If I have an object that only allows move-only semantics - is it possible to move items from a set? I can't seem to find a way to do this.
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1@cppguy The elements of a set are immutable, so swapping or moving shouldn't work. – juanchopanza Aug 26 '14 at 21:26
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@juanchopanza [This seem to work?](http://ideone.com/Z97Eve) – Felix Glas Aug 26 '14 at 21:30
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3That works because `std::string` can fallback to copy semantics. – sircodesalot Aug 26 '14 at 21:31
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@juanchopanza Heh, I trust you :) – Felix Glas Aug 26 '14 at 21:45
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@Snps Here's [a better example](http://ideone.com/e0hNIX). It shows that the move calls the copy constructor, because the iterator is a `const_iterator` so you cannot get a non-const reference from it. If you comment out the copy constructor, it doesn't compile. – juanchopanza Aug 26 '14 at 21:57
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2C++17 can do this via `extract` and `merge` – Mark K Cowan Feb 02 '17 at 23:55
2 Answers
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C++17 added a function std::set<>::extract that allows moving an object out of a set:
std::set<MoveOnlyType> s;
s.emplace(arg0, arg1, arg2); // only way to insert such move-only objects, since C++11
auto internal_node = s.extract(s.begin()); // internal_node no longer part of set, we can do with it what we want
MoveOnlyType m = std::move(internal_node.value()); // finally get the actual object out
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No, it is not possible. There is no way to get non-const access to elements in a set, and move requires non-const references. Allowing non-const access would make it trivially easy to break the invariants for set.
Nevin
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