这个需要看你使用的是哪种操作系统，对象是子对象还是一个完整的对象
https://en.cppreference.com/w/cpp/language/types
fundmental types/ data model
Data models
The choices made by each implementation about the sizes of the fundamental types are collectively known as data model. Four data models found wide acceptance:
32 bit systems:
LP32 or 2/4/4 (int is 16-bit, long and pointer are 32-bit)
Win16 API
ILP32 or 4/4/4 (int, long, and pointer are 32-bit);
Win32 API
Unix and Unix-like systems (Linux, Mac OS X)
64 bit systems:
LLP64 or 4/4/8 (int and long are 32-bit, pointer is 64-bit)
Win64 API
LP64 or 4/8/8 (int is 32-bit, long and pointer are 64-bit)
Unix and Unix-like systems (Linux, Mac OS X)
Other models are very rare. For example, ILP64 (8/8/8: int, long, and pointer are 64-bit) only appeared in some early 64-bit Unix systems (e.g. Unicos on Cray).
standard paper
Object types have alignment requirements (6.7.1, 6.7.2) which place restrictions on the addresses at which an
object of that type may be allocated. An alignment is an
implementation-defined integer value representing
the number of bytes between successive addresses at which a given object can be allocated. An object type
imposes an alignment requirement on every object of that type; stricter alignment can be requested using the
alignment specifier (10.6.2).
2 A fundamental alignment is represented by an alignment less than or equal to the greatest alignment supported
by the implementation in all contexts, which is equal to alignof(std::max_align_t) (21.2). The alignment
required for
a type might be different when it is used as the type of a complete object and when it is used as
the type of a subobject. [ Example:
struct B { long double d; };
struct D : virtual B { char c; };
When D is the type of a complete object, it will have a subobject of type B, so it must be aligned appropriately
for a long double. If D appears as a subobject of another object that also has B as a virtual base class, the
B subobject might be part of a different subobject, reducing the alignment requirements on the D subobject.
—end example ] The result of the alignof operator reflects the alignment requirement of the type in the
complete-object case.
