Why is C++11’s POD “standard layout” definition the way it is?

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You are allowed to cast a standard layout class object address to a pointer to its first member and back by one of the later paragraphs, which is also often done in C:

struct A { int x; };
A a;

// "px" is guaranteed to point to a.x
int *px = (int*) &a;

// guaranteed to point to a
A *pa = (A*)px;

For that to work, the first member and the complete object have to have the same address (the compiler cannot adjust the int pointer by any bytes because it can’t know whether it’s a member of an A or not).

Finally, what would go wrong if more than one constituent class had data members?

Within a class, members are allocated in increasing addresses according to the declaration order. However C++ doesn’t dictate the order of allocation for data members across classes. If both the derived class and base class had data members, the Standard doesn’t define an order for their addresses on purpose, so as to give an implementation full flexibility in layouting memory. But for the above cast to work, you need to know what is the “first” member in allocation order!

What would go wrong if the first data member was also a base class?

If the base class has the same type as the first data member, implementations that place the base classes before the derived class objects in memory would need to have a padding byte before the derived class object data members in memory (base class would have size one), to avoid having the same address for both the base class and the first data member (in C++, two distinct objects of the same type always have different addresses). But that would again make impossible to cast the address of the derived class object to the type of its first data member.

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