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POPointer-to-member-func, template & inheritance mixup
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2010-06-24T16:30:22.953
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2011-07-04T16:28:05.980
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2011-07-04T16:28:05.980
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Body
I am trying to create a generic "callback" object that will hold arbitrary data and invoke member functions of related classes. Due to internal policy, I cannot use Boost. The callback object looks like this: <pre><code>template<typename Object, typename Data> class Callback { public: typedef void (Object::*PHandler)(Callback*); Callback(Object* obj, PHandler handler) : pObj(obj), pHandler(handler) {} Callback& set(PHandler handler) { pHandler = handler; return *this; } void run() { (pObj->*pHandler)(this); } public: Data data; protected: Object* pObj; PHandler pHandler; }; </code></pre> And the class it works on: <pre><code>struct Object1 { struct Data { int i; }; typedef Callback<Object1, Data> Callback1; void callback(Callback1* pDisp) { printf("%cb\n", pDisp->data.i); } void test() { Callback1 cb(this, &Object1::callback); cb.data.i = 1; cb.run(); } }; </code></pre> The following test works as expected: <pre><code>Object1 obj1; obj1.test(); </code></pre> So far so good. However, when a coworker tried to derive from the <code>Callback</code> class instead of using a <code>typedef</code>, they got compilation errors due to incompatible pointers: <pre><code>struct Object2 { struct Data { int i; Data(int j) { i = j; } }; class Callback2 : public Callback<Object2, Data> { Callback2(Object2* obj, PHandler handler, int i) : Callback(obj, handler) { data.i = i; } }; void callback(Callback2* pDisp) { printf("%cb\n", pDisp->data.i); } void test() { Callback2 cb(this, &Object2::callback, 2); cb.run(); } }; </code></pre> I tried using the "curiously recurring template pattern" in the Callback class and managed to get derived classes working, but it broke code that used the <code>typedef</code> method. My question is: How can I modify the <code>Callback</code> class to work with both cases, and without requiring extra work on the part of the user of the class?
Tags
<c++><templates><derived-class><member-function-pointers>
Title
Pointer-to-member-func, template & inheritance mixup
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