CODEWITHSUNDEEP
c++
Rico
Rico

Reputation: 1

How does the derived class occupy the space of base class in C++

Case 1:

class A {
 private:
  uint32_t a1 : 8;
};

class B : public A {
 private:
  uint32_t a2 : 2;
  uint32_t a3 : 22;
};

size(B) prints 4 Bytes

Case 2:

class A {
 private:
  uint32_t a1 : 8;
  uint32_t a2 : 2;
};

class B : public A {
 private:
  uint32_t a3 : 22;
};

size(B) prints 8 Bytes

So Why these two cases corresponding to 2 different results?

Upvotes: 0

Views: 65

Answers (1)

datenwolf
datenwolf

Reputation: 162164

So Why these two cases corresponding to 2 different results?

Because the compiler is allowed to place struct/class members at any offset it sees fit, as long as the order of members is preserved. And while it's somewhat "predictable" how things are padded and where they end up for "normal" types, any notion of "rules of thumb" go out of the window as soon as bitfields and inheritance are mixed.

Upvotes: 1

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