Why are there restrictions on C 11's POD standard layout?

Understanding the Restrictions on C 11's POD Standard Layout

The relaxed definition of Plain Old Data (POD) structures in C 11 introduces specific constraints to ensure standard layout and compatibility. The restrictions revolve around the following key points:

Unique Access Control for Data Members:

Permitting data members with varying access controls would invalidate the assumption that all data members can be accessed through a common pointer, which is crucial for conversions between object addresses and pointers to member functions.

Ordering of Base Classes and Data Members:

If the first data member is also a base class (e.g., struct Bad), it creates ambiguity in determining the order of allocation for data members within the derived class and the base class, undermining the predictability of member addresses.

Multiple Classes with Data Members:

When multiple constituent classes have data members (e.g., derived class and base class), the Standard intentionally leaves the order of allocation across these classes undefined, providing flexibility for memory layout optimizations. However, for the conversion between object addresses and member pointers to work correctly, the first member in allocation order must be known, which is not guaranteed in such scenarios.

In summary, these restrictions in the POD standard layout definition prevent potential inconsistencies and ensure the reliability of pointer conversions and memory management in C 11.

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