对于std::optional<int>对应的内存布局为
struct optional_mem {
int _M_payload;
bool _M_engaged;
};
可以通过godbolt通过pahole工具查看
在汇编窗口选择 pahole右侧会显示数据结构
class exception_ptr {
void * _M_exception_object; /* 0 8 */
public:
/* size: 8, cachelines: 1, members: 1 */
/* last cacheline: 8 bytes */
};
struct nullopt_t {
/* size: 1, cachelines: 0, members: 0 */
/* padding: 1 */
/* last cacheline: 1 bytes */
};
class optional<int> : _Optional_base<int, true, true>, _Enable_copy_move<true, true, true, true, std::optional<int> > {
/* struct _Optional_base<int, true, true> <ancestor>; */ /* 0 8 */
/* XXX 65528 bytes hole, try to pack */
/* struct _Enable_copy_move<true, true, true, true, std::optional<int> > <ancestor>; */ /* 0 0 */
/* XXX last struct has 1 byte of padding */
public:
/* size: 8, cachelines: 1, members: 2 */
/* padding: 8 */
/* paddings: 1, sum paddings: 1 */
/* last cacheline: 8 bytes */
/* BRAIN FART ALERT! 8 bytes != 0 (member bytes) + 0 (member bits) + 65528 (byte holes) + 0 (bit holes), diff = -524224 bits */
};
class _Optional_base_impl<int, std::_Optional_base<int, true, true> > {
protected:
/* size: 1, cachelines: 0, members: 0 */
/* padding: 1 */
/* last cacheline: 1 bytes */
};
struct _Empty_byte {
/* size: 1, cachelines: 0, members: 0 */
/* padding: 1 */
/* last cacheline: 1 bytes */
};
union _Storage<int, true> {
struct _Empty_byte _M_empty; /* 0 0 */
int _M_value; /* 0 4 */
};
struct _Optional_payload_base<int> {
union _Storage<int, true> _M_payload; /* 0 4 */
bool _M_engaged; /* 4 1 */
/* size: 8, cachelines: 1, members: 2 */
/* padding: 3 */
/* last cacheline: 8 bytes */
};
struct _Optional_payload<int, true, true, true> : _Optional_payload_base<int> {
/* struct _Optional_payload_base<int> <ancestor>; */ /* 0 8 */
/* XXX last struct has 3 bytes of padding */
/* size: 8, cachelines: 1, members: 1 */
/* paddings: 1, sum paddings: 3 */
/* last cacheline: 8 bytes */
/* BRAIN FART ALERT! 8 bytes != 0 (member bytes) + 0 (member bits) + 0 (byte holes) + 0 (bit holes), diff = 64 bits */
};
struct _Optional_base<int, true, true> : _Optional_base_impl<int, std::_Optional_base<int, true, true> > {
/* class _Optional_base_impl<int, std::_Optional_base<int, true, true> > <ancestor>; */ /* 0 0 */
/* XXX last struct has 1 byte of padding */
struct _Optional_payload<int, true, true, true> _M_payload; /* 0 8 */
/* size: 8, cachelines: 1, members: 2 */
/* paddings: 1, sum paddings: 1 */
/* last cacheline: 8 bytes */
};
struct _Enable_copy_move<true, true, true, true, std::optional<int> > {
/* size: 1, cachelines: 0, members: 0 */
/* padding: 1 */
/* last cacheline: 1 bytes */
};
class bad_optional_access : public exception {
public:
/* class exception <ancestor>; */ /* 0 0 */
/* size: 8, cachelines: 1, members: 1 */
/* padding: 8 */
/* last cacheline: 8 bytes */
};
可以看到最终数据存储在
struct _Optional_payload_base<int> {
union _Storage<int, true> _M_payload; /* 0 4 */
bool _M_engaged; /* 4 1 */
/* size: 8, cachelines: 1, members: 2 */
/* padding: 3 */
/* last cacheline: 8 bytes */
};
中，_M_payload中存储实际数据