How can I have a pair with reference inside vector?

Question

I really need to make std::vector of std::pair with reference (&) inside, but it breaks inside the function when I try to pu

Answer 1

Since C++14 std::make_pair is defined as

template< class T1, class T2 >
std::pair<V1,V2> make_pair( T1&& t, T2&& u );

where V1 and V2 are std::decay<T1>::type and std::decay<T2>::type respectively.

This means that your make_pair<const int&, int> calls do not really produce pairs with references as their first elements (contrary to what you apparently believed). They actually produce temporaries of pair<int, int> type. At this point you lose any attachment to the original int object stored in your unique_ptr.

When you pass these pair<int, int> temporaries to push_back, they get implicitly converted to temporaries of pair<const int&, int> type, which is your vector's element type. Through this mechanism you attach references inside your vector's element to int members of those pair<int, int> temporaries produced by make_pair (not to int objects stored in your unique_ptrs). Once the temporaries expire, the references go sour.

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In this case you can eliminate this specific problem by avoiding make_pair altogether and simply directly constructing std::pair objects of proper type, e.g.

vector.push_back(std::pair<const int&, int>(*ptr, 11));

but you might run into other problems caused by raw references later.