What are the main difference between TYPE and CLASS?

Dear all,

A quick question, modern Fortran have type and class, but,
What are the main difference between TYPE and CLASS ?

I feel that to some extent it seems they are interchangeable, is it?
I mean I define a type call it AAA, Now if I want a variable B with type AAA, in many cases it seems that, I can do both

type(AAA) :: B

and

class(AAA) :: B

Is it?

Thanks much in advance!

PS, I found a S.O topic,

But I do not fully understand the explanation.
Can someone explain the difference in some simple language? Thanks.

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Well, please note you cannot “do both” generally.

With the polymorphic option i.e., the one with CLASS declaration the object needs to have either the ALLOCATABLE or the POINTER attribute when B is not a dummy argument:

class(AAA), allocatable :: B ! or class(AAA), pointer :: B

Perhaps this suffices as your “main difference”!!

But then should B be a dummy argument, then yes you can “do both”.

Anyways a book such as Modern Fortran Explained can better help answer such questions.

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I wrote about this at Doctor Fortran in “Not My TYPE” - Doctor Fortran (stevelionel.com)

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Intuitively, I guess it might be useful to think class(T) as something like type(<:T) or type(<= T) (= anything that is a subtype of T), in contrast to type(T) (= exactly the type T). So, with type(T) :: foo, we know how to generate actual data or an object on memory, but with class(T) :: foo, we don’t know what to generate actually (on memory)…

(The symbol <: seems to be explained here Subtyping - Wikipedia )

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