What is a scalar Object in C++?

As far as I understand it fundamental types are Scalar and Arrays are aggregate but what about user defined types? By what criteria would I divide them into the two categories?

struct S { int i; int j };
class C { public: S s1_; S s2_ };
std::vector<int> V;
std::vector<int> *pV = &v;

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.

I think this would be a more comprehensive answer:

original document:
http://howardhinnant.github.io/TypeHiearchy.pdf

a scalar is a fundamental except it cannot be void, but it can be a pointer type, or an enum type.

And a fundamental has a keyword in the language. it is easy to recognize when said like that.