checked_result(r)</code></td>
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<td align="left"><code class="computeroutput">checked_result<R></code></td>
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<td align="left">constructor with valid instance of R</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">checked_result<R>(t)</code></td>
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<td align="left"><code class="computeroutput">checked_result<R></code></td>
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<td align="left">constructor with <code class="computeroutput">checked_result<T></code>
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where T is not R. T must be convertible to R.</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">checked_result(e, msg)</code></td>
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<td align="left"><code class="computeroutput">checked_result<R></code></td>
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<td align="left">constructor with error information</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">static_cast<R>(c)</code></td>
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<td align="left">R</td>
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<td align="left">extract wrapped value - compile time error if not
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possible</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">static_cast<<code class="computeroutput"><a class="link" href="exception.html#safe_numerics.safe_numerics_error" title="enum class safe_numerics_error"><code class="computeroutput">safe_numerics_error</code></a></code>>(c)</code></td>
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<td align="left"><a class="link" href="exception.html#safe_numerics.safe_numerics_error" title="enum class safe_numerics_error"><code class="computeroutput">safe_numerics_error</code></a></td>
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<td align="left">extract wrapped value - may return <a class="link" href="exception.html#safe_numerics.safe_numerics_error" title="enum class safe_numerics_error"><code class="computeroutput">safe_numerics_error</code></a><code class="computeroutput">::success</code>
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if there is no error</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">static_cast<const char *>(c)</code></td>
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<td align="left"><code class="computeroutput">const char *</code></td>
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<td align="left">returns pointer to the included error message</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">c.exception()</code></td>
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<td align="left"><code class="computeroutput">bool</code></td>
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<td align="left">true if <code class="computeroutput">checked_result</code> contains an error
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condition.</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput"><table border="0" summary="Simple list" class="simplelist">
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<tr><td>c1 < c2</td></tr>
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<tr><td>c1 >= c2</td></tr>
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<tr><td>c1 > c2</td></tr>
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<tr><td>c1 <= c2</td></tr>
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<tr><td>c1 == c2</td></tr>
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<tr><td>c1 != c2</td></tr>
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</table></code></td>
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<td align="left"><code class="computeroutput">boost::logic::tribool</code></td>
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<td align="left">compare the wrapped values of two checked_result
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instances. If the values are such that the result of such a
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comparison cannot be reasonably defined, The result of the
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comparison is
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<code class="computeroutput">boost::logic::tribool::indeterminant</code>.</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput"><table border="0" summary="Simple list" class="simplelist">
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<tr><td>c1 + c2</td></tr>
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<tr><td>c1 - c2</td></tr>
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<tr><td>c1 * c2</td></tr>
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<tr><td>c1 / c2</td></tr>
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<tr><td>c1 % c2</td></tr>
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<tr><td>c1 | c2</td></tr>
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<tr><td>c1 & c2</td></tr>
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<tr><td>c1 ^ c2</td></tr>
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<tr><td>c1 << c2</td></tr>
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<tr><td>c1 >> c2</td></tr>
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</table></code></td>
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<td align="left"><code class="computeroutput">checked_result<R></code></td>
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<td align="left">returns a new instance of
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<code class="computeroutput">checked_result<R>.</code>
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</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput"><table border="0" summary="Simple list" class="simplelist"><tr><td>os << c</td></tr></table></code></td>
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<td align="left"><code class="computeroutput">OS</code></td>
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<td align="left">writes result to output stream. If the result is an error
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it writes the string corresponding to the error message.
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Otherwise, it writes the numeric value resulting from the
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operation. Returns reference to output stream.</td>
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</tr>
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</tbody>
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</table></div>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="idm130202483424"></a>Example of use</h4></div></div></div>
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<pre class="programlisting"><span class="comment">// Copyright (c) 2018 Robert Ramey</span>
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<span class="comment">//</span>
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<span class="comment">// Distributed under the Boost Software License, Version 1.0. (See</span>
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<span class="comment">// accompanying file LICENSE_1_0.txt or copy at</span>
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<span class="comment">// http://www.boost.org/LICENSE_1_0.txt)</span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">safe_numerics</span><span class="special">/</span><span class="identifier">checked_result</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">safe_numerics</span><span class="special">/</span><span class="identifier">checked_result_operations</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</span><span class="special">{</span>
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<span class="keyword">using</span> <span class="identifier">ext_uint</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">safe_numerics</span><span class="special">::</span><span class="identifier">checked_result</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">int</span><span class="special">></span><span class="special">;</span>
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<span class="keyword">const</span> <span class="identifier">ext_uint</span> <span class="identifier">x</span><span class="special">{</span><span class="number">4</span><span class="special">}</span><span class="special">;</span>
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<span class="keyword">const</span> <span class="identifier">ext_uint</span> <span class="identifier">y</span><span class="special">{</span><span class="number">3</span><span class="special">}</span><span class="special">;</span>
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<span class="comment">// operation is a success!</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"success! x - y = "</span> <span class="special"><<</span> <span class="identifier">x</span> <span class="special">-</span> <span class="identifier">y</span><span class="special">;</span>
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<span class="comment">// subtraction would result in -1, and invalid result for an unsigned value</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"problem: y - x = "</span> <span class="special"><<</span> <span class="identifier">y</span> <span class="special">-</span> <span class="identifier">x</span><span class="special">;</span>
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<span class="keyword">const</span> <span class="identifier">ext_uint</span> <span class="identifier">z</span> <span class="special">=</span> <span class="identifier">y</span> <span class="special">-</span> <span class="identifier">x</span><span class="special">;</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"z = "</span> <span class="special"><<</span> <span class="identifier">z</span><span class="special">;</span>
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<span class="comment">// sum of two negative overflows is a negative overflow.</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"z + z"</span> <span class="special"><<</span> <span class="identifier">z</span> <span class="special">+</span> <span class="identifier">z</span><span class="special">;</span>
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<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
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<span class="special">}</span>
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</pre>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="idm130202416320"></a>See Also</h4></div></div></div>
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<p><a class="link" href="exception_policy.html" title="ExceptionPolicy<EP>">ExceptionPolicy</a></p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="idm130202414608"></a>Header</h4></div></div></div>
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<p><a href="../include/checked_result.hpp" target="_top"><code class="computeroutput">#include
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<boost/numeric/safe_numerics/checked_result.hpp></code></a></p>
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<p><a href="../include/checked_result_operations.hpp" target="_top"><code class="computeroutput">#include
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<boost/numeric/safe_numerics/checked_result_operations.hpp>
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</code></a></p>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2012-2018 Robert Ramey<p><a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">Subject to Boost
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Software License</a></p>
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</div></td>
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</tr></table>
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<hr>
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