*this object 的尺寸

Question

Code:

#include <cstdio>

class myc {
    int dummy;
public:
    int si(){return sizeof(*this);}
};

class d_myc : public myc {
    int d_dummy;
};

int main() {
    myc a;
    d_myc b;
    printf("%d %d\n%d %d", a.si(), b.si(), sizeof(a), sizeof(b));
    return 0;
}

output:

4 4
4 8

I expected:

4 8
4 8

Why were my expectations wrong?

Answer 1

sizeof is resolved at compile-time, not run-time. So sizeof(*this) is equivalent to sizeof(myc) .

Answer 2

I do not know "Why were your expectations wrong" because I cannot read minds:). But if you wrote sizeof(myc), sizeof(d_myc) the compiler would generate exactly the same code as for what you have coded above. myc has 1 int, assuming 32bit, so 4 bytes. d_myc has 2 ints => 8 bytes.

Answer 3

This is resolved at compile time:

class myc {
    int dummy;
public:
    int si(){return sizeof(*this);}
};

ie *this is always myc and will never be d_myc . To get what you want you will have to override the function in d_myc to do the same in the derived as the base. This is because sizeof(d_myc) includes the base class too.