Nonplainolddata

Nonplainolddata, often written as non-POD, is a label used in C++ to describe types that do not qualify as plain old data. Plain old data historically referred to simple aggregates that behave like C structs: no user-provided constructors or destructors, no virtual functions, and a predictable memory layout. In modern C++, POD is defined as a type that is both standard-layout and trivially copyable; therefore any type that fails one of these properties is considered non-POD.

Non-POD types include classes with user-provided constructors, destructors, or copy/move operations, as well as classes with virtual functions or non-standard layouts. By contrast, POD types have a straightforward memory representation and can be copied with simple bitwise operations in many contexts. The distinction matters for low-level operations such as binary serialization, memory-mapped I/O, or interfacing with C APIs, where the exact layout and trivial copy behavior are assumed.

Implications of being non-POD include the need to properly construct and destroy objects, and to rely on the type’s copy and move semantics rather than assuming trivial copying. Raw memory operations like memcpy or treating an object as a blob are unsafe for non-POD types. When interoperability or performance relies on a flat memory layout, developers often wrap non-POD data in POD-structs or provide explicit serialization routines.

Notes: In modern C++, POD is defined as the conjunction of standard-layout and trivially copyable. The is_pod trait is deprecated in favor of checking std::is_standard_layout and std::is_trivially_copyable.