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POFunction template parameter deduction
 

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  1. POFunction template parameter deduction
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    <p>I am writing the small C++ message-based communication framework with using of the Reactor pattern. The problem which I have met is the serialization of the application (user-defined) messages. To prevent API from redudant information I make an assumption about the serialization function. There is an Archive class which holds the serialized form of the message, however it is the template, so the user could choose the binary form of its. The assumption is that for each message there will be only one serialization function available (defined), so the type deduction of the binary type could be cleary deduced from the function signature. Let's see the code:</p> <pre><code>template &lt;typename T&gt; struct Archive { T t; }; // struct Archive template &lt;typename Message, typename T&gt; void serialize(const Message&amp; msg, Archive&lt;T&gt;* const ar); struct Signal { void* payload; }; template &lt;typename T&gt; struct Wrapper { Signal* pack() { Signal* s = new Signal; archive(&amp;serialize, &amp;s-&gt;payload); return s; } template &lt;typename P&gt; void archive(void(*f)(const T&amp;, Archive&lt;P&gt;* const), void** payload) { Archive&lt;P&gt; ar; f(t, &amp;ar); P* p = new P; *p = ar.t; *payload = p; } T t; }; struct TestMsg { int i; }; template &lt;&gt; void serialize(const TestMsg&amp; msg, Archive&lt;int&gt;* const ar) { ar-&gt;t = msg.i; } int main() { Wrapper&lt;TestMsg&gt; msg; msg.pack(); return 0; } </code></pre> <p>Compiler claims that it cannot deduce the P type. Is there any other way (without traits) to help compiler with such deduction?</p> <p>Kind Regard, Gracjan</p> <p>EDIT(14-05-2013 15:42): According to the request in comment I attach the Traits solution:</p> <pre><code>/****** Library part *******/ template &lt;typename T&gt; struct Archive { T t; }; // struct Archive template &lt;typename T&gt; struct MessageTrait {}; template &lt;typename Message, typename T&gt; void serialize(const Message&amp; msg, Archive&lt;T&gt;* const ar); struct Signal { void* payload; }; template &lt;typename T&gt; struct Wrapper { Signal* pack() { typedef Archive&lt;typename MessageTrait&lt;T&gt;::ArchType&gt; ArchiveType; Signal* s = new Signal; ArchiveType ar; serialize(t, &amp;ar); return s; } T t; }; /****** Application part ******/ struct TestMsg { int i; }; template&lt;&gt; struct MessageTrait&lt;TestMsg&gt; { typedef int ArchType; }; template &lt;&gt; void serialize(const TestMsg&amp; msg, Archive&lt;int&gt;* const ar) { ar-&gt;t = msg.i; } int main() { Wrapper&lt;TestMsg&gt; msg; msg.pack(); return 0; } </code></pre>
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    1. CO`s->payload` is a `void*`. This doesn't match with `Archive<P>* const`. What do you expect to happen? Also your template specialisation syntax for `serialize` is incorrect. And you're taking the address of a template function without giving it template parameters when you do `&serialize` which is wrong.
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    2. COSpecialization for serialize method is correct. What I expect is that the compiler find the best match to the expected function with using of knowledge of the first parameter type. To be independent of the definition lookup I have used the using directive whitch provide the base definition. So the serialize function in other scopes could not be templated, to be used correctly.
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    3. COCurrently, to omit this problem (homefuly temporary) I have introduced the MessageTrait: `template <typename T> struct MessageTrait {}; template<> struct MessageTrait<TestMsg> { typdef int ArchiveType; };`
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