What is a scalar Object in C++?

Short version: Types in C++ are:

• Object types: scalars, arrays, classes, unions

• Reference types

• Function types

• (Member types) [see below]

• void

Long version

• Object types

  • Scalars

    1. arithmetic (integral, float)

    2. pointers: T * for any type T

    3. enum

    4. pointer-to-member

    5. nullptr_t

  • Arrays: T[] or T[N] for any complete, non-reference type T

  • Classes: class Foo or struct Bar

    1. Trivial classes

    2. Aggregates

    3. POD classes

    4. (etc. etc.)

  • Unions: union Zip

• References types: T &, T && for any object or free-function type T

• Function types

  • Free functions: R foo(Arg1, Arg2, ...)

  • Member functions: R T::foo(Arg1, Arg2, ...)

• void

Member types work like this. A member type is of the form T::U, but you can’t have objects or variables of member type. You can only have member pointers. A member pointer has type T::* U, and it is a pointer-to-member-object if U is a (free) object type, and a pointer-to-member-function if U is a (free) function type.

All types are complete except void, unsized arrays and declared-but-not-defined classes and unions. All incomplete types except void can be completed.

All types can be const/volatile qualified.

The <type_traits> header provides trait classes to check for each of these type characteristics.