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Why Does C 11's 'Standard Layout' Require Strict Data Member Constraints?

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Nov 25, 2024 pm 04:25 PM

Why Does C 11's

Understanding C 11's POD "Standard Layout" Definition

C 11 introduced a relaxed definition for Plain Old Data (POD) types, known as "standard layout." This definition specifies strict criteria that a class must satisfy to be considered standard layout. One of the surprising aspects of the definition is its restrictions on data member access controls, data members sharing the same type as base classes, and the presence of multiple data members in constituent classes.

Why Are Varying Access Controls Prohibited?

Allowing data members with varying access controls would disrupt the crucial requirement that the first member and the complete object have the same address. This is necessary for the valid conversion of a standard layout object address to a pointer to its first member.

Why is a Shared Type Between First Data Member and Base Class Prohibited?

If the first data member and the base class share the same type, it introduces ambiguity in memory layout. Implementations that place base classes before derived classes would require a padding byte to prevent the collision of addresses. This would once again hinder the reliable conversion between the object address and its first member pointer.

Consequences of Multiple Data Members in Constituent Classes

Within a single class, members are sequentially allocated in declaration order. However, across classes, standard layout does not specify a fixed allocation order. Having multiple data members in constituent classes introduces uncertainty in determining the "first" member in memory, which is essential for the aforementioned address conversion.

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