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| 4 | <!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> | |
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| 10 | <title>THE BOOST MPL LIBRARY: Broken Integral Constant Expressions</title> | |
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| 15 | <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> | |
| 16 | </tr></table><div class="header-separator"></div> | |
| 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 << 0) | |
| 26 | , flag2 = (1 << 1) | |
| 27 | , flag3 = (1 << 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 "<a class="reference" href="http://www.boost.org/more/int_const_guidelines.htm" target="_top">Coding Guidelines for Integral Constant | |
| 48 | Expressions</a>" 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< typename N1, typename N2 > struct less | |
| 68 | : bool_< (N1::value < N2::value) > // 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<N1,N2>' 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 "something's wrong between the | |
| 85 | parentheses", and that plus the "syntax error" 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 "leak" 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> | |
| 113 | </div> | |
| 114 | </div> | |
| 115 | ||
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