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| 7 | <title>THE BOOST MPL LIBRARY: ETI</title> |
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| 12 | <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="./eti.html" class="navigation-link">ETI</a></td> |
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| 13 | </tr></table><div class="header-separator"></div> |
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| 14 | <div class="section" id="eti"> |
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| 15 | <h1><a class="toc-backref" href="./portability.html#id76" name="eti">ETI</a></h1> |
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| 16 | <p>In context of C++ template problems, ETI is an abbreviation for "Early |
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| 17 | Template Instantiation" — a Microsoft Visual C++ - specific issue that |
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| 18 | has been a barrier to any serious work with templates on this platform until |
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| 19 | Microsoft developers fixed it in Visual C++ 7.1 (2003 .NET). Although the |
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| 20 | problem is relatively easy to work around if the right techniques |
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| 21 | are applied systematically through the codebase, the approach is definitely |
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| 22 | tedious and time-consuming. So, if one day you discover that you are spending |
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| 23 | too much time dealing with the issue, consider upgrading to the |
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| 24 | newer version of the compiler. In fact, seriously consider it regardless. |
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| 25 | The benefits of saved time, money and frustration are well worth the price.</p> |
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| 26 | <div class="section" id="eti-the-problem"> |
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| 27 | <h2><a name="eti-the-problem">The Problem</a></h2> |
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| 28 | <p>Here is a short demonstration of the issue with MSVC 6.x:</p> |
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| 29 | <pre class="literal-block"> |
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| 30 | template< typename F, typename T > struct apply1 |
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| 31 | { |
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| 32 | typedef typename F::template apply<T>::type type; |
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| 33 | }; |
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| 34 | </pre> |
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| 35 | <p>Trying to compiling this innocent-looking code, we get:</p> |
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| 36 | <pre class="literal-block"> |
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| 37 | portability.cpp(4) : error C2903: 'apply' : symbol is neither a class template |
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| 38 | nor a function template |
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| 39 | portability.cpp(5) : see reference to class template instantiation |
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| 40 | 'apply1<F,T>' being compiled |
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| 41 | portability.cpp(4) : error C2143: syntax error : missing ',' before '<' |
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| 42 | portability.cpp(5) : see reference to class template instantiation |
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| 43 | 'apply1<F,T>' being compiled |
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| 44 | portability.cpp(4) : error C2059: syntax error : '<' |
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| 45 | portability.cpp(5) : see reference to class template instantiation |
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| 46 | 'apply1<F,T>' being compiled |
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| 47 | </pre> |
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| 48 | <p>The "symbol is neither a class template nor a function template" part of the |
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| 49 | diagnostics is actually often an indication of ETI-related problems. Another |
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| 50 | typical error message usually says something about nested type such-and-such |
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| 51 | not being a member of a global namespace.</p> |
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| 52 | <p>Both cases are two sides of the same compiler bug, which we call |
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| 53 | "Early template instantiation": the compiler, for internal |
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| 54 | purposes, in order to process class template definitions, |
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| 55 | instantiates class templates with dummy template parameters |
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| 56 | (<tt class="literal"><span class="pre">int</span></tt>'s). That can happen both during parsing of template |
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| 57 | definitions (and such errors are most easy to identify and fix — |
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| 58 | the template definition itself just doesn't compile; the example |
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| 59 | above falls into this category), or later during template |
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| 60 | instantiation, and these one are hard to detect — the bug will |
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| 61 | only be triggered in some particular context.</p> |
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| 62 | <!-- namespace-scope: nested templates are immune? --> |
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| 63 | <p>ETI is always performed during parsing of the namespace-scope |
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| 64 | template definition, which basically means that any template |
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| 65 | definition that is rendered invalid by substituting its template |
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| 66 | parameters by <tt class="literal"><span class="pre">int</span></tt>s might not compile, as it happened with our |
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| 67 | example:</p> |
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| 68 | <pre class="literal-block"> |
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| 69 | template< typename F, typename T > struct apply1 |
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| 70 | { |
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| 71 | // typedef typename F::template apply<T>::type type; |
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| 72 | // ETI generates this: |
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| 73 | typedef typename int::template apply<int>::type type; |
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| 74 | }; |
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| 75 | </pre> |
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| 76 | <p>If you compile this, you'll get <em>exactly</em> the same diagnostics as we've just seen.</p> |
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| 77 | <p>Note that we've said "might not compile", because... well, the short answer is, |
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| 78 | "it depends". We haven't analyzed things to the point that we could tell you the |
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| 79 | exact condition when ETI leads to an error and when it doesn't, but |
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| 80 | that's not very important anyway — if it's an error, you just fix it (we'll show you how |
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| 81 | in a second), and if it's not, then you leave things as is. If one day the |
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| 82 | potential issue turns into a real one, then you apply the workaround we are about |
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| 83 | to give you.</p> |
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| 84 | </div> |
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| 85 | <div class="section" id="eti-the-symptoms"> |
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| 86 | <h2><a name="eti-the-symptoms">The Symptoms</a></h2> |
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| 87 | <p>We've already looked at the typical diagnostics, so we won't repeat |
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| 88 | ourselves. Instead we'll |
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| 89 | just mention that many MSVC's INTERNAL COMPILER ERRORs (ICEs) are, in fact, caused |
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| 90 | by an ETI-related problem somewhere deep down the instantiation stack.</p> |
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| 91 | </div> |
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| 92 | <div class="section" id="eti-the-solution"> |
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| 93 | <h2><a name="eti-the-solution">The Solution</a></h2> |
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| 94 | <p>There is no way we can change the compiler's behavior in this case, so what we have |
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| 95 | to do is to adjust to it and still make our templates do what we want. Surprisingly, |
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| 96 | in most cases it's quite simple to achieve:</p> |
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| 97 | <pre class="literal-block"> |
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| 98 | // potentially unsafe |
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| 99 | template< typename F > struct apply0 |
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| 100 | { |
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| 101 | typedef typename F::type type; |
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| 102 | }; |
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| 103 | |
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| 104 | // now ETI-safe |
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| 105 | template<> struct apply0<int> |
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| 106 | { |
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| 107 | typedef int type; |
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| 108 | }; |
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| 109 | </pre> |
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| 110 | <p>Since the original template could have never been instantiated with <tt class="literal"><span class="pre">int</span></tt>, |
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| 111 | providing a stub <tt class="literal"><span class="pre">int</span></tt> specialization is completely innocent.</p> |
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| 112 | <!-- Looks like you're missing lots of stuff, like ETI_BASE - - no? --> |
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| 113 | </div> |
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| 114 | </div> |
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| 115 | |
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