Why does C 11 define a strict Standard Layout Sequence for classes?

Understanding the Rational Behind C 11's POD Standard Layout Definition

The Standard Layout Sequence (SLS) of a class in C 11 is meticulously defined to facilitate efficient data transfer operations. The rationale behind its stringent constraints lies in the following considerations:

Consistent Access Control for Data Members

The requirement for uniform access control (public or private) among all non-static data members ensures that when an object's address is cast to a pointer to its first member, the access level of the member can be reliably determined.

Simplified Data Member Ordering

The restriction on having multiple base classes with non-static data members ensures a consistent and deterministic allocation order for data members within an object. Without this rule, the compiler would lack the ability to determine which member would be allocated first, making the cast from an object address to a pointer to its first member unreliable.

Avoiding Address Conflicts

The prohibition against base classes having the same type as the first non-static data member prevents potential address conflicts. In memory layouts where base classes are placed before derived class objects, a padding byte would be required to separate the base class and the derived class data member, complicating the casting process.

To illustrate the potential consequences of violating these constraints:

• Varying Access Controls: If data members could have different access levels, it would be challenging to determine the appropriate access level when casting an object address to a pointer to its first member. This could lead to unpredictable or erroneous results.
• First Data Member as Base Class: With this construction, the base class and the first data member would have the same address, violating the assumption that distinct objects of the same type have unique addresses. This would make the cast from an object address to a pointer to its first member unreliable.

By adhering to these restrictions, C 11's SLS ensures the reliable and consistent casting of object addresses, facilitating efficient data transfer operations.

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