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POType mapping in object factory
 

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  1. POType mapping in object factory
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    <p>Suppose that I have two sets of associated types, for example <code>Animal</code>s and their <code>Offspring</code>:</p> <pre><code>/* Animal types */ struct Animal { virtual string getType() const = 0; }; struct Cat : public Animal { virtual string getType() const { return "Cat"; } }; struct Dog : public Animal { virtual string getType() const { return "Dog"; } }; /* Offspring types */ struct Offspring { virtual string getType() const = 0; }; struct Kitten : public Offspring { virtual string getType() const { return "Kitten"; } }; struct Puppy : public Offspring { virtual string getType() const { return "Puppy"; } }; </code></pre> <p>I am trying to implement a factory which, given an <code>Animal</code> will return an object of the associated <code>Offspring</code> type (e.g. if the <code>Animal</code> is in fact a <code>Dog</code>, the factory will return a <code>Puppy</code>).</p> <p>My first attempt at implementing such a factory looks like this:</p> <pre><code>// First attempt at OffspringFactory class OffspringFactory1 { static Offspring* createKitten() { return new Kitten(); } static Offspring* createPuppy() { return new Puppy(); } public: // Create an Offspring according to the Animal type static Offspring* getOffspring(const Animal&amp; a) { // Static mapping of Animal types to Offspring factory functions static map&lt;string, Offspring* (*)()&gt; factoryMap; if (factoryMap.empty()) { factoryMap["Dog"] = &amp;createPuppy; factoryMap["Cat"] = &amp;createKitten; } // Lookup our Offspring factory function map&lt;string, Offspring* (*)()&gt;::const_iterator fnIt = factoryMap.find(a.getType()); if (fnIt != factoryMap.end()) return fnIt-&gt;second(); else throw "Bad animal type"; } }; </code></pre> <p>It works fine, but I've resorted to a string-based mapping rather than something purely type-based. In trying to move towards a more type-based implementation I arrived at this:</p> <pre><code>// Second attempt at OffspringFactory class OffspringFactory2 { // Mapping Animal types to Offspring types template&lt;typename TAnimal&gt; struct OffspringMapper; template&lt;&gt; struct OffspringMapper&lt;Cat&gt; { typedef Kitten offspring_type; }; template&lt;&gt; struct OffspringMapper&lt;Dog&gt; { typedef Puppy offspring_type; }; // Factory method template&lt;typename TAnimal&gt; static Offspring* create() { return new OffspringMapper&lt;TAnimal&gt;::offspring_type(); } public: // Create an Offspring according to the Animal type static Offspring* getOffspring(const Animal&amp; a) { // Static mapping of Animal type strings to Offspring factory functions static map&lt;string, Offspring* (*)()&gt; factoryMap; if (factoryMap.empty()) { factoryMap["Dog"] = &amp;create&lt;Dog&gt;; factoryMap["Cat"] = &amp;create&lt;Cat&gt;; } // Lookup our Offspring factory function map&lt;string, Offspring* (*)()&gt;::const_iterator fnIt = factoryMap.find(a.getType()); if (fnIt != factoryMap.end()) return fnIt-&gt;second(); else throw "Bad animal type"; } }; </code></pre> <p>Frankly, I'm not sure I've improved anything here: I still have my string mapping, along with quite a few more lines of less readable code... </p> <p>Is there any merit in the second implementation over the first, and is there any way I can get rid of that map?</p>
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    1. USatkins
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    1. COi am not quite sure what you are trying to achieve? is there some kind of real life problem you are trying to solve?
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      1. USAnders K.
    2. COCan you add `static Offspring* Animal::createOffspring() =0;`? That would make this _REALLY_ easy. Otherwise you're going to have to simply replace your strings with an enum.
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      1. USMooing Duck
    3. CO@MooingDuck I think you meant `virtual` not `static` in your comment.
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      1. USKurt Stutsman
 

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