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POC++ function dispatch with template parameters
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  1. POC++ function dispatch with template parameters
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    copied!<p>I'm in the process of refactoring a large class -- let's call it <code>Big</code> -- that has a huge amount of copy-paste code. Much of this copy-paste code exists in <code>switch</code> <code>case</code>s where only the types involved end up being different. The code is switching based on an <code>enum</code> member variable of the class whose value is known only at runtime.</p> <p>My attempt to fix this involves having a <code>Dispatcher</code> class that looks up appropriately typed functions via a <code>static</code> function called <code>lookup()</code>. The functions that do the actual work are always called <code>go()</code> and have to be defined in a wrapper class template (whose sole parameter is the runtime <code>enum</code> value currently being switched on). The <code>go()</code> functions may or may not be template functions themselves.</p> <p>Here is a distilled version of the code. My apologies for the length, but this was as short as I could get it without losing important context.</p> <pre><code>#include &lt;cassert&gt; class Big { public: enum RuntimeValue { a, b }; Big(RuntimeValue rv) : _rv(rv) { } bool equals(int i1, int i2) { return Dispatcher&lt;Equals, bool(int, int)&gt;::lookup(_rv)(i1, i2); } template&lt;typename T&gt; bool isConvertibleTo(int i) { return Dispatcher&lt;IsConvertibleTo, bool(int)&gt;::lookup&lt;T&gt;(_rv)(i); } private: template&lt;RuntimeValue RV&gt; struct Equals { static bool go(int i1, int i2) { // Pretend that this is some complicated code that relies on RV // being a compile-time constant. return i1 == i2; } }; template&lt;RuntimeValue RV&gt; struct IsConvertibleTo { template&lt;typename T&gt; static bool go(int i) { // Pretend that this is some complicated code that relies on RV // being a compile-time constant. return static_cast&lt;T&gt;(i) == i; } }; template&lt;template&lt;RuntimeValue&gt; class FunctionWrapper, typename Function&gt; struct Dispatcher { static Function * lookup(RuntimeValue rv) { switch (rv) { case a: return &amp;FunctionWrapper&lt;a&gt;::go; case b: return &amp;FunctionWrapper&lt;b&gt;::go; default: assert(false); return 0; } } template&lt;typename T&gt; static Function * lookup(RuntimeValue rv) { switch (rv) { case a: return &amp;FunctionWrapper&lt;a&gt;::go&lt;T&gt;; case b: return &amp;FunctionWrapper&lt;b&gt;::go&lt;T&gt;; default: assert(false); return 0; } } // And so on as needed... template&lt;typename T1, typename T2&gt; static Function * lookup(RuntimeValue rv); }; RuntimeValue _rv; }; int main() { Big big(Big::a); assert(big.equals(3, 3)); assert(big.isConvertibleTo&lt;char&gt;(123)); } </code></pre> <p>This mostly works, except that:</p> <ol> <li>It builds and works fine under Visual C++ 9 (2008), but under GCC 4.8 it results in compilation errors in the function-template overload of <code>lookup()</code>.</li> <li>It requires that a new function-template overload of <code>lookup()</code> be written for every new number of function template parameters that we want to support in <code>go()</code>.</li> <li>It's cumbersome and confusing to use.</li> </ol> <p>Here are the errors that occur under GCC:</p> <pre><code>Big.cpp: In static member function 'static Function* Big::Dispatcher&lt;FunctionWrapper, Function&gt;::lookup(Big::RuntimeValue)': Big.cpp(66,65) : error: expected primary-expression before '&gt;' token case a: return &amp;FunctionWrapper&lt;a&gt;::go&lt;T&gt;; ^ Big.cpp(66,66) : error: expected primary-expression before ';' token case a: return &amp;FunctionWrapper&lt;a&gt;::go&lt;T&gt;; ^ Big.cpp(67,65) : error: expected primary-expression before '&gt;' token case b: return &amp;FunctionWrapper&lt;b&gt;::go&lt;T&gt;; ^ Big.cpp(67,66) : error: expected primary-expression before ';' token case b: return &amp;FunctionWrapper&lt;b&gt;::go&lt;T&gt;; ^ </code></pre> <p>My question is twofold:</p> <ol> <li>Why is this failing to build under GCC, and how do I fix it?</li> <li>Is there a better (i.e., less cumbersome and confusing) way to do this?</li> </ol> <p>The code has to be compilable under Visual C++ 9 (2008), so I can't use anything C++11-specific.</p>
 

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