What is a scalar Object in C++?

31
votes

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;
3
c++typestheory
I think you may want to clean up the spelling and syntax of your introductory paragraph, and possibly expanind on it a bit if you want useful answers – tletnes
Do you mean "types" and "aggregate"? – us2012
@us2012: "Tapes and Arrogates" should be an indie college band. – Kerrek SB

3 Answers

46
votes

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.

34
votes

I think this would be a more comprehensive answer:

enter image description here

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.

7
votes

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.

C++ Type Categories