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