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PODefine std::hash<std::function>
 

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  1. PODefine std::hash<std::function>
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    <p>I need to create a templated class that can hold pointers to elements of type <code>T</code> and then performs functions on them. The functions will come from different places, so I need a container to store them, so I can call them later. I decided to use an <code>std::unordered_set</code>, because it offers speed and restricts duplication due to it being implemented as a hash table. I have a whole class written, but it doesn't compile due to there not being a hash function defined for my <code>std::function</code> that takes a pointer of type <code>T</code> and returns <code>void</code>. It's easy enough to specify it with <code>struct hash&lt;std::function&lt;void(MyCustomType*)&gt;&gt;</code> (and overloading the <code>()</code> operator, too) for each type I use, but how do I actually hash the function?</p> <p>Here is a watered-down excerpt from my class with the relevant members and methods:</p> <pre><code>template &lt;typename T&gt; class Master { private: std::unordered_set&lt;std::function&lt;void(T*)&gt;&gt; functions; protected: registerFunction(std::function&lt;void(T*)&gt; function) { this-&gt;functions.insert(function); } unregisterFunction(std::function&lt;void(T*)&gt; function) { this-&gt;functions.erase(function); } }; </code></pre> <p>I'm not completely bound to using an <code>std::unordered_set</code>, but it seems to offer everything that I'd need to get this piece (and the rest of my code) working well.</p> <p>Am I thinking about this the wrong way? Is it completely impossible to hash a <code>std::function</code>?</p>
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    1. USAndrew Larsson
    1. USAndrew Larsson
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    1. COYou would also need to define some op==, which is about impossible to do.
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      1. PODefine std::hash<std::function>
      1. USPlasmaHH
    2. COThe problem you face is that a `std::function` could hold *any* functor of any compatible type. That's the whole point of it, type erasure. Without access to that thing it contains, you can't hash it (or compare values). And even if `std::function` wanted to give you access it basically can't: you can't write a function that does anything with it because the type could be anything.
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      1. PODefine std::hash<std::function>
      1. USSteve Jessop
 

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