123 lines
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123 lines
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HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
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<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
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<title>THE BOOST MPL LIBRARY: ETI</title>
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<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./incomplete-support-for.html" class="navigation-link">Prev</a> <a href="./resources.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./incomplete-support-for.html" class="navigation-link">Back</a> Along</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="./eti.html" class="navigation-link">ETI</a></td>
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</tr></table><div class="header-separator"></div>
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<div class="section" id="eti">
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<h1><a class="toc-backref" href="./portability.html#id76" name="eti">ETI</a></h1>
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<p>In context of C++ template problems, ETI is an abbreviation for "Early
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Template Instantiation" — a Microsoft Visual C++ - specific issue that
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has been a barrier to any serious work with templates on this platform until
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Microsoft developers fixed it in Visual C++ 7.1 (2003 .NET). Although the
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problem is relatively easy to work around if the right techniques
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are applied systematically through the codebase, the approach is definitely
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tedious and time-consuming. So, if one day you discover that you are spending
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too much time dealing with the issue, consider upgrading to the
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newer version of the compiler. In fact, seriously consider it regardless.
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The benefits of saved time, money and frustration are well worth the price.</p>
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<div class="section" id="eti-the-problem">
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<h2><a name="eti-the-problem">The Problem</a></h2>
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<p>Here is a short demonstration of the issue with MSVC 6.x:</p>
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<pre class="literal-block">
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template< typename F, typename T > struct apply1
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{
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typedef typename F::template apply<T>::type type;
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};
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</pre>
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<p>Trying to compiling this innocent-looking code, we get:</p>
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<pre class="literal-block">
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portability.cpp(4) : error C2903: 'apply' : symbol is neither a class template
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nor a function template
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portability.cpp(5) : see reference to class template instantiation
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'apply1<F,T>' being compiled
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portability.cpp(4) : error C2143: syntax error : missing ',' before '<'
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portability.cpp(5) : see reference to class template instantiation
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'apply1<F,T>' being compiled
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portability.cpp(4) : error C2059: syntax error : '<'
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portability.cpp(5) : see reference to class template instantiation
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'apply1<F,T>' being compiled
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</pre>
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<p>The "symbol is neither a class template nor a function template" part of the
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diagnostics is actually often an indication of ETI-related problems. Another
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typical error message usually says something about nested type such-and-such
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not being a member of a global namespace.</p>
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<p>Both cases are two sides of the same compiler bug, which we call
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"Early template instantiation": the compiler, for internal
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purposes, in order to process class template definitions,
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instantiates class templates with dummy template parameters
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(<tt class="literal"><span class="pre">int</span></tt>'s). That can happen both during parsing of template
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definitions (and such errors are most easy to identify and fix —
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the template definition itself just doesn't compile; the example
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above falls into this category), or later during template
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instantiation, and these one are hard to detect — the bug will
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only be triggered in some particular context.</p>
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<!-- namespace-scope: nested templates are immune? -->
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<p>ETI is always performed during parsing of the namespace-scope
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template definition, which basically means that any template
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definition that is rendered invalid by substituting its template
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parameters by <tt class="literal"><span class="pre">int</span></tt>s might not compile, as it happened with our
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example:</p>
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<pre class="literal-block">
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template< typename F, typename T > struct apply1
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{
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// typedef typename F::template apply<T>::type type;
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// ETI generates this:
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typedef typename int::template apply<int>::type type;
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};
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</pre>
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<p>If you compile this, you'll get <em>exactly</em> the same diagnostics as we've just seen.</p>
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<p>Note that we've said "might not compile", because... well, the short answer is,
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"it depends". We haven't analyzed things to the point that we could tell you the
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exact condition when ETI leads to an error and when it doesn't, but
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that's not very important anyway — if it's an error, you just fix it (we'll show you how
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in a second), and if it's not, then you leave things as is. If one day the
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potential issue turns into a real one, then you apply the workaround we are about
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to give you.</p>
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</div>
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<div class="section" id="eti-the-symptoms">
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<h2><a name="eti-the-symptoms">The Symptoms</a></h2>
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<p>We've already looked at the typical diagnostics, so we won't repeat
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ourselves. Instead we'll
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just mention that many MSVC's INTERNAL COMPILER ERRORs (ICEs) are, in fact, caused
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by an ETI-related problem somewhere deep down the instantiation stack.</p>
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</div>
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<div class="section" id="eti-the-solution">
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<h2><a name="eti-the-solution">The Solution</a></h2>
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<p>There is no way we can change the compiler's behavior in this case, so what we have
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to do is to adjust to it and still make our templates do what we want. Surprisingly,
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in most cases it's quite simple to achieve:</p>
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<pre class="literal-block">
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// potentially unsafe
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template< typename F > struct apply0
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{
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typedef typename F::type type;
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};
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// now ETI-safe
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template<> struct apply0<int>
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{
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typedef int type;
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};
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</pre>
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<p>Since the original template could have never been instantiated with <tt class="literal"><span class="pre">int</span></tt>,
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providing a stub <tt class="literal"><span class="pre">int</span></tt> specialization is completely innocent.</p>
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<!-- Looks like you're missing lots of stuff, like ETI_BASE - - no? -->
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</div>
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</div>
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<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./incomplete-support-for.html" class="navigation-link">Prev</a> <a href="./resources.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./incomplete-support-for.html" class="navigation-link">Back</a> Along</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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