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POunordered_set using hash of value object's address
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Id
19347862
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AcceptedAnswerId
19357581
AnswerCount
3
ClosedDate
CommentCount
1
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CreationDate
2013-10-13T17:03:58.773
FavoriteCount
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2013-10-14T20:52:04.633
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2013-10-14T20:52:04.633
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1571944
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1
Score
2
ViewCount
872
LastEditorDisplayName
text
Body
I have a class which needs to have a <code>std::unordered_set</code> which holds non-copyable, non-moveable entity objects, and whose hash function hashes the instance's address. Something like the following: <pre><code>class A { public: A(); A(const A&) = delete; A(A&&) = delete; void operator=(const A&) = delete; void operator=(A&&) = delete; bool operator==(const A& other) { return this == &other; } }; template<> struct std::hash<A> { size_t operator()(const A& obj) const { return std::hash<A*>()(&obj); } }; class B { private: std::unordered_set<A> entities; }; </code></pre> If <code>emplace()</code> is always used instead of <code>insert()</code>, is it safe to use <code>unordered_set</code> in this way? Does the standard specify that an implementation can't move node objects after they are constructed? What about if <code>A</code> were moveable? Is it guaranteed that the hash function will be called on the object owned by the set, or since the standard library prefers to think of everything as value-objects, is it allowed to hash an <code>insert</code>ed object before storage is allocated for it? As a final thought, I know I could get around all this by using <code>std::unordered_set<std::unique_ptr<A>></code>, but I'd like to use a custom allocator for the <code>A</code> objects, and I don't want to override <code>new</code> and <code>delete</code> for <code>A</code>.
Tags
<c++><c++11><hash><c++-standard-library>
Title
unordered_set using hash of value object's address
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1. PO
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1. USbcrist
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1. USbcrist
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1. COYou don't have to override `new` and `delete`, you can use `std::unique_ptr<A, D>` with a custom deleter `D`.
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