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POFunction signature-like expressions as C++ template arguments
 

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    <p>I was looking at Don Clugston's <a href="http://www.codeproject.com/KB/cpp/FastDelegate.aspx">FastDelegate</a> mini-library and noticed a weird syntactical trick with the following structure:</p> <pre><code>TemplateClass&lt; void( int, int ) &gt; Object; </code></pre> <p>It almost appears as if a function signature is being used as an argument to a template instance declaration.</p> <p>This technique (whose presence in FastDelegate is apparently due to one Jody Hagins) was used to simplify the declaration of template instances with a semi-arbitrary number of template parameters.</p> <p>To wit, it allowed this something like the following:</p> <pre><code>// A template with one parameter template&lt;typename _T1&gt; struct Object1 { _T1 m_member1; }; // A template with two parameters template&lt;typename _T1, typename _T2&gt; struct Object2 { _T1 m_member1; _T2 m_member2; }; // A forward declaration template&lt;typename _Signature&gt; struct Object; // Some derived types using "function signature"-style template parameters template&lt;typename _Dummy, typename _T1&gt; struct Object&lt;_Dummy(_T1)&gt; : public Object1&lt;_T1&gt; {}; template&lt;typename _Dummy, typename _T1, typename _T2&gt; struct Object&lt;_Dummy(_T1, _T2)&gt; : public Object2&lt;_T1, _T2&gt; {}; // A. "Vanilla" object declarations Object1&lt;int&gt; IntObjectA; Object2&lt;int, char&gt; IntCharObjectA; // B. Nifty, but equivalent, object declarations typedef void UnusedType; Object&lt; UnusedType(int) &gt; IntObjectB; Object&lt; UnusedType(int, char) &gt; IntCharObjectB; // C. Even niftier, and still equivalent, object declarations #define DeclareObject( ... ) Object&lt; UnusedType( __VA_ARGS__ ) &gt; DeclareObject( int ) IntObjectC; DeclareObject( int, char ) IntCharObjectC; </code></pre> <p>Despite the real whiff of hackiness, I find this kind of spoofy emulation of variadic template arguments to be pretty mind-blowing.</p> <p>The real meat of this trick seems to be the fact that I can pass textual constructs like "Type1(Type2, Type3)" as arguments to templates. So here are my questions: How exactly does the compiler interpret this construct? Is it a function signature? Or, is it just a text pattern with parentheses in it? If the former, then does this imply that any arbitrary function signature is a valid type as far as the template processor is concerned?</p> <p>A follow-up question would be that since the above code sample is valid code, why doesn't the C++ standard just allow you to do something like the following, which does not compile?</p> <pre><code>template&lt;typename _T1&gt; struct Object { _T1 m_member1; }; // Note the class identifier is also "Object" template&lt;typename _T1, typename _T2&gt; struct Object { _T1 m_member1; _T2 m_member2; }; Object&lt;int&gt; IntObject; Object&lt;int, char&gt; IntCharObject; </code></pre>
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    1. CO+1, great question and I'm a big fan of the library in question.
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    2. COSee paragraph "C++ Function Types As A DSL" from [this article](http://cpp-next.com/archive/2010/11/expressive-c-fun-with-function-composition/)
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    3. COFor another way to emulate variadic template arguments see http://templog.svn.sourceforge.net/viewvc/templog/code/trunk/tuples.h?revision=47&view=markup
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