Using a std::unordered_set of std::unique_ptr

Question

Assume I have a set of unique_ptr:

std::unordered_set > my_set;

I\'m not sure what\'s the safe way to

Answer 1

Note that the ability to do heterogenous lookups on standard containers is subject of some proposals.

http://cplusplus.github.io/LWG/lwg-proposal-status.html lists

• N3465 Adding heterogeneous comparison lookup to associative containers for TR2 (Rev 2) [Handle with N3573]
• N2882 id.
• N3573 Heterogenous extensions to unordered containers [Handle with N3465]

Especially the latter looks like it would cover your use case.

For now, here is an IMO not very pretty but working alternative workaround (O(n)):

#include 
#include 
#include 

#include 
#include 

#include 

struct MyClass {};

template 
struct RawEqualTo
{
    RawEqualTo(T const* raw) : raw(raw) {}

    bool operator()(T const* p) const  
        { return raw == p; }
    bool operator()(std::unique_ptr const& up) const  
        { return raw == up.get(); }

  private:
    T const* raw;
};


using namespace std;
int main()
{
    std::unordered_set > my_set;

    my_set.insert(std::unique_ptr(new MyClass));
    my_set.insert(std::unique_ptr(new MyClass));

    auto raw = my_set.begin()->get();

    bool found = end(my_set) != std::find_if(begin(my_set), end(my_set), RawEqualTo(raw));
    assert(found);

    raw = new MyClass;

    found = end(my_set) != std::find_if(begin(my_set), end(my_set), RawEqualTo(raw));
    assert(!found);

    delete raw;
}

Warning It's also very inefficient, of course.