Struct Inheritance - Memory requirements

[beakie]

How many bytes of memory will each of these examples require once getNum() has finished executing?

Assuming a char is 1 byte and a pointer is 4?


1) 1 byte? a

struct parentstruct {
  char a;

  char getNum() { return a; };
};

parentstruct temp;

void main() {
  temp.getNum();
}

2) 1 byte? a

struct parentstruct {
  char a;

  virtual char getNum() { return a; };
};

parentstruct temp;

void main() {
  temp.getNum();
}

3) 2 bytes? a + b

struct parentstruct {
  char a;

  virtual char getNum() { return a; };
};

struct childstruct_one : public parentstruct {
  char b;

  char getNum() { return a + b; };
};

childstruct_one temp;

void main() {
  temp.getNum();
}

4) 6 bytes? a + b + pointer

struct parentstruct {
  char a;

  virtual char getNum() { return a; };
};

struct childstruct_one : public parentstruct {
  char b;

  char getNum() { return a + b; };
};

parentstruct * temp;

void main() {
  temp = new childstruct_one;
  temp.getNum();
}

Is this right, please?

[helios]

1. Probably.
2. No. The virtual table takes up space. Exactly how much will depend on the implementation.
3. No. See #2.
4. Doubly no. There's an inherent space overhead when using dynamic allocation, the exact value of which again depends on the implementation. Also see #2.

Although I know what you meant, I'd just like to point out that asking about space requirements for things that are not dynamically allocated is somewhat pointless.

[BlackSheep]

Note that per instance of a derived class you don't have a full virtual table, just a pointer to the table. (Or multiple pointers in the case of multiple inheritance)

[beakie]

Thanks. I appreciate your responses.