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POConditional compile-time inclusion/exclusion of code based on template argument(s)?
 

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  1. POConditional compile-time inclusion/exclusion of code based on template argument(s)?
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    <p>Consider the following class, with the inner struct <code>Y</code> being used as a type, eg. in templates, later on:</p> <pre><code>template&lt;int I&gt; class X{ template&lt;class T1&gt; struct Y{}; template&lt;class T1, class T2&gt; struct Y{}; }; </code></pre> <p>Now, this example will obviously not compile, with the error that the second <code>X&lt;I&gt;::Y</code> has already been defined or that it has too many template parameters.<br> I'd like to resolve that without (extra) partial specialization, since the <code>int I</code> parameter isn't the only one and the position of it can differ in different partial specializations (my actual struct looks <a href="http://ideone.com/9R2IS" rel="noreferrer">more like this</a>, the above is just for simplicity of the question), so I'd like <code>one class fits every I</code> solution.</p> <hr> <p>My first thought was obviously <code>enable_if</code>, but that seems to fail on me, eg. I still get the same errors:</p> <pre><code>// assuming C++11 support, else use boost #include &lt;type_traits&gt; template&lt;int I&gt; class X{ template&lt;class T1, class = std::enable_if&lt;I==1&gt;::type&gt; struct Y{}; template&lt;class T1, class T2, class = std::enable_if&lt;I==2&gt;::type&gt; struct Y{}; }; </code></pre> <hr> <p>So, since <code>enable_if</code> fails, I hope there is another way to achieve the following compile time check:</p> <pre><code>template&lt;int I&gt; class X{ __include_if(I == 1){ template&lt;class T1&gt; struct Y{}; } __include_if(I == 2){ template&lt;class T1, class T2&gt; struct Y{}; } }; </code></pre> <hr> <p>It would just be to save me <strong>a lot</strong> of code duplication, but I'd be really happy if it was somehow possible.<br> Edit: Sadly, I can't use the obvious: variadic templates, as I'm using Visual Studio 2010, so only the C++0x stuff that is supported there I can use. :/</p>
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    1. CO+1. Interesting question. Will try to answer it after office time :D
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    2. COI'm awaiting it with pleasure. :) My line of thought is that it should be possible as the compiler knows everything it needs to know at, well, compile-time.
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    3. CO@Xeo: Are you allowed to use C++0x feautures?
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