What is a scalar Object in C++?

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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.

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