How to move a unique_ptr from one set to another? (C++) - c++

Question

The code below is giving the error: Call to deleted constructor of 'std::unique_ptr<int>' 'unique_ptr' has been explicitly marked deleted here passing argument to parameter 'item' here.
Could someone please explain why this is? I would have thought everything would be fine because I'm using std::move in the call to foo.add.
#include <iostream>
#include <memory>
#include <set>
class Foo {
public:
void add(std::unique_ptr<int> item) {
set.emplace(std::move(item));
}
private:
std::set<std::unique_ptr<int>> set;
};
int main() {
Foo foo;
std::set<std::unique_ptr<int>> set;
set.emplace(std::make_unique<int>(1));
set.emplace(std::make_unique<int>(2));
set.emplace(std::make_unique<int>(3));
for (auto &item : set) {
foo.add(std::move(item)); // error on this line
}
return 0;
}

Answer 1

Use c++ 17 extract() function.
example
#include <set>
#include <memory>
#include <iostream>
int main() {
auto s = std::set<std::unique_ptr<int>>{};
s.insert(std::make_unique<int>(10));
std::cout << s.size() << "\n";
auto it = s.extract(s.begin());
// Pointer type here just for clarification
std::unique_ptr<int> new_ptr = std::move(it.value());
std::cout << s.size() << "\n";
std::cout << *new_ptr << "\n";
}
Then instead of your for each loop you might use a while loop:
while (!set.empty()) {
auto it = set.extract(set.begin());
foo.add(std::move(it.value());
}