What are POD types in C++?

Question

I've come across this term POD-type a few times. What does it mean?

Answer 1

The concept of POD and the type trait std::is_pod will be deprecated in C++20. See this question for further information.

Answer 2

A POD (plain old data) object has one of these data types--a fundamental type, pointer, union, struct, array, or class--with no constructor. Conversely, a non-POD object is one for which a constructor exists. A POD object begins its lifetime when it obtains storage with the proper size for its type and its lifetime ends when the storage for the object is either reused or deallocated.

PlainOldData types also must not have any of:

• Virtual functions (either their own, or inherited)
• Virtual base classes (direct or indirect).

A looser definition of PlainOldData includes objects with constructors; but excludes those with virtual anything. The important issue with PlainOldData types is that they are non-polymorphic. Inheritance can be done with POD types, however it should only be done for ImplementationInheritance (code reuse) and not polymorphism/subtyping.

A common (though not strictly correct) definition is that a PlainOldData type is anything that doesn't have a VeeTable.

Answer 3

POD stands for Plain Old Data - that is, a class (whether defined with the keyword struct or the keyword class) without constructors, destructors and virtual members functions. Wikipedia's article on POD goes into a bit more detail and defines it as:

A Plain Old Data Structure in C++ is an aggregate class that contains only PODS as members, has no user-defined destructor, no user-defined copy assignment operator, and no nonstatic members of pointer-to-member type.

Greater detail can be found in this answer for C++98/03. C++11 changed the rules surrounding POD, relaxing them greatly, thus necessitating a follow-up answer here.