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POC++: inheritance problem
 

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  1. POC++: inheritance problem
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    <p>It's quite hard to explain what I'm trying to do, I'll try: Imagine a base <code>class A</code> which contains some variables, and a set of classes deriving from <code>A</code> which all implement some method <code>bool test()</code> that operates on the variables inherited from <code>A</code>.</p> <pre><code>class A { protected: int somevar; // ... }; class B : public A { public: bool test() { return (somevar == 42); } }; class C : public A { public: bool test() { return (somevar &gt; 23); } }; // ... more classes deriving from A </code></pre> <p>Now I have an instance of <code>class A</code> and I have set the value of <code>somevar</code>.</p> <pre><code>int main(int, char* []) { A a; a.somevar = 42; </code></pre> <p>Now, I need some kind of container that allows me to iterate over the elements <code>i</code> of this container, calling <code>i::test()</code> in the context of <code>a</code>... that is:</p> <pre><code> std::vector&lt;...&gt; vec; // push B and C into vec, this is pseudo-code vec.push_back(&amp;B); vec.push_back(&amp;C); bool ret = true; for(i = vec.begin(); i != vec.end(); ++i) { // call B::test(), C::test(), setting *this to a ret &amp;= ( a .* (&amp;(*i)::test) )(); } return ret; } </code></pre> <p>How can I do this? I've tried two methods:</p> <ol> <li>forcing a cast from B::* to A::*, adapting a pointer to call a method of a type on an object of a different type (works, but seems to be bad);</li> <li>using std::bind + the solution above, ugly hack;</li> <li>changing the signature of <code>bool test()</code> so that it takes an argument of type <code>const A&amp;</code> instead of inheriting from A, I don't really like this solution because somevar must be public.</li> </ol> <p><strong>EDIT:</strong></p> <p>Solution (1) is:</p> <pre><code>typedef bool (A::*)() mptr; std::vector&lt;mptr&gt; vec; vec.push_back(static_cast&lt;mptr&gt;(&amp;T::test)); std::vector&lt;mptr&gt;::iterator i; for(i = vec.begin(); i != vec.end(); ++i) { (a .* (*i))(); } </code></pre> <p>I'm not sure the static cast is safe.</p>
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    1. USGiovanni Funchal
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    1. COManipulating parent state within overridden child virtual methods seems sketchy: What problem are you *actually* trying to solve here? Side note regarding your comment about making somevar public, it's already protected which is just a slightly restricted form of public in that it already exposes your class's internal state to arbitrary modification.
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      1. USMark B
    2. COUsually when you have a problem like this it's a clue you're probably going about it the wrong way. That doesn't help you figure out the right way but at least you get a clue to start looking for it.
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