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POwhy can't I perform a dynamic cast? Strategy pattern in C
 

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  1. POwhy can't I perform a dynamic cast? Strategy pattern in C
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    <p>I'm trying to implement the strategy pattern in C++ to make my code more flexible (and get to learn some OO programing). The dynamic cast showing up on the 3rd line in main() fails. Also I get some quite wierd error messages. Could you explain to me what I'm doing wrong? I'm not used to using design patterns in C++. </p> <p>The error message I get from the compiler:</p> <pre><code>/* /tmp/cc8b482g.o: In function `individual::setObjectiveFunction(int)': main.cpp:(.text+0x20b): undefined reference to `ObjectiveFunctionhyperBowl::ObjectiveFunctionhyperBowl()' /tmp/cc8b482g.o: In function `main': main.cpp:(.text+0x25e): undefined reference to `ObjectiveFunctionhyperBowl::ObjectiveFunctionhyperBowl()' main.cpp:(.text+0x344): undefined reference to `ObjectiveFunctionhyperBowl::~ObjectiveFunctionhyperBowl()' main.cpp:(.text+0x3ac): undefined reference to `ObjectiveFunctionhyperBowl::~ObjectiveFunctionhyperBowl()' */ </code></pre> <p>Edit: There was a correct suggestion from "6502" that I was using a class before it was declared (this is a trimmed version of the whole code)</p> <p>The constructor of ObjectiveFunctionhyperBowl are there, arent they? </p> <p>What am I missing, the declaration is now right above from where it is used and it still doesn't work...</p> <p>Thanx in advance again!</p> <p>And many thanx for the fast responses! - This is my first question in Stack Overflow, I promise I'll repay the deed helping other programers when I become a better programer!</p> <pre><code>#include &lt;iostream&gt; #include &lt;sstream&gt; #include &lt;random&gt; #include &lt;assert.h&gt; #include &lt;vector&gt; class individual; class objectiveFunctionStrategy; /* * This is my abstract class implementation for a strategy. */ class objectiveFunctionStrategy { public: objectiveFunctionStrategy(); virtual ~objectiveFunctionStrategy(); virtual float evaluate(individual)=0; }; /* * The class individual should contain strategy objects */ class individual{ private: std::vector&lt;float&gt; featureVector; objectiveFunctionStrategy * ofstrategy; unsigned int ndim; public: /*Constructors*/ individual(std::vector&lt;float&gt;); float getObjectiveFunction(); void setObjectiveFunction(int k); std::vector&lt;float&gt; getFeatureVector(); }; individual::individual(std::vector&lt;float&gt; fv){ for(unsigned int i=0;i&lt;fv.size() ; i++){ featureVector.push_back(fv[i]); } this -&gt; ndim = featureVector.size(); } std::vector&lt;float&gt; individual::getFeatureVector(){ return this-&gt;featureVector; } float individual::getObjectiveFunction(){ /* *Here I'm planning of passing a concrete strategy object to *make the objective functions interchangeable. * *So the calculation would be ofstrategy.evaluate() */ float Fc=0; if(false == false){ std::vector&lt;float&gt; vv = this-&gt;featureVector; Fc = ofstrategy-&gt;evaluate(vv); } return Fc; } /* * Now this one is a concrete strategy object class: */ class ObjectiveFunctionhyperBowl: public objectiveFunctionStrategy { public: /* * could have a reference to the individual. * might do things a bit more complicated. */ ObjectiveFunctionhyperBowl(); ~ObjectiveFunctionhyperBowl(); float evaluate(individual ind){ float Fc =0 ; std::vector&lt;float&gt; v = ind.getFeatureVector(); for(unsigned int i=0;i&lt;v.size();i++){ Fc += v[i]*v[i]; } return Fc; } }; void individual::setObjectiveFunction(int i){ /* * here the strategy is defined inside the class by an integer */ this-&gt;ofstrategy = new ObjectiveFunctionhyperBowl; } int main(){ std::vector&lt;float&gt; v; ObjectiveFunctionhyperBowl hb; objectiveFunctionStrategy* ofs; ofs = dynamic_cast&lt;objectiveFunctionStrategy*&gt; (&amp;hb); individual indiv(v); /* * Now the above still does not compile... * error message: * * * */ v.push_back(2.1); v.push_back(22.1); hb.evaluate(indiv); } </code></pre>
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    1. COThere will be more errors after you fix your original question. `hb.evaluate(indiv);` uses `indiv` variable, which is not defined. Plus, you pass it by value to `hb.evaluate`. Const reference will be more efficient.
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      1. POwhy can't I perform a dynamic cast? Strategy pattern in C
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    2. COThanx! This is a heavilly trimmed version of my code that reproduces the problem. I thought of that, my first approach was to keep a reference to the "individual" inside the strategy object and then just call bd.evaluate(). I'm not that confident right now though to write that in C++...
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