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<section id="safe_numerics.pending_issues">
  <title>Pending Issues</title>

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  <para>The library is under development. There are a number of issues still
  pending.</para>

  <section>
    <title><code>safe_base</code> Only Works for Scalar Types</title>

    <para>The following is paraphrased from an issue raised by Andrzej
    Krzemieński as a <ulink
    url="https://github.com/robertramey/safe_numerics/issues/44">github
    issue</ulink>. It touches upon fundamental ideas behind the library and
    how these ideas as the implementation of the library collided with
    reality.</para>

    <para><quote>In the current implementation safe&lt;T&gt; will only work
    with T being a C++ scalar type. Therefore making a general type
    requirements that say what operations are allowed is superfluous, and
    confusing (because it implies that safe&lt;&gt; is more
    generic.</quote></para>

    <para>When I started out, It became clear that I wanted "safe" types to
    look like "numeric" types. It also became clear pretty soon that there was
    going to be significant template meta-programming in the implementation.
    Normal type traits like std::is_integer are defined in the std namespace
    and one is discouraged from extending it. Also I needed some compile time
    "max" and "lowest" values. This lead me to base the design on
    std::numeric_limits. But std::numeric limits is inherently extensible to
    any "numeric" type. For example, money is a numeric type but not an
    intrinsic types. So it seemed that I needed to define a "numeric" concept
    which required that there be an implementation of std::numeric_limits for
    any type T - such as money in this case. When I'm doubt - I tend to think
    big.</para>

    <para>For now though I'm not going to address it. For what it's worth, my
    preference would be to do something like: <programlisting>template&lt;typename T&gt;
struct range {
    T m_lowest;
    T m_highest;
    // default implementation
    range(
        const &amp; T t_min, 
        const &amp; T t_max
    ) :
        m_lowest(std::numeric_limits&lt;T&gt;::lowest(t_min),
        m_highest(std::numeric_limits&lt;T&gt;::max(t_max)
    {}
};</programlisting></para>

    <para>Then redeclare <code>safe_base</code>, etc., accordingly.</para>

    <para>Also not that for C++20, template value parameters are no longer
    restricted to integer primitive types buy maybe class types as well. This
    means the library maybe extended to user class types without changing the
    current template signatures.</para>
  </section>

  <section>
    <title>Concepts are Defined but Not Enforced.</title>

    <para>The following is paraphrased from an issue raised by Andrzej
    Krzemieński as a <ulink
    url="https://github.com/robertramey/safe_numerics/issues/46">github
    issue</ulink>.</para>

    <para><quote>You do not need a concept to constrain anything with it, in
    your library. Or is the purpose of the Type requirements to show in detail
    what it means that safe&lt;T&gt; is a 'drop-in replacement for
    T?</quote></para>

    <para>Right - currently I don't use the concept to constrain anything.
    They are currently a purely "conceptual" tool to keep the design from
    getting off track. This is common with other libraries such as the C++
    standard library where the concepts are defined but not enforced by
    compile time predicates. Hopefully in future that might change - see
    below</para>

    <para><quote>If you want to extend safe&lt;T&gt; for other integer types,
    Type requirement still need to be fixed:</quote></para>

    <para>Hmmmm - I'm not quite sure that this is true. One thing that IS true
    is the the interface and implementation of the library will need to be
    enhanced to permit "safe" to be applied to user defined types. This is
    apparent now, but as my brain can only comprehend the library one piece at
    a time, this design feature was lost during the implementation. In
    implementing co-existence of floats with safe integers, I did refactor the
    implementation in a way which I believe my eventually permit the
    application to any user supplied T which implements all the required
    operations of Numeric types. So as it is now this is pending. If the
    library were to become widely used, there might be motivation to do this.
    Time will tell. So for now I'm leaving these in the documentation and
    code, even though they are not actually used.</para>
  </section>

  <section>
    <title><code>Safe</code> values Can Be Uninitialized</title>

    <para>Currently the library permits a <code>safe&lt;int&gt;</code> value
    to be uninitialized. This supports the goal of "drop-in" replacement of
    C++/C built-in types with safe counter parts. On the other hand, this
    breaks the "always valid" guarantee. The recent improvements in the
    exception policy implementation will permit a user to specify how this is
    to be handled.</para>
  </section>

  <section>
    <title>Other Pending Issues</title>

    <para><itemizedlist>
        <listitem>
          <para>The library is currently limited to integers. If there is
          interest, it could be extended to floats and possible to user
          defined types.</para>
        </listitem>

        <listitem>
          <para>Although care has been taken to make the library portable, at
          least some parts of the implementation - particularly
          <code>checked</code> integer arithmetic - depend upon two's
          complement representation of integers. Hence the library is probably
          not currently portable to all other possible C++ architectures.
          These days, this is unlikely to be a limitation in practice.
          Starting with C++20, integer arithmetic will be guaranteed by the
          C++ standard to be two's complement.</para>
        </listitem>

        <listitem>
          <para><code>std::common_type</code> is used in a variety of generic
          libraries, including std::chrono. Without a specialization for
          <code>safe&lt;T&gt;</code>s one cannot use the safe wrappers e.g. as
          a representation for <code>std::chrono::duration</code>.</para>
        </listitem>
      </itemizedlist></para>
  </section>
</section>
