Making POD classes movable - c++

Question

I have a POD class and I want to make it movable for efficiency. I keep all the data in a std::array member object, and I make my public member variables references to parts of this std::array object. By doing this, now I am able to move the entire data by moving the std::array instance in the move constructor (I know that it is not literally a POD class anymore after writing constructors.).
Is this a good method of doing this? Does it actually move the data? See the code output below: After moving the std::array, I observe that both objects have the same values. It looks like it doesn't move, but it copies the data. What is the problem here?
#include <array>
class MyPodClass
{
private:
typedef double TYPE_x;
typedef double TYPE_y;
typedef double TYPE_z;
typedef int TYPE_p;
typedef int TYPE_r;
typedef int TYPE_s;
typedef char TYPE_k;
typedef char TYPE_l;
typedef char TYPE_m;
typedef float TYPE_a;
typedef float TYPE_b;
typedef float TYPE_c;
enum TypeSizes
{
STARTING_POSITION_x = 0,
STARTING_POSITION_y = STARTING_POSITION_x + sizeof(TYPE_x),
STARTING_POSITION_z = STARTING_POSITION_y + sizeof(TYPE_y),
STARTING_POSITION_p = STARTING_POSITION_z + sizeof(TYPE_z),
STARTING_POSITION_r = STARTING_POSITION_p + sizeof(TYPE_p),
STARTING_POSITION_s = STARTING_POSITION_r + sizeof(TYPE_r),
STARTING_POSITION_k = STARTING_POSITION_s + sizeof(TYPE_s),
STARTING_POSITION_l = STARTING_POSITION_k + sizeof(TYPE_k),
STARTING_POSITION_m = STARTING_POSITION_l + sizeof(TYPE_l),
STARTING_POSITION_a = STARTING_POSITION_m + sizeof(TYPE_m),
STARTING_POSITION_b = STARTING_POSITION_a + sizeof(TYPE_a),
STARTING_POSITION_c = STARTING_POSITION_b + sizeof(TYPE_b),
END_POSITION = STARTING_POSITION_c + sizeof(TYPE_c),
};
std::array<unsigned char, END_POSITION> MovableBulkData;
public:
MyPodClass()
: //x(*static_cast<TYPE_x*>(&MovableBulkData[STARTING_POSITION_x])), // ERROR: Invalid type conversion. Why?
x(*(TYPE_x*)(&MovableBulkData[STARTING_POSITION_x])),
y(*(TYPE_y*)(&MovableBulkData[STARTING_POSITION_y])),
z(*(TYPE_z*)(&MovableBulkData[STARTING_POSITION_z])),
p(*(TYPE_p*)(&MovableBulkData[STARTING_POSITION_p])),
r(*(TYPE_r*)(&MovableBulkData[STARTING_POSITION_r])),
s(*(TYPE_s*)(&MovableBulkData[STARTING_POSITION_s])),
k(*(TYPE_k*)(&MovableBulkData[STARTING_POSITION_k])),
l(*(TYPE_l*)(&MovableBulkData[STARTING_POSITION_l])),
m(*(TYPE_m*)(&MovableBulkData[STARTING_POSITION_m])),
a(*(TYPE_a*)(&MovableBulkData[STARTING_POSITION_a])),
b(*(TYPE_b*)(&MovableBulkData[STARTING_POSITION_b])),
c(*(TYPE_c*)(&MovableBulkData[STARTING_POSITION_c]))
{
}
MyPodClass(MyPodClass && RValue)
: MovableBulkData(std::move(RValue.MovableBulkData)),
x(*(TYPE_x*)(&MovableBulkData[STARTING_POSITION_x])),
y(*(TYPE_y*)(&MovableBulkData[STARTING_POSITION_y])),
z(*(TYPE_z*)(&MovableBulkData[STARTING_POSITION_z])),
p(*(TYPE_p*)(&MovableBulkData[STARTING_POSITION_p])),
r(*(TYPE_r*)(&MovableBulkData[STARTING_POSITION_r])),
s(*(TYPE_s*)(&MovableBulkData[STARTING_POSITION_s])),
k(*(TYPE_k*)(&MovableBulkData[STARTING_POSITION_k])),
l(*(TYPE_l*)(&MovableBulkData[STARTING_POSITION_l])),
m(*(TYPE_m*)(&MovableBulkData[STARTING_POSITION_m])),
a(*(TYPE_a*)(&MovableBulkData[STARTING_POSITION_a])),
b(*(TYPE_b*)(&MovableBulkData[STARTING_POSITION_b])),
c(*(TYPE_c*)(&MovableBulkData[STARTING_POSITION_c]))
{
}
const MyPodClass & operator=(MyPodClass && RValue)
{
MovableBulkData = std::move(RValue.MovableBulkData);
return *this;
}
TYPE_x & x;
TYPE_y & y;
TYPE_z & z;
TYPE_p & p;
TYPE_r & r;
TYPE_s & s;
TYPE_k & k;
TYPE_l & l;
TYPE_m & m;
TYPE_a & a;
TYPE_b & b;
TYPE_c & c;
};
int wmain(int argc, wchar_t *argv[], wchar_t *envp[])
{
MyPodClass PodObject1, PodObject2;
PodObject1.y = 3.4;
PodObject1.s = 4;
PodObject1.m = 'm';
PodObject1.a = 2.3f;
std::cout << "PodObject1.y = " << PodObject1.y << std::endl;
std::cout << "PodObject1.s = " << PodObject1.s << std::endl;
std::cout << "PodObject1.m = " << PodObject1.m << std::endl;
std::cout << "PodObject1.a = " << PodObject1.a << std::endl << std::endl;
std::cout << "PodObject2.y = " << PodObject2.y << std::endl;
std::cout << "PodObject2.s = " << PodObject2.s << std::endl;
std::cout << "PodObject2.m = " << PodObject2.m << std::endl;
std::cout << "PodObject2.a = " << PodObject2.a << std::endl << std::endl;
std::cout << "Moving PodObject1 to PodObject2..." << std::endl << std::endl;
PodObject2 = std::move(PodObject1);
std::cout << "PodObject1.y = " << PodObject1.y << std::endl;
std::cout << "PodObject1.s = " << PodObject1.s << std::endl;
std::cout << "PodObject1.m = " << PodObject1.m << std::endl;
std::cout << "PodObject1.a = " << PodObject1.a << std::endl << std::endl;
std::cout << "PodObject2.y = " << PodObject2.y << std::endl;
std::cout << "PodObject2.s = " << PodObject2.s << std::endl;
std::cout << "PodObject2.m = " << PodObject2.m << std::endl;
std::cout << "PodObject2.a = " << PodObject2.a << std::endl << std::endl;
std::cout << "Modifying PodObject1 and PodObject2..." << std::endl << std::endl;
PodObject1.s = 5;
PodObject2.m = 'n';
std::cout << "PodObject1.y = " << PodObject1.y << std::endl;
std::cout << "PodObject1.s = " << PodObject1.s << std::endl;
std::cout << "PodObject1.m = " << PodObject1.m << std::endl;
std::cout << "PodObject1.a = " << PodObject1.a << std::endl << std::endl;
std::cout << "PodObject2.y = " << PodObject2.y << std::endl;
std::cout << "PodObject2.s = " << PodObject2.s << std::endl;
std::cout << "PodObject2.m = " << PodObject2.m << std::endl;
std::cout << "PodObject2.a = " << PodObject2.a << std::endl << std::endl;
std::cout << std::endl;
_wsystem(L"timeout /t 60 /nobreak");
return 0;
}
Output:
PodObject1.y = 3.4
PodObject1.s = 4
PodObject1.m = m
PodObject1.a = 2.3
PodObject2.y = -9.25596e+61
PodObject2.s = -858993460
PodObject2.m = ╠
PodObject2.a = -1.07374e+08
Moving PodObject1 to PodObject2...
PodObject1.y = 3.4
PodObject1.s = 4
PodObject1.m = m
PodObject1.a = 2.3
PodObject2.y = 3.4
PodObject2.s = 4
PodObject2.m = m
PodObject2.a = 2.3
Modifying PodObject1 and PodObject2...
PodObject1.y = 3.4
PodObject1.s = 5
PodObject1.m = m
PodObject1.a = 2.3
PodObject2.y = 3.4
PodObject2.s = 4
PodObject2.m = n
PodObject2.a = 2.3

Answer 1

This is a misuse of move semantics. Since your class contains a number of simple data members like int and float, there is really nothing to move. You'd be better off with memcpy(), which is probably close to what your compiler gives you for free if you just write the class the normal, naive way, with no std::array and no pointer gymnastics.
Move semantics would have been useful here if your class contained e.g. a std::string, because std::string uses dynamically allocated memory which can be "moved" (read: adopted) into the target of a move.
The above of course means that you could "fix" your problem by dynamically allocating the array, which would allow you to move it. But in the end this would be a baroque way to achieve the effect of using a trivial POD class with no gymnastics and storing it in a std::unique_ptr, which of course enables move semantics.