c++ - Insert into vector having objects without copy constructor

Question

I have a class whose copy constructors are explicitly deleted (because A uses pointers internally and I don't want to fall into shallow copy pitfalls):

class A {
  public:
    A(const A&) = delete;
    A& operator=(const A&) = delete;

    A(const B& b, const C& c);
}

Now I have a vector of type vector<A> aVector; and I want to insert elements into it - so I use emplace_back:

aVector.emplace_back(b, c);

However, this fails to compile using gcc and I get the error -

third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_construct.h: In instantiation of 'void std::_Construct(_T1*, _Args&& ...)
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:77:3:   required from 'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator)  
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:119:41:   required from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) 
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:260:63:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) 
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:283:67:   required from '_ForwardIterator std::__uninitialized_move_if_noexcept_a(_InputIterator, _InputIterator, _ForwardIterator, _Allocator&)
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/vector.tcc:410:6:   required from 'void std::vector<_Tp, _Alloc>::_M_emplace_back_aux(_Args&& ...) 
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/vector.tcc:102:4:   required from 'void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...)

What is the reason for this error and how can it be fixed without removing the deletion of the copy constructors? Do I need a move constructor - does it need to be explicitly defined?

Answer 1

You should add move constructor - because std::vector::emplace_back may do relocation which requires copy/move constructor. Or just use std::deque.

LIVE DEMO

#include <vector>
#include <deque>
using namespace std;

struct NoCopyNoMove
{
    NoCopyNoMove(const NoCopyNoMove&) = delete;
    NoCopyNoMove& operator=(const NoCopyNoMove&) = delete;
    NoCopyNoMove(NoCopyNoMove&&) = delete;
    NoCopyNoMove& operator=(NoCopyNoMove&&) = delete;

    NoCopyNoMove(int){};
};

struct OnlyMove
{
    OnlyMove(const OnlyMove&) = delete;
    OnlyMove& operator=(const OnlyMove&) = delete;
    OnlyMove(OnlyMove&&) noexcept {}
    OnlyMove& operator=(OnlyMove&&) noexcept {}

    OnlyMove(int){};
};

int main()
{
    deque<NoCopyNoMove> x;
    x.emplace_back(1);

    vector<OnlyMove> y;
    y.emplace_back(1);
}

---

§ 23.2.3 Table 101 — Optional sequence container operations

a.emplace_back(args) [...]

Requires: T shall be EmplaceConstructible into X from args. For vector, T shall also be MoveInsertable into X.