Default value for allocatable derived type component

[jacobwilliams]

Allow for the default state of allocatable variables to be allocated with a specified value:

type my_type
  integer :: i = 1  ! this has "always" been possible
  character(len=:),allocatable :: str = 'default'  ! this is not currently possible
end type my_type

I'd also like to be able to define parameters with allocatable components like so:

  type(my_type),parameter :: t1 = my_type(1,'option 1')
  type(my_type),parameter :: t2 = my_type(2,'option 2')
  type(my_type),dimension(2),parameter :: list_of_ts = [t1,t2]

[certik]

@jacobwilliams thanks for reporting it. This is related to #22 a bit. I thought that when you write integer :: i = 1, it is setting the save attribute automatically, which is rarely what you want.

[aradi]

I think, as long as it is an assignment in a derived type declaration, it does not have any direct implications for the save attribute of eventual instances of it. However, the spirit of this kind of assignment could be also carried over to proposal #22 by allowing default values for optional allocatable dummy arguments as well.

[milancurcic]

Implicit save behavior is relevant only in procedures.

However, now I realize that in the context of default value for derived type components, which I've been happily using for years, the type constructor uses the same style as proposed in #22. For example:

type my_type
  integer :: i = 1  
end type my_type

type(my_type) :: t1 = my_type()  ! this works, t1 % i is 1
type(my_type) :: t2 = my_type(3) ! this works also, t2 % i is 3

So, the proposal #22 (sorry to hijack the thread :)) is consistent with the current syntax for default values of derived type components, except there we need an extra attribute (default) to work around the implicit save behavior.

I like this proposal also. It could be merged with #22, or have two separate smaller proposals. We should make whatever is easier to digest by the committee.

[jacobwilliams]

Frankly, I think the implicit save behavior should be removed. It is a malicious anti-feature of the language. But, of course it will be be with us until the end of time (like implicit typing I suppose).

[certik]

@jacobwilliams I created #40 for removing the implicit save behavior.