求助这段C语言代码

buaaspy

typedef struct ManagedObject {
    VTable *vt_raw;
    union {
    U_32 obj_info;
    POINTER_SIZE_INT padding;
    };
    VTable *vt_unsafe() { return vt_raw; }
    VTable *vt() { assert(vt_raw); return vt_unsafe(); }
    static VTable *allocation_handle_to_vtable(Allocation_Handle ah) {
        return (VTable *) ah;
    }
    static bool are_vtable_pointers_compressed() { return false; }

    static unsigned header_offset() { return (unsigned)(POINTER_SIZE_INT)(&((ManagedObject*)NULL)->obj_info); }

    /// returns the size of constant object header part (vt pointer and obj_info)
    static size_t get_constant_header_size() { return sizeof(ManagedObject); }
    /// returns the size of object header including dynamically enabled fields.
    static size_t get_size() {
        return get_constant_header_size() + (_tag_pointer ? sizeof(void*) : 0);
    }

    U_32 get_obj_info() { return obj_info; }
    void set_obj_info(U_32 value) { obj_info = value; }
    U_32* get_obj_info_addr() {
        return (U_32*)((char*)this + header_offset());
    }

    /**
     * returns address of a tag pointer field in a _non-array_ object.
     * For array objects use VM_Vector::get_tag_pointer_address().
     */
    void** get_tag_pointer_address() {
        assert(_tag_pointer);
        return (void**)((char*)this + get_constant_header_size());
    }

    VMEXPORT static bool _tag_pointer;
} ManagedObject;


typedef struct VM_Vector
{
    ManagedObject object;
    I_32 length;

    static size_t length_offset() { return (size_t)(&((VM_Vector*)NULL)->length); }
    I_32 get_length() { return length; }
    void set_length(I_32 len) { length = len; }

    void** get_tag_pointer_address() {
        assert(ManagedObject::_tag_pointer);
        int offset = sizeof(VM_Vector);
        return (void**)((char*)this + offset);
    }

} VM_Vector;


这段代码是一个开源JVM中关于对象分布的代码，可是看不出它是怎么容纳不同大小的对象数据的
Thanks for any help

Fatihyang

union 结构 好好msdn一下 也算基础吧  其他就不多说