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94 lines
4.0 KiB
XML
94 lines
4.0 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE section PUBLIC "-//Boost//DTD BoostBook XML V1.1//EN"
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"http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
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<section id="safe_numerics.pending_issues">
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<title>Pending Issues</title>
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<?dbhtml stop-chunking?>
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<para>The library is under development. There are a number of issues still
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pending.</para>
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<section>
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<title><code>safe_base</code> Only Works for Scalar Types</title>
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<para>The following is paraphrased from an issue raised by Andrzej
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Krzemieński as a <ulink
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url="https://github.com/robertramey/safe_numerics/issues/44">github
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issue</ulink>. It touches upon fundamental ideas behind the library and
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how these ideas as the implementation of the library collided with
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reality.</para>
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<para><quote>In the current implementation safe<T> will only work
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with T being a C++ scalar type. Therefore making a general type
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requirements that say what operations are allowed is superfluous, and
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confusing (because it implies that safe<> is more
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generic.</quote></para>
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<para>When I started out, It became clear that I wanted "safe" types to
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look like "numeric" types. It also became clear pretty soon that there was
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going to be significant template meta-programming in the implementation.
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Normal type traits like std::is_integer are defined in the std namespace
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and one is discouraged from extending it. Also I needed some compile time
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"max" and "lowest" values. This lead me to base the design on
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std::numeric_limits. But std::numeric limits is inherently extensible to
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any "numeric" type. For example, money is a numeric type but not an
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intrinsic types. So it seemed that I needed to define a "numeric" concept
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which required that there be an implementation of std::numeric_limits for
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any type T - such as money in this case. When I'm doubt - I tend to think
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big.</para>
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<para>For now though I'm not going to address it. For what it's worth, my
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preference would be to do something like: <programlisting>template<typename T>
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struct range {
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T m_lowest;
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T m_highest;
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// default implementation
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range(
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const & T t_min,
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const & T t_max
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) :
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m_lowest(std::numeric_limits<T>::lowest(t_min),
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m_highest(std::numeric_limits<T>::max(t_max)
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{}
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};</programlisting></para>
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<para>Then redeclare <code>safe_base</code>, etc., accordingly.</para>
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<para>Also note that for C++20, template value parameters are no longer
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restricted to integer primitive types but may be class types as well. This
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means the library maybe extended to user class types without changing the
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current template signatures.</para>
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</section>
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<section>
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<title>Other Pending Issues</title>
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<para><itemizedlist>
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<listitem>
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<para>The library is currently limited to integers. If there is
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interest, it could be extended to floats and possible to user
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defined types.</para>
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</listitem>
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<listitem>
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<para>Although care has been taken to make the library portable, at
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least some parts of the implementation - particularly
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<code>checked</code> integer arithmetic - depend upon two's
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complement representation of integers. Hence the library is probably
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not currently portable to all other possible C++ architectures.
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These days, this is unlikely to be a limitation in practice.
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Starting with C++20, integer arithmetic will be guaranteed by the
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C++ standard to be two's complement.</para>
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</listitem>
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<listitem>
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<para><code>std::common_type</code> is used in a variety of generic
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libraries, including std::chrono. Without a specialization for
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<code>safe<T></code>s one cannot use the safe wrappers e.g. as
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a representation for <code>std::chrono::duration</code>.</para>
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</listitem>
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</itemizedlist></para>
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</section>
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</section>
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