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