Why does the size of a derived class include private members from the base class?

Question

I have the following code:

class A {
  private:
    int i;
 };

class B : public A {
 private:
  int j;
 };

When I check sizeof(B)

Answer 1

You either misunderstand sizeof or your misunderstand the layout (in memory) of C++ objects.

For performance reason (to avoid the cost of indirection), compilers will often implement Derivation using Composition:

// A
+---+
| i |
+---+

// B
+---+---+
| A | j |
+---+---+

Note that if private, B cannot peek in A even though it contains it.

The sizeof operator will then return the size of B, including the necessary padding (for alignment correction) if any.

If you want to learn more, I heartily recommend Inside the C++ Object Model by Stanley A. Lippman. While compiler dependent, many compilers do in fact use the same basic principles.