ceph/src/boost/libs/mpl/doc/tutorial/broken-integral-constant.html

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  10 <title>THE BOOST MPL LIBRARY: Broken Integral Constant Expressions</title>
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  17 <div class="section" id="broken-integral-constant">
  18 <h1><a class="toc-backref" href="./portability.html#id74" name="broken-integral-constant">Broken Integral Constant Expressions</a></h1>
  19 <p>This is probably the most surprising of the portability issues
  20 we're going to discuss, not least because for many C++ programmers, their everyday
  21 experience seems to indicate no problems in this area whatsoever. After all,
  22 integer compile-time computations along the lines of:</p>
  23 <pre class="literal-block">
  24 enum flags {
  25       flag1     = (1 &lt;&lt; 0)
  26     , flag2     = (1 &lt;&lt; 1)
  27     , flag3     = (1 &lt;&lt; 2)
  28     ...
  29     };
  30 </pre>
  31 <p>are <em>very</em> commonplace in C++, and there is hardly a compiler out
  32 there that cannot handle this correctly.  While arithmetic by
  33 itself is indeed rarely problematic, when you are trying to mix it
  34 with templates on certain deficient compilers, all kinds of new
  35 issues arise. Fortunately, as with the rest of the portability
  36 issues we're discussing here, the problem fades into past as new
  37 compiler versions are released. The majority of most recent
  38 compilers of many vendors are already free from these issues.</p>
  39 <div class="section" id="the-problem">
  40 <h2><a name="the-problem">The Problem</a></h2>
  41 <p>The problem is in fact multi-faceted; there are a number of
  42 different subissues.  Some are present in one set of compilers,
  43 some are in another, and it's not uncommon for a code that works
  44 for one compiler to break another one and vice-versa. If this
  45 sounds like a maintenance nightmare to you, it is!  If you are
  46 interested in the specific list of issues, please refer to John
  47 Maddock's excellent &quot;<a class="reference" href="http://www.boost.org/more/int_const_guidelines.htm" target="_top">Coding Guidelines for Integral Constant
  48 Expressions</a>&quot; summary.  For the purpose of our discission here, it
  49 is sufficient to say that if your code has to work on one of the
  50 compilers listed as problematic in this area, you can safely assume
  51 that if you decide to fight them on a case-by-case basis, chances
  52 are that you won't be able to maintain your sanity for very long.</p>
  53 </div>
  54 <div class="section" id="the-symptoms">
  55 <h2><a name="the-symptoms">The Symptoms</a></h2>
  56 <p>On the positive side, when you have an issue with integral
  57 arithmetic, the diagnostics are almost always straightforward:
  58 usually the error message refers you to the exact place in the code
  59 that is causing problems, and the essence of issue is obvious from
  60 the error's text, or it becomes obvious once you've encountered the
  61 same error a couple of times.  For instance, if we throw this
  62 well-formed fragment at MSVC 7.1 (otherwise an excellent compiler!)</p>
  63 <pre class="literal-block">
  64 void value();
  65
  66 // compares two Integral Constants
  67 template&lt; typename N1, typename N2 &gt; struct less
  68     : bool_&lt; (N1::value &lt; N2::value) &gt; // line #8
  69 {
  70 };
  71 </pre>
  72 <p>we get:</p>
  73 <pre class="literal-block">
  74 portability.cpp(8) : warning C4346: 'N2::value' : dependent name is not a type
  75         prefix with 'typename' to indicate a type
  76         portability.cpp(10) : see reference to class template instantiation 'less&lt;N1,N2&gt;' being compiled
  77 portability.cpp(8) : error C2143: syntax error : missing ',' before ')'
  78 portability.cpp(9) : error C2143: syntax error : missing ';' before '{'
  79 portability.cpp(10) : error C2143: syntax error : missing ';' before '}'
  80 portability.cpp(10) : fatal error C1004: unexpected end of file found
  81 </pre>
  82 <p>The errors themselves are far from being ideal, but at least we are
  83 pointed at the correct line and even the correct part of the
  84 line. The above basically reads as &quot;something's wrong between the
  85 parentheses&quot;, and that plus the &quot;syntax error&quot; part is usually
  86 enough of the clue.</p>
  87 </div>
  88 <div class="section" id="the-solution">
  89 <h2><a name="the-solution">The Solution</a></h2>
  90 <p>Despite the fact the problems are so numerous and multi-faceted and
  91 the workarounds are conflicting, the problems can be hidden
  92 reliably beneath a library abstraction layer.  The underlaying idea
  93 is very simple: we can always wrap the constants in types and pass
  94 those around. Then all that is left is to implement algebraic
  95 metafunctions that operate on such wrappers, once, and we are home
  96 safe.</p>
  97 <p>If this sounds familiar to you, probably it's because you have
  98 already took a look at the MPL and know that the approach we just
  99 described is in fact <em>the</em> standard way of doing arithmetic in the
 100 library.  Although it's motivated by more general observations,
 101 this fact comes very handy for the library users that care about
 102 portability of their numerically-heavy metaprograms.  The MPL
 103 primitives are already there, and more importantly, they already
 104 implement the necessary workarounds, so your numeric code just
 105 works.  In fact, if you stay within the library's type-wrapper
 106 idioms, these particular problems never &quot;leak&quot; out of its
 107 abstraction layer.</p>
 108 <p>On a final note, there is a price of avoiding built-in arithmetics
 109 altogether, namely decreased readability and, on some compilers,
 110 increased compile-time overhead. Still, in majority of cases, the
 111 benefits of type-based arithmetics overweight its small
 112 shortcomings.</p>
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