What is a scalar Object in C++?

问题

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;

回答1:

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.

回答2:

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.

回答3:

There is a series of library classes that used for test the type of variables. std::is_scalar can be used to test if an object is a scalar.

A scalar type is a type that has built-in functionality for the addition operator without overloads (arithmetic, pointer, member pointer, enum and std::nullptr_t).

Also a table from here.

来源：https://stackoverflow.com/questions/14821936/what-is-a-scalar-object-in-c

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