1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> 4<title>Pending Issues</title> 5<link rel="stylesheet" href="boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="index.html" title="Safe Numerics"> 8<link rel="up" href="index.html" title="Safe Numerics"> 9<link rel="prev" href="rationale.html" title="Rationale and FAQ"> 10<link rel="next" href="acknowledgements.html" title="Acknowledgements"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img href="index.html" height="164px" src="pre-boost.jpg" alt="Library Documentation Index"></td> 15<td><h2>Safe Numerics</h2></td> 16</tr></table> 17<div class="spirit-nav"> 18<a accesskey="p" href="rationale.html"><img src="images/prev.png" alt="Prev"></a><a accesskey="u" href="index.html"><img src="images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="images/home.png" alt="Home"></a><a accesskey="n" href="acknowledgements.html"><img src="images/next.png" alt="Next"></a> 19</div> 20<div class="section"> 21<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 22<a name="safe_numerics.pending_issues"></a>Pending Issues</h2></div></div></div> 23<div class="toc"><dl class="toc"> 24<dt><span class="section"><a href="pending_issues.html#idm130201646352"><code class="computeroutput">safe_base</code> Only Works for Scalar Types</a></span></dt> 25<dt><span class="section"><a href="pending_issues.html#idm130201605904">Concepts are Defined but Not Enforced.</a></span></dt> 26<dt><span class="section"><a href="pending_issues.html#idm130201600176">Other Pending Issues</a></span></dt> 27</dl></div> 28<p>The library is under development. There are a number of issues still 29 pending.</p> 30<div class="section"> 31<div class="titlepage"><div><div><h3 class="title"> 32<a name="idm130201646352"></a><code class="computeroutput">safe_base</code> Only Works for Scalar Types</h3></div></div></div> 33<p>The following is paraphrased from an issue raised by Andrzej 34 Krzemieński as a <a href="https://github.com/robertramey/safe_numerics/issues/44" target="_top">github 35 issue</a>. It touches upon fundamental ideas behind the library and 36 how these ideas as the implementation of the library collided with 37 reality.</p> 38<p><span class="quote">“<span class="quote">In the current implementation safe<T> will only work 39 with T being a C++ scalar type. Therefore making a general type 40 requirements that say what operations are allowed is superfluous, and 41 confusing (because it implies that safe<> is more 42 generic.</span>”</span></p> 43<p>When I started out, It became clear that I wanted "safe" types to 44 look like "numeric" types. It also became clear pretty soon that there was 45 going to be significant template meta-programming in the implementation. 46 Normal type traits like std::is_integer are defined in the std namespace 47 and one is discouraged from extending it. Also I needed some compile time 48 "max" and "lowest" values. This lead me to base the design on 49 std::numeric_limits. But std::numeric limits is inherently extensible to 50 any "numeric" type. For example, money is a numeric type but not an 51 intrinsic types. So it seemed that I needed to define a "numeric" concept 52 which required that there be an implementation of std::numeric_limits for 53 any type T - such as money in this case. When I'm doubt - I tend to think 54 big.</p> 55<p>For now though I'm not going to address it. For what it's worth, my 56 preference would be to do something like: </p> 57<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 58<span class="keyword">struct</span> <span class="identifier">range</span> <span class="special">{</span> 59 <span class="identifier">T</span> <span class="identifier">m_lowest</span><span class="special">;</span> 60 <span class="identifier">T</span> <span class="identifier">m_highest</span><span class="special">;</span> 61 <span class="comment">// default implementation</span> 62 <span class="identifier">range</span><span class="special">(</span> 63 <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">T</span> <span class="identifier">t_min</span><span class="special">,</span> 64 <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">T</span> <span class="identifier">t_max</span> 65 <span class="special">)</span> <span class="special">:</span> 66 <span class="identifier">m_lowest</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span><span class="special">::</span><span class="identifier">lowest</span><span class="special">(</span><span class="identifier">t_min</span><span class="special">)</span><span class="special">,</span> 67 <span class="identifier">m_highest</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span><span class="special">::</span><span class="identifier">max</span><span class="special">(</span><span class="identifier">t_max</span><span class="special">)</span> 68 <span class="special">{</span><span class="special">}</span> 69<span class="special">}</span><span class="special">;</span></pre> 70<p>Then redeclare <code class="computeroutput">safe_base</code>, etc., accordingly.</p> 71<p>Also not that for C++20, template value parameters are no longer 72 restricted to integer primitive types buy maybe class types as well. This 73 means the library maybe extended to user class types without changing the 74 current template signatures.</p> 75</div> 76<div class="section"> 77<div class="titlepage"><div><div><h3 class="title"> 78<a name="idm130201605904"></a>Concepts are Defined but Not Enforced.</h3></div></div></div> 79<p>The following is paraphrased from an issue raised by Andrzej 80 Krzemieński as a <a href="https://github.com/robertramey/safe_numerics/issues/46" target="_top">github 81 issue</a>.</p> 82<p><span class="quote">“<span class="quote">You do not need a concept to constrain anything with it, in 83 your library. Or is the purpose of the Type requirements to show in detail 84 what it means that safe<T> is a 'drop-in replacement for 85 T?</span>”</span></p> 86<p>Right - currently I don't use the concept to constrain anything. 87 They are currently a purely "conceptual" tool to keep the design from 88 getting off track. This is common with other libraries such as the C++ 89 standard library where the concepts are defined but not enforced by 90 compile time predicates. Hopefully in future that might change - see 91 below</p> 92<p><span class="quote">“<span class="quote">If you want to extend safe<T> for other integer types, 93 Type requirement still need to be fixed:</span>”</span></p> 94<p>Hmmmm - I'm not quite sure that this is true. One thing that IS true 95 is the the interface and implementation of the library will need to be 96 enhanced to permit "safe" to be applied to user defined types. This is 97 apparent now, but as my brain can only comprehend the library one piece at 98 a time, this design feature was lost during the implementation. In 99 implementing co-existence of floats with safe integers, I did refactor the 100 implementation in a way which I believe my eventually permit the 101 application to any user supplied T which implements all the required 102 operations of Numeric types. So as it is now this is pending. If the 103 library were to become widely used, there might be motivation to do this. 104 Time will tell. So for now I'm leaving these in the documentation and 105 code, even though they are not actually used.</p> 106</div> 107<div class="section"> 108<div class="titlepage"><div><div><h3 class="title"> 109<a name="idm130201600176"></a>Other Pending Issues</h3></div></div></div> 110<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 111<li class="listitem"><p>The library is currently limited to integers. If there is 112 interest, it could be extended to floats and possible to user 113 defined types.</p></li> 114<li class="listitem"><p>Although care has been taken to make the library portable, at 115 least some parts of the implementation - particularly 116 <code class="computeroutput">checked</code> integer arithmetic - depend upon two's 117 complement representation of integers. Hence the library is probably 118 not currently portable to all other possible C++ architectures. 119 These days, this is unlikely to be a limitation in practice. 120 Starting with C++20, integer arithmetic will be guaranteed by the 121 C++ standard to be two's complement.</p></li> 122<li class="listitem"><p><code class="computeroutput">std::common_type</code> is used in a variety of generic 123 libraries, including std::chrono. Without a specialization for 124 <code class="computeroutput">safe<T></code>s one cannot use the safe wrappers e.g. as 125 a representation for <code class="computeroutput">std::chrono::duration</code>.</p></li> 126</ul></div> 127</div> 128</div> 129<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 130<td align="left"></td> 131<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 132 Software License</a></p> 133</div></td> 134</tr></table> 135<hr> 136<div class="spirit-nav"> 137<a accesskey="p" href="rationale.html"><img src="images/prev.png" alt="Prev"></a><a accesskey="u" href="index.html"><img src="images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="images/home.png" alt="Home"></a><a accesskey="n" href="acknowledgements.html"><img src="images/next.png" alt="Next"></a> 138</div> 139</body> 140</html> 141
