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POHow to achieve dynamic polymorphism (run-time call dispatch) on unrelated types?
 

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  1. POHow to achieve dynamic polymorphism (run-time call dispatch) on unrelated types?
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    <p><strong>GOAL:</strong></p> <p>I would like to achieve <strong>type-safe dynamic polymorphism</strong> (i.e. run-time dispatch of a function call) <strong>on unrelated types</strong> - i.e. on types which <em>do not have a common base class</em>. It seems to me that this is achievable, or at least theoretically sound. I will try to define my problem more formally.</p> <p><strong>PROBLEM DEFINITION:</strong></p> <p>Given the following:</p> <ul> <li>two or more <strong>unrelated</strong> types <code>A1, ..., An</code>, each of which has a method called <code>f</code>, possibly with different signatures, but <strong>with the same return type</strong> <code>R</code>; and</li> <li>a <code>boost::variant&lt;A1*, ..., An*&gt;</code> object <code>v</code> (or whatever other type of variant) which can and <em>must</em> assume at any time one value of any of those types;</li> </ul> <p>My goal is to write instructions conceptually equivalent to <code>v.f(arg_1, ..., arg_m);</code> that would get <strong>dispatched at run-time</strong> to function <code>Ai::f</code> if the <em>actual type</em> of the value contained in <code>v</code> is <code>Ai</code>. If the call arguments are not compatible with the formal parameters of <strong>each</strong> function <code>Ai</code>, the compiler should raise an error. </p> <p>Of course I do not need to stick to the syntax <code>v.f(arg_1, ..., arg_m)</code>: for instance, something like <code>call(v, f, ...)</code> is also acceptable.</p> <p>I tried to achieve this in C++, but so far I have failed to come up with a <strong>good</strong> solution (I do have a bunch of bad ones). Below I clarify what I mean by "good solution".</p> <p><strong>CONSTRAINTS:</strong></p> <p>A <strong>good solution</strong> is anything that lets me mimic the <code>v.f(...)</code> idiom, e.g. <code>call_on_variant(v, f, ...);</code>, and <em>satisfies the following constraints</em>:</p> <ol> <li>does not require any sort of <strong>separate declaration</strong> for each function <code>f</code> that must be called this way (e.g. <code>ENABLE_CALL_ON_VARIANT(f)</code>) or for any list of unrelated types <code>A1, ..., An</code> that can be treated polymorphically (e.g. <code>ENABLE_VARIANT_CALL(A1, ..., An)</code>) somewhere else in the code, especially on global scope;</li> <li>does not require to <strong>explicitly name the types of the input arguments</strong> when doing the call (e.g. <code>call_on_variant&lt;int, double, string&gt;(v, f, ...)</code>). Naming the <em>return</em> type is OK, so for instance <code>call_on_variant&lt;void&gt;(v, f, ...)</code> is acceptable.</li> </ol> <p>Follows a demonstrative example that hopefully clarifies my wish and requirements.</p> <p><strong>EXAMPLE:</strong></p> <pre><code>struct A1 { void f(int, double, string) { cout &lt;&lt; "A"; } }; struct A2 { void f(int, double, string) { cout &lt;&lt; "B"; } }; struct A3 { void f(int, double, string) { cout &lt;&lt; "C"; } }; using V = boost::variant&lt;A1, A2, A3&gt;; // Do not want anything like the following here: // ENABLE_VARIANT_CALL(foo, &lt;whatever&gt;) int main() { A a; B b; C c; V v = &amp;a; call_on_variant(v, f, 42, 3.14, "hello"); // Do not want anything like the following here: // call_on_variant&lt;int, double, string&gt;(v, f, 42, 3.14, "hello"); V v = &amp;b; call_on_variant(v, f, 42, 3.14, "hello"); V v = &amp;c; call_on_variant(v, f, 42, 3.14, "hello"); } </code></pre> <p>The output of this program should be: <code>ABC</code>.</p> <p><strong>BEST (FAILED) ATTEMPT:</strong></p> <p>The closest I got to the desired solution is this macro:</p> <pre><code>#define call_on_variant(R, v, f, ...) \ [&amp;] () -&gt; R { \ struct caller : public boost::static_visitor&lt;void&gt; \ { \ template&lt;typename T&gt; \ R operator () (T* pObj) \ { \ pObj-&gt;f(__VA_ARGS__); \ } \ }; \ caller c; \ return v.apply_visitor(c); \ }(); </code></pre> <p>Which would work perfectly, <strong>if only template members were allowed in local classes</strong> (see <a href="https://stackoverflow.com/questions/4299314/member-template-in-local-class">this question</a>). Does anybody have an idea how to fix this, or suggest an alternative approach?</p>
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    1. USAndy Prowl
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      1. POHow to achieve dynamic polymorphism (run-time call dispatch) on unrelated types?
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    1. COSounds like a job for type erasure. Is that acceptable?
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      1. POHow to achieve dynamic polymorphism (run-time call dispatch) on unrelated types?
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    2. CO@KerrekSB: indeed, i tried pretty much everythin i know, but with no success so far
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      1. USAndy Prowl
    3. COSomehow it looks to me like it wouldn't be possible, but I'd love to see otherwise.
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      1. POHow to achieve dynamic polymorphism (run-time call dispatch) on unrelated types?
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