Question

#include <vector>
#include <memory>

using namespace std;

class A {
public:
    A(): i(new int) {}
    A(A const& a) = delete;
    A(A &&a): i(move(a.i)) {}

    unique_ptr<int> i;
};

class AGroup {
public:
    void                    AddA(A &&a) { a_.emplace_back(move(a)); }

    vector<A> a_;
};

int main() {
    AGroup ag;
    ag.AddA(A());
    return 0;
}

does not compile… (says that unique_ptr’s copy constructor is deleted)

I tried replacing move with forward. Not sure if I did it right, but it didn’t work for me.

---

[~/nn/src] g++ a.cc -o a -std=c++0x
/opt/local/include/gcc44/c++/bits/unique_ptr.h: In member function 'A& A::operator=(const A&)':
a.cc:6:   instantiated from 'void std::vector<_Tp, _Alloc>::_M_insert_aux(__gnu_cxx::__normal_iterator<typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer, std::vector<_Tp, _Alloc> >, _Args&& ...) [with _Args = A, _Tp = A, _Alloc = std::allocator<A>]'
/opt/local/include/gcc44/c++/bits/vector.tcc:100:   instantiated from 'void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = A, _Tp = A, _Alloc = std::allocator<A>]'
a.cc:17:   instantiated from here
/opt/local/include/gcc44/c++/bits/unique_ptr.h:219: error: deleted function 'std::unique_ptr<_Tp, _Tp_Deleter>& std::unique_ptr<_Tp, _Tp_Deleter>::operator=(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = int, _Tp_Deleter = std::default_delete<int>]'
a.cc:6: error: used here
In file included from /opt/local/include/gcc44/c++/vector:69,
                 from a.cc:1:
/opt/local/include/gcc44/c++/bits/vector.tcc: In member function 'void std::vector<_Tp, _Alloc>::_M_insert_aux(__gnu_cxx::__normal_iterator<typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer, std::vector<_Tp, _Alloc> >, _Args&& ...) [with _Args = A, _Tp = A, _Alloc = std::allocator<A>]':
/opt/local/include/gcc44/c++/bits/vector.tcc:100:   instantiated from 'void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = A, _Tp = A, _Alloc = std::allocator<A>]'
a.cc:17:   instantiated from here
/opt/local/include/gcc44/c++/bits/vector.tcc:314: note: synthesized method 'A& A::operator=(const A&)' first required here

Answer 1

Probably your standard library doesn’t (yet) define unique_ptr<T>::unique_ptr(unique_ptr &&). I checked my headers in 4.5 and it’s there, so maybe try upgrading.

When it fails to find the move constructor, it would look for the copy constructor and find it deleted.

I get other errors when I compile that, though.

EDIT: Got it to work. I don’t understand why you have to move an object which is already an rvalue reference, but you do. The only problem was a missing assigment operator.

#include <vector>
#include <memory>

using namespace std;

class A {
public:
    A(): i(new int) {}
    A(A const& a) = delete;
    A &operator=(A const &) = delete;
    A(A &&a): i(move(a.i)) {}
    A &operator=(A &&a ) { i = move(a.i); }

    unique_ptr<int> i;
};

class AGroup {
public:
    void                    AddA(A &&a) { a_.emplace_back(move(a)); }

    vector<A> a_;
};

int main() {
    AGroup ag;
    ag.AddA(A());
    return 0;
}