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<section id="safenumerics.checked_result">
  <title>checked_result&lt;R&gt;</title>

  <?dbhtml stop-chunking?>

  <section>
    <title>Description</title>

    <para>checked_result is a special kind of variant class designed to hold
    the result of some operation. It can hold either the result of the
    operation or information on why the operation failed to produce a valid
    result. It is similar to other types proposed for and/or included to the
    C++ standard library or Boost such as<code> expected</code>,
    <code>variant</code>, <code>optional</code> and <code>outcome</code>. In
    some circumstances it may be referred to as a "monad".</para>

    <para><itemizedlist>
        <listitem>
          <para>All instances of <code>checked_result&lt;R&gt;</code> are
          immutable. That is, once constructed, they cannot be altered.</para>
        </listitem>

        <listitem>
          <para>There is no default constructor.</para>
        </listitem>

        <listitem>
          <para><code>checked_result&lt;R&gt;</code> is never empty.</para>
        </listitem>

        <listitem>
          <para>Binary operations supported by type R are guaranteed to be
          supported by checked_result&lt;R&gt;.</para>
        </listitem>

        <listitem>
          <para>Binary operations can be invoked on a pair of
          <code>checked_result&lt;R&gt;</code> instances if and only if the
          underlying type (R) is identical for both instances. They will
          return a value of type <code>checked_result&lt;R&gt;</code>.</para>
        </listitem>

        <listitem>
          <para>Unary operations can be invoked on
          <code>checked_result&lt;R&gt;</code> instances. They will return a
          value of type <code>checked_result&lt;R&gt;</code>.</para>
        </listitem>

        <listitem>
          <para>Comparison operations will return a
          <code>boost::logic::tribool</code>. Other binary operations will a
          value of the same type as the arguments.</para>
        </listitem>
      </itemizedlist></para>

    <para>Think of <code>checked&lt;R&gt;</code> as an "extended" version of R
    which can hold all the values that R can hold in addition other "special
    values". For example, consider checked&lt;int&gt;.</para>
  </section>

  <section>
    <title>Notation</title>

    <informaltable>
      <tgroup cols="2">
        <colspec align="left" colwidth="1*"/>

        <colspec align="left" colwidth="4*"/>

        <thead>
          <row>
            <entry align="left">Symbol</entry>

            <entry align="left">Description</entry>
          </row>
        </thead>

        <tbody>
          <row>
            <entry><code>R</code></entry>

            <entry>Underlying type</entry>
          </row>

          <row>
            <entry><code>r</code></entry>

            <entry>An instance of type R</entry>
          </row>

          <row>
            <entry><code>c, c1, c2</code></entry>

            <entry>an instance of checked_result&lt;R&gt;</entry>
          </row>

          <row>
            <entry><code>t</code></entry>

            <entry>an instance of checked_result&lt;T&gt; for some type T not
            necessarily the same as R</entry>
          </row>

          <row>
            <entry><code>e</code></entry>

            <entry>An instance of type <link
            linkend="safe_numerics.safe_numerics_error"><code>safe_numerics_error</code></link></entry>
          </row>

          <row>
            <entry><code>msg</code></entry>

            <entry>An instance of type const char *</entry>
          </row>

          <row>
            <entry><code>OS</code></entry>

            <entry>A type convertible to <link
            linkend="safe_numerics.safe_numerics_error"><code>std::basic_ostream</code></link></entry>
          </row>

          <row>
            <entry><code>os</code></entry>

            <entry>An instance of type convertible to <link
            linkend="safe_numerics.safe_numerics_error"><code>std::basic_ostream</code></link></entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>
  </section>

  <section>
    <title>Template Parameters</title>

    <para>R must model the type requirements of <link
    linkend="safe_numerics.numeric">Numeric</link></para>

    <informaltable>
      <tgroup cols="2">
        <colspec align="left" colwidth="1*"/>

        <colspec align="left" colwidth="4*"/>

        <thead>
          <row>
            <entry align="left">Parameter</entry>

            <entry>Description</entry>
          </row>
        </thead>

        <tbody>
          <row>
            <entry><code>R</code></entry>

            <entry>Underlying type</entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>
  </section>

  <section>
    <title>Model of</title>

    <para><link linkend="safe_numerics.numeric">Numeric</link></para>
  </section>

  <section>
    <title>Valid Expressions</title>

    <para>All expressions are <code>constexpr</code>.</para>

    <para><informaltable>
        <tgroup cols="3">
          <colspec align="left" colwidth="2*"/>

          <colspec align="left" colwidth="1*"/>

          <colspec align="left" colwidth="3*"/>

          <thead>
            <row>
              <entry align="left">Expression</entry>

              <entry>Return Type</entry>

              <entry>Semantics</entry>
            </row>
          </thead>

          <tbody>
            <row>
              <entry><code>checked_result(r)</code></entry>

              <entry><code>checked_result&lt;R&gt;</code></entry>

              <entry>constructor with valid instance of R</entry>
            </row>

            <row>
              <entry><code>checked_result&lt;R&gt;(t)</code></entry>

              <entry><code>checked_result&lt;R&gt;</code></entry>

              <entry>constructor with <code>checked_result&lt;T&gt;</code>
              where T is not R. T must be convertible to R.</entry>
            </row>

            <row>
              <entry><code>checked_result(e, msg)</code></entry>

              <entry><code>checked_result&lt;R&gt;</code></entry>

              <entry>constructor with error information</entry>
            </row>

            <row>
              <entry><code>static_cast&lt;R&gt;(c)</code></entry>

              <entry>R</entry>

              <entry>extract wrapped value - compile time error if not
              possible</entry>
            </row>

            <row>
              <entry><code>static_cast&lt;<code><link
              linkend="safe_numerics.safe_numerics_error"><code>safe_numerics_error</code></link></code>&gt;(c)</code></entry>

              <entry><link
              linkend="safe_numerics.safe_numerics_error"><code>safe_numerics_error</code></link></entry>

              <entry>extract wrapped value - may return <link
              linkend="safe_numerics.safe_numerics_error"><code>safe_numerics_error</code></link><code>::success</code>
              if there is no error</entry>
            </row>

            <row>
              <entry><code>static_cast&lt;const char *&gt;(c)</code></entry>

              <entry><code>const char *</code></entry>

              <entry>returns pointer to the included error message</entry>
            </row>

            <row>
              <entry><code>c.exception()</code></entry>

              <entry><code>bool</code></entry>

              <entry>true if <code>checked_result</code> contains an error
              condition.</entry>
            </row>

            <row>
              <entry><code><simplelist>
                  <member>c1 &lt; c2</member>

                  <member>c1 &gt;= c2</member>

                  <member>c1 &gt; c2</member>

                  <member>c1 &lt;= c2</member>

                  <member>c1 == c2</member>

                  <member>c1 != c2</member>
                </simplelist></code></entry>

              <entry><code>boost::logic::tribool</code></entry>

              <entry>compare the wrapped values of two checked_result
              instances. If the values are such that the result of such a
              comparison cannot be reasonably defined, The result of the
              comparison is
              <code>boost::logic::tribool::indeterminant</code>.</entry>
            </row>

            <row>
              <entry><code><simplelist>
                  <member>c1 + c2</member>

                  <member>c1 - c2</member>

                  <member>c1 * c2</member>

                  <member>c1 / c2</member>

                  <member>c1 % c2</member>

                  <member>c1 | c2</member>

                  <member>c1 &amp; c2</member>

                  <member>c1 ^ c2</member>

                  <member>c1 &lt;&lt; c2</member>

                  <member>c1 &gt;&gt; c2</member>
                </simplelist></code></entry>

              <entry><code>checked_result&lt;R&gt;</code></entry>

              <entry>returns a new instance of
              <code>checked_result&lt;R&gt;.</code></entry>
            </row>

            <row>
              <entry><code><simplelist>
                  <member>os &lt;&lt; c</member>
                </simplelist></code></entry>

              <entry><code>OS</code></entry>

              <entry>writes result to output stream. If the result is an error
              it writes the string corresponding to the error message.
              Otherwise, it writes the numeric value resulting from the
              operation. Returns reference to output stream.</entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable></para>
  </section>

  <section>
    <title>Example of use</title>

    <programlisting><xi:include href="../../example/example20.cpp"
        parse="text" xmlns:xi="http://www.w3.org/2001/XInclude"/></programlisting>
  </section>

  <section>
    <title>See Also</title>

    <para><link
    linkend="safe_numerics.exception_policy">ExceptionPolicy</link></para>
  </section>

  <section>
    <title>Header</title>

    <para><ulink url="../include/checked_result.hpp"><code>#include
    &lt;boost/numeric/safe_numerics/checked_result.hpp&gt;</code></ulink></para>

    <para><ulink url="../include/checked_result_operations.hpp"><code>#include
    &lt;boost/numeric/safe_numerics/checked_result_operations.hpp&gt;
    </code></ulink></para>
  </section>
</section>
