C++ std::deque copy constructor issue

Question

#include <deque>
#include <vector>

struct A
{
    A(int* const p) : m_P(p) {}
    A(A&& rhs)      : m_P(rhs.m_P) { rhs.m_P = nullptr; }
    A& operator=(A&& rhs) { delete m_P; m_P = rhs.m_P; rhs.m_P = nullptr; }
    ~A() { delete m_P; }

    A(A const& rhs)            = delete;
    A& operator=(A const& rhs) = delete;

    int* m_P;
};

int main()
{
#ifdef DDDEQUE
    std::vector<std::pair<int, std::deque<A> > >  vd;
    vd.emplace(vd.end(), 1, std::deque<A>());
#endif // #ifdef DDDEQUE

    std::vector<std::pair<int, std::vector<A> > > vv;
    vv.emplace(vv.end(), 1, std::vector<A>());
}

If compiling with g++ 4.8.5, 5.2.0, 5.3.0 and -DDDDEQUE I get a verbose error message ending with

.../bits/stl_construct.h:75:7: error: use of deleted function ‘A::A(const A&)’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^
gcc.cpp:11:5: note: declared here
      A(A const& rhs)            = delete;

without -D... compiles OK. With VC2015, VC2012 both versions compile OK. Does deque (but not vector ) need the copy constructor for gcc?

Answer 1

This appears to be specific to libstdc++ (gcc); given the code below;

struct A
{
    A() {};
    A(A&&) noexcept { }
    A& operator=(A&&) noexcept  { return *this; }
    ~A() { }
};

int main()
{
    std::vector<A> a;
    a.push_back(A{}); // or emplace(a.end()... etc.
    std::vector<std::deque<A>> b;
    b.push_back(std::deque<A>());
    std::vector<std::pair<int,A>> c;
    c.push_back(std::pair<int,A>{});
    std::vector<std::pair<int,std::deque<A>>> d;
    d.push_back(std::pair<int,std::deque<A>>{});
}

G++ fails to compile b , and d , clang compiles all 4 (except possibly d depending on the version of libc++ used) and MSVC compiles all 4 cases (using their own associated standard library; with libstdc++, clang also fails b and d ).

Does deque (but not vector ) need the copy constructor for gcc?

It appears as though, yes gcc does still require the copy constructor.

---

In more formal terms; in C++03 std::deque required the type used in the container to be Copy Constructible and Copy Assignable . This changed in C++11, the requirements were relaxed, albeit a complete type is generally still required - given the OP sample, your standard library still requires the copy construction and assignment.

From the linked reference;

T - The type of the elements.

T must meet the requirements of CopyAssignable and CopyConstructible . (until C++11)

The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is required that element type is a complete type and meets the requirements of Erasable , but many member functions impose stricter requirements. (since C++11)