Plain Old Data Structures

In computer science and object-oriented programming, a passive data structure (PDS, not to be confused with IBM's partitioned data sets; also termed a plain old data structure, or plain old data (POD)), is a term for a record, to contrast with objects. It is a data structure that is represented only as passive collections of field values (instance variables), without using object-oriented features.

Passive data structures are appropriate when there is a part of a system where it should be clearly indicated that the detailed logic for data manipulation and integrity are elsewhere. PDSs are often found at the boundaries of a system, where information is being moved to and from other systems or persistent storage and the problem domain logic that is found in other parts of the system is irrelevant. For example, PDS would be convenient for representing the field values of objects that are being constructed from external data, in a part of the system where the semantic checks and interpretations needed for valid objects are not applied yet.

A PDS type in C++, or Plain Old C++ Object, is defined as either a scalar type or a PDS class. A PDS class has no user-defined copy assignment operator, no user-defined destructor, and no non-static data members that are not themselves PDS. Moreover, a PDS class must be an aggregate, meaning it has no user-declared constructors, no private nor protected non-static data, no virtual base classes and no virtual functions. The standard includes statements about how PDS must behave in C++. The type_traits library in the C++ Standard Library provides a function named is_pod that can be used to determine whether a given type is a POD.

In some contexts, C++ allows only PDS types to be used. For example, a union in C++98 cannot contain a class that has virtual functions or nontrivial constructors or destructors. This restriction is imposed because the compiler cannot determine which constructor or destructor should be called for a union. PDS types can also be used for interfacing with C, which supports only PDS.

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