CODEWITHSUNDEEP
c++c++11c++14
tmlen
tmlen

Reputation: 9090

Assumptions on memory layout of objects

For which categories of types (standard layout, POD, trivial type, alignment constraints...) are the following assumptions valid?

B is a subclass of A, and B* b a pointer to a B object.

  1. A* a = b has the same address as b, i.e.

    static_cast<A*>(b) == reinterpret_cast<A*>(b)

  2. B bs[n] is an array of B objects.

    &bs[i] == static_cast<B*>(reinterpret_cast<unsigned char*>(bs) + i * sizeof(B))

  3. offsetof can be used to access data members of A and of B, from b:

    int i = *static_cast<int*>(
    reinterpret_cast<unsigned char*>(b) + offsetof(B, m_i));

EDIT: Changed void* to unsigned char* for pointer arithmetic (with 1 byte unit)

Upvotes: 4

Views: 156

Answers (1)

Kerrek SB
Kerrek SB

Reputation: 477040

If A and B are both standard-layout and b points to a most-derived object of class B, then the first assertion should be true.

The offsetof macro can be used on members of standard-layout types.

The second point of the question is true for any type B because that's how arrays are defined.

Upvotes: 4

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