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120 lines
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<title>THE BOOST MPL LIBRARY: Broken Integral Constant Expressions</title>
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<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./portability.html" class="navigation-link">Prev</a> <a href="./incomplete-support-for.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./incomplete-support-for.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./portability.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
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<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./technical-details.html" class="navigation-link">Technical Details</a> / <a href="./portability.html" class="navigation-link">Portability</a> / <a href="./broken-integral-constant.html" class="navigation-link">Broken Integral Constant Expressions</a></td>
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<div class="section" id="broken-integral-constant">
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<h1><a class="toc-backref" href="./portability.html#id74" name="broken-integral-constant">Broken Integral Constant Expressions</a></h1>
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<p>This is probably the most surprising of the portability issues
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we're going to discuss, not least because for many C++ programmers, their everyday
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experience seems to indicate no problems in this area whatsoever. After all,
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integer compile-time computations along the lines of:</p>
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<pre class="literal-block">
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enum flags {
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flag1 = (1 << 0)
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, flag2 = (1 << 1)
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, flag3 = (1 << 2)
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...
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};
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</pre>
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<p>are <em>very</em> commonplace in C++, and there is hardly a compiler out
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there that cannot handle this correctly. While arithmetic by
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itself is indeed rarely problematic, when you are trying to mix it
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with templates on certain deficient compilers, all kinds of new
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issues arise. Fortunately, as with the rest of the portability
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issues we're discussing here, the problem fades into past as new
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compiler versions are released. The majority of most recent
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compilers of many vendors are already free from these issues.</p>
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<div class="section" id="the-problem">
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<h2><a name="the-problem">The Problem</a></h2>
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<p>The problem is in fact multi-faceted; there are a number of
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different subissues. Some are present in one set of compilers,
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some are in another, and it's not uncommon for a code that works
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for one compiler to break another one and vice-versa. If this
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sounds like a maintenance nightmare to you, it is! If you are
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interested in the specific list of issues, please refer to John
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Maddock's excellent "<a class="reference" href="http://www.boost.org/more/int_const_guidelines.htm" target="_top">Coding Guidelines for Integral Constant
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Expressions</a>" summary. For the purpose of our discission here, it
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is sufficient to say that if your code has to work on one of the
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compilers listed as problematic in this area, you can safely assume
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that if you decide to fight them on a case-by-case basis, chances
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are that you won't be able to maintain your sanity for very long.</p>
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</div>
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<div class="section" id="the-symptoms">
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<h2><a name="the-symptoms">The Symptoms</a></h2>
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<p>On the positive side, when you have an issue with integral
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arithmetic, the diagnostics are almost always straightforward:
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usually the error message refers you to the exact place in the code
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that is causing problems, and the essence of issue is obvious from
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the error's text, or it becomes obvious once you've encountered the
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same error a couple of times. For instance, if we throw this
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well-formed fragment at MSVC 7.1 (otherwise an excellent compiler!)</p>
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<pre class="literal-block">
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void value();
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// compares two Integral Constants
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template< typename N1, typename N2 > struct less
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: bool_< (N1::value < N2::value) > // line #8
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{
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};
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</pre>
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<p>we get:</p>
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<pre class="literal-block">
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portability.cpp(8) : warning C4346: 'N2::value' : dependent name is not a type
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prefix with 'typename' to indicate a type
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portability.cpp(10) : see reference to class template instantiation 'less<N1,N2>' being compiled
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portability.cpp(8) : error C2143: syntax error : missing ',' before ')'
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portability.cpp(9) : error C2143: syntax error : missing ';' before '{'
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portability.cpp(10) : error C2143: syntax error : missing ';' before '}'
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portability.cpp(10) : fatal error C1004: unexpected end of file found
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</pre>
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<p>The errors themselves are far from being ideal, but at least we are
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pointed at the correct line and even the correct part of the
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line. The above basically reads as "something's wrong between the
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parentheses", and that plus the "syntax error" part is usually
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enough of the clue.</p>
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</div>
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<div class="section" id="the-solution">
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<h2><a name="the-solution">The Solution</a></h2>
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<p>Despite the fact the problems are so numerous and multi-faceted and
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the workarounds are conflicting, the problems can be hidden
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reliably beneath a library abstraction layer. The underlaying idea
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is very simple: we can always wrap the constants in types and pass
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those around. Then all that is left is to implement algebraic
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metafunctions that operate on such wrappers, once, and we are home
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safe.</p>
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<p>If this sounds familiar to you, probably it's because you have
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already took a look at the MPL and know that the approach we just
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described is in fact <em>the</em> standard way of doing arithmetic in the
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library. Although it's motivated by more general observations,
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this fact comes very handy for the library users that care about
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portability of their numerically-heavy metaprograms. The MPL
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primitives are already there, and more importantly, they already
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implement the necessary workarounds, so your numeric code just
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works. In fact, if you stay within the library's type-wrapper
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idioms, these particular problems never "leak" out of its
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abstraction layer.</p>
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<p>On a final note, there is a price of avoiding built-in arithmetics
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altogether, namely decreased readability and, on some compilers,
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increased compile-time overhead. Still, in majority of cases, the
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benefits of type-based arithmetics overweight its small
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shortcomings.</p>
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</div>
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