C ++-应用于对象类的sizeof函数

Question

I received the following question which was related to sizeof() :

class C
{
public:
    C();
    virtual ~C();

    unsigned char _member0 s[4];
    static long _member1 d;
}

int main()
{
    C vc;
    cout << sizeof(vc);
}

Can someone explain how the sizeof() function is evaluated in this case?

Answer 1

The exact answer could vary from compiler to compiler so in strict sense the answer to your question is this is Implementation Defined .
Considering this to be an interview Q( saw your previous Q ), You should have pointed out the following points:

• A compiler is allowed to add padding bytes to a structure/class,this might add to the size.
• A compiler might add vptr to an class instance,this might add to the size.
• The class members will occupy memory.
• static members do not contribute towards the size of an class object because they do not belong to an instance of class but to the class.

Answer 2

It gives the size of vc. vc is of class C. Each object of class C contains metadata (pointer to vtable), since C contains virtual methods. In addition, C has a data field (the character array).

Hence the size of vc should be the size of a pointer plus four bytes (plus padding, see comment below, thanks).

d is not component of an object of class C, since it is static, hence it does not count.

So we have:

------vc---------             ----vtable for C----           ----statics----
| ptr to vtable | ----------> | pointer to ~C    |           | C::d        |       
|---------------|             | ...              |           | ...         |
| char [4]      |             --------------------           ---------------
-----------------