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    <p>I don't work with templates, but I think:</p> <p>(1) You cannot use templates inside a class, templates are more like global types or global variables.</p> <p>(2) In O.O.P., the same problem you present, and that you are trying to solve by using templates, can be solved by using inheritance.</p> <p>Classes work similar to templates, you can extended by adding new things, or replace things of classes with pointers, pointers to objects (A.K.A. "references") and overriding virtual functions.</p> <pre><code>#include &lt;iostream&gt; struct Animal { virtual void eat(int amount ) { std::cout &lt;&lt; "I eat like a generic Animal." &lt;&lt; std::endl; } virtual ~Animal() { } }; #if 0 // example 1 struct Wolf : Animal { virtual void eat(int amount) { std::cout &lt;&lt; "I eat like a wolf!" &lt;&lt; std::endl; } }; struct Fish : Animal { virtual void eat(int amount) { std::cout &lt;&lt; "I eat like a fish!" &lt;&lt; std::endl; } }; #else // example 2 struct AnimalFood { virtual int readAmount() { return 5; } virtual void showName() { std::cout &lt;&lt; "I'm generic animal food" &lt;&lt; std::endl; } }; struct PredatorFood : AnimalFood { virtual int readAmount() { return 500; } virtual void showName() { std::cout &lt;&lt; "I'm food for a predator" &lt;&lt; std::endl; } }; struct Fish : Animal { virtual void eat(AnimalFood* aFood) { if (aFood-&gt;readAmount() &lt; 50) { std::cout &lt;&lt; "OK food, vitamines: " &lt;&lt; aFood-&gt;readAmount() &lt;&lt; std::endl; } else { std::cout &lt;&lt; "too much food, vitamines: " &lt;&lt; aFood-&gt;readAmount() &lt;&lt; std::endl; } } }; struct Shark : Fish { virtual void eat(AnimalFood* aFood) { if (aFood-&gt;readAmount() &lt; 250) { std::cout &lt;&lt; "too litle food for a shark, Im very hungry, vitamines: " &lt;&lt; aFood-&gt;readAmount() &lt;&lt; std::endl; } else { std::cout &lt;&lt; "OK, vitamines: " &lt;&lt; aFood-&gt;readAmount() &lt;&lt; std::endl; } } }; struct Wolf : Fish { virtual void eat(AnimalFood* aFood) { if (aFood-&gt;readAmount() &lt; 150) { std::cout &lt;&lt; "too litle food for a wolf, Im very hungry, vitamines: " &lt;&lt; aFood-&gt;readAmount() &lt;&lt; std::endl; } else { std::cout &lt;&lt; "OK, vitamines: " &lt;&lt; aFood-&gt;readAmount() &lt;&lt; std::endl; } } }; #endif int main() { // find animals Wolf* loneWolf = new Wolf(); Fish* goldenFish = new Fish(); Shark* sharky = new Shark(); // prepare food AnimalFood* genericFood = new AnimalFood(); PredatorFood* bigAnimalFood = new PredatorFood(); // give food to animals loneWolf-&gt;eat(genericFood); loneWolf-&gt;eat(bigAnimalFood); goldenFish-&gt;eat(genericFood); goldenFish-&gt;eat(bigAnimalFood); sharky-&gt;eat(genericFood); sharky-&gt;eat(bigAnimalFood); delete bigAnimalFood; delete genericFood; delete sharky; delete goldenFish; delete loneWolf; } </code></pre> <p>Cheers.</p>
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    1. POHow to achieve "virtual template function" in C++
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    1. USsehe
    1. USumlcat
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    1. CODouble underscores are reserved for the implementation in all cases, and due to various rules I suggest avoiding leading underscores completely as well.
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      1. USMark B
    2. CO(1) is incorrect -- you can have template methods inside of a class.
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      1. USRichard
    3. CO@Mark B Good Observation. But, double underscores are also used for "pseudokeywords", words that are treated as keywords by an specific compilers / linkers. "Override" is used as a keyword in some compilers, and some not, I think last version C++ standard mark as a keyword, and underscore not need it anymore.
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