hkB*_*sai 2 c++ pod move-semantics
我有一个POD课程,我想让它可以提高效率.我将所有数据保存在std::array成员对象中,并使我的公共成员变量引用此std::array对象的部分内容.通过这样做,现在我可以通过std::array在移动构造函数中移动实例来移动整个数据(我知道在编写构造函数之后它不再是字面上的POD类.).
这是一个很好的方法吗？它真的会移动数据吗？请参阅下面的代码输出:移动后std::array,我观察到两个对象具有相同的值.看起来它不会移动,但它会复制数据.这里有什么问题？
#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;
}
输出:
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
这是对移动语义的误用.由于您的类包含许多像int和的简单数据成员float,因此实际上无需移动.memcpy()如果你只是以正常,天真的方式写课std::array,没有指针体操,你可能会更好地接受你的编译器给你的东西.
如果你的类包含例如std :: string,那么移动语义在这里会很有用,因为std :: string使用动态分配的内存,可以"移动"(读取:采用)到移动的目标中.
上面当然意味着您可以通过动态分配数组来"修复"您的问题,这将允许您移动它.但最终这将是一种巴洛克式的方式来实现使用一个没有体操的普通POD课程并将其存储在a中的效果std::unique_ptr,这当然能够实现移动语义.