sizeof *this object

Question

Code:

#include 

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

class d_myc : public myc {
    int d_dummy;
};

int mai         


        

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);}
};

i.e. *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.