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  1. PO
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    <p>To create a hash function, you can use <a href="http://en.cppreference.com/w/cpp/utility/hash" rel="nofollow">std::hash</a>, which is defined for integers. Then, you can combine them "as the boost guys does" (because doing a good hash is something non trivial) as explained here : <a href="http://en.cppreference.com/w/cpp/utility/hash" rel="nofollow">http://en.cppreference.com/w/cpp/utility/hash</a>.</p> <p>Here is a hash_combine method :</p> <pre><code>inline void hash_combine(std::size_t&amp; seed, std::size_t v) { seed ^= v + 0x9e3779b9 + (seed &lt;&lt; 6) + (seed &gt;&gt; 2); } </code></pre> <p>So the "guideline" is more or less what's is shown on cppreference.</p> <p>You CAN'T be sure your hash function is colision free. Colision free means that you do not loose data (or you restrict yourself to a small set of possibilities for your class). If any int32 value is allowed for each fields, a collision free hash is a monstrously big index, and it won't fit in a small table. Let unordered_map take care of collisions, and combine std::hash hash as explained above.</p> <p>In you case, it will look something like</p> <pre><code>std::size_t hash() const { std::size_t h1 = std::hash&lt;CEditLine*&gt;()(editLine); //Your int32 type is probably a typedef of a hashable type. Otherwise, // you'll have to static_cast&lt;&gt; it to a type supported by std::hash. std::size_t h2 = std::hash&lt;int32&gt;()(nDisplayLine); std::size_t h3 = std::hash&lt;int32&gt;()(start); std::size_t h4 = std::hash&lt;int32&gt;()(len); std::size_t hash = 0; hash_combine(hash, h1); hash_combine(hash, h2); hash_combine(hash, h3); hash_combine(hash, h4); return hash; } </code></pre> <p>Then, you can specialize the std::hash operator for your class.</p> <pre><code>namespace std { template&lt;&gt; struct hash&lt;CacheKey&gt; { public: std::size_t operator()(CacheKey const&amp; s) const { return s.hash(); } }; } </code></pre>
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    1. POHow construct hash function for a user defined type?
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    1. USJeremy Cochoy
    1. USJeremy Cochoy
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    1. COThis is a good guideline but please add a short summary to your answer to prevent link rot.
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      1. USKonrad Rudolph
 

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