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POHow to achieve "virtual template function" in C++
 

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  1. POHow to achieve "virtual template function" in C++
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    <p>first off: I have read and I know now that a virtual template member function is not (yet?) possible in C++. A workaround would be to make the class a template and then use the template-argument also in the member-function.</p> <p>But in the context of OOP, I find that the below example would not be very "natural" if the class was actually a template. Please note that the code is actually not working, but the gcc-4.3.4 reports: <code>error: templates may not be ‘virtual’</code></p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; class Animal { public: template&lt; class AMOUNT &gt; virtual void eat( AMOUNT amount ) const { std::cout &lt;&lt; "I eat like a generic Animal." &lt;&lt; std::endl; } virtual ~Animal() { } }; class Wolf : public Animal { public: template&lt; class AMOUNT &gt; void eat( AMOUNT amount) const { std::cout &lt;&lt; "I eat like a wolf!" &lt;&lt; std::endl; } virtual ~Wolf() { } }; class Fish : public Animal { public: template&lt; class AMOUNT &gt; void eat( AMOUNT amount) const { std::cout &lt;&lt; "I eat like a fish!" &lt;&lt; std::endl; } virtual ~Fish() { } }; class GoldFish : public Fish { public: template&lt; class AMOUNT &gt; void eat( AMOUNT amount) const { std::cout &lt;&lt; "I eat like a goldfish!" &lt;&lt; std::endl; } virtual ~GoldFish() { } }; class OtherAnimal : public Animal { virtual ~OtherAnimal() { } }; int main() { std::vector&lt;Animal*&gt; animals; animals.push_back(new Animal()); animals.push_back(new Wolf()); animals.push_back(new Fish()); animals.push_back(new GoldFish()); animals.push_back(new OtherAnimal()); for (std::vector&lt;Animal*&gt;::const_iterator it = animals.begin(); it != animals.end(); ++it) { (*it)-&gt;eat(); delete *it; } return 0; } </code></pre> <p>So creating a "Fish&lt; Amount > foo" is kind of strange. However, it seems desirable to me to provide an arbitrary amount of food to eat for each animal.</p> <p>Thus, I am searching a solution about how to achieve something like</p> <pre><code>Fish bar; bar.eat( SomeAmount food ); </code></pre> <p>This becomes particularly useful when looking at the for-loop. One might like to feed a specific amount (FoodAmount) to all of the different animals (via eat() and bind1st() e.g.), it could not be done that easily, although I wound find this very inuitive (and thus to some extent "natural). While some might want to argue now that this is due to the "uniform"-character of a vector, I think/wish that it should be possible to achieve this and I really would like to know how, as this is puzzling me for quite some time now...</p> <p><strong>[EDIT]</strong></p> <p>To perhaps clarify the motivation behind my question, I want to program an Exporter-class and let different, more specialized classes derive from it. While the top-level Exporter-class is generally only for cosmetic/structural purpose, a GraphExporter-class is derived, that should again serve as a base-class for even more specialzed export. However, similar to the Animal-example, I would like to be able to define GraphExporter* even on specialized/derived classes (e.g. on SpecialGraphExplorer) but when calling "write( out_file )", it should call the appropriate member function for SpecialGraphExporter instead of GraphExporter::write( out_file).</p> <p>Maybe this makes my situation and intentions clearer.</p> <p>Best,</p> <p>Shadow</p>
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    1. COWhat does `fish.eat("string")` do? What does `wolf.eat(Fish())` do? Can you use an abstract base class `Amount` instead of trying to make it a template parameter?
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    2. CO`Amount` sounds an awful lot like a value, rather than a type, which makes me very confused about why you feel a template is necessary here.
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    3. COMay I ask why do you want to describe a quantity as a class? It seems for me that quantity type can well be known at compile time, i.e. `size_t`. That as well will allow you to pass different values. Or perhaps another example can demonstrate the need better.
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