Fortran constructor of abstract class initializing private variables - constructor

Question

I'm beginning with Fortran and I wanted to try a pretty simple example - one abstract class Connection, then its derived class IntervalConnection, while Connection class would have a constructor defined and IntervalConnection would inherit this constructor.
In this constructor I have several private variables, which I need to initialize.
The problem is, I'm getting following error, when trying to create a new object:
Connection.f08:152:10:
con = IntervalConnection(n1_p, n2_p, 5.0)
1
Error: Component ‘input_neuron’ at (1) is a PRIVATE component of ‘connection’
I suppose I'm not defining the constructor new_connection correctly. Could you, please, tell me, how should I implement it in a better way?
My code
module Connection_mod
implicit none
public
! TODO smazat
type :: Neuron
real, private :: state
contains
procedure :: get_state => get_state_impl
procedure :: set_state => set_state_impl
end type Neuron
!------------------!------------------------------------------------------------------------
! Type definitions !
!------------------!
type, abstract :: Connection
class(Neuron), pointer, private :: input_neuron
class(Neuron), pointer, private :: output_neuron
real, private :: weight
contains
procedure :: adjust_weight => adjust_weight_impl
! Getters and setters
procedure :: get_input_neuron => get_input_neuron_impl
procedure :: get_output_neuron => get_output_neuron_impl
procedure :: get_weight => get_weight_impl
end type Connection
type, extends(Connection) :: IntervalConnection
contains
procedure :: pass_signal => pass_signal_impl
end type IntervalConnection
!------------!------------------------------------------------------------------------------
! Interfaces !
!------------!
interface Connection
module procedure new_connection
end interface Connection
contains
!------------------------!------------------------------------------------------------------
! Method implementations !
! -----------------------!
!--------------!
! class Neuron !
!--------------!
! TODO smazat
function get_state_impl(this) result(state)
class(Neuron), intent(in) :: this
real :: state
state = this%state
end function get_state_impl
! TODO smazat
function set_state_impl(this, new_state) result(ret_this)
class(Neuron), target :: this
real, intent(in) :: new_state
class(Neuron), pointer :: ret_this
ret_this => this
this%state = new_state
end function set_state_impl
!------------------!
! class Connection !
!------------------!
subroutine adjust_weight_impl(this, added_value)
class(Connection), intent(inout) :: this
real, intent(in) :: added_value
this%weight = this%weight + added_value
end subroutine adjust_weight_impl
!--------------------------!
! class IntervalConnection !
!--------------------------!
subroutine pass_signal_impl(this)
! TODO dokoncit
class(IntervalConnection), intent(in) :: this
real :: a
class(Neuron), pointer :: dummy
a=this%weight * this%input_neuron%get_state()
dummy => this%output_neuron%set_state(5.0)
!this%output_neuron%set_state(this%weight * this%input_neuron%get_state())
end subroutine pass_signal_impl
!--------------!------------------------------------------------------------------------
! Constructors !
!--------------!
function new_connection(this, input_neuron, output_neuron, weight) result(ret_this)
class(Connection), target :: this
class(Connection), pointer :: ret_this
class(Neuron), pointer :: input_neuron
class(Neuron), pointer :: output_neuron
real, intent(in) :: weight
ret_this => this
this%input_neuron => input_neuron
this%output_neuron => output_neuron
this%weight = weight
end function new_connection
!-------------------!-------------------------------------------------------------------
! Getters & Setters !
!-------------------!
function get_input_neuron_impl(this) result (input_neuron)
class(Connection), target, intent(in) :: this
class(Neuron), pointer :: input_neuron
input_neuron => this%input_neuron
end function get_input_neuron_impl
function get_output_neuron_impl(this) result (output_neuron)
class(Connection), target, intent(in) :: this
class(Neuron), pointer :: output_neuron
output_neuron => this%output_neuron
end function get_output_neuron_impl
function get_weight_impl(this) result (weight)
class(Connection), intent(in) :: this
real :: weight
weight = this%weight
end function get_weight_impl
end module Connection_mod
program a
use Connection_mod
type(Neuron), target :: n1
type(Neuron), target :: n2
type(Neuron), pointer :: n1_p
type(Neuron), pointer :: n2_p
type(IntervalConnection) :: con
n1_p => n1
n2_p => n2
con = IntervalConnection(n1_p, n2_p, 5.0)
end program a

Answer 1

The function you created as a constructor takes four arguments
function new_connection(this, input_neuron, output_neuron, weight)
but you call it with three arguments
class(IntervalConnection) :: con
con = IntervalConnection(n1_p, n2_p, 5.0)
I moved the assignment out. It is not strictly necessary.
Therefore the generic resolution (TKR) cannot be successful and the default constructor will be tried.
And you also overloaded the Constructor or Connection, but then you call IntervalConection(). These are different!
You probably misunderstood how a constructor works with the arguments and return values. There is no place for any this argument. A constructor is NOT a type-bound procedure. It is not inherited. But the child connection can call the parent constructor if needed. Sometimes you want that, sometimes not. Notice the returned value is type, not class!
function new_interval_connection(input_neuron, output_neuron, weight) result(ret)
type(IntervalConnection) :: ret
class(Neuron), pointer :: input_neuron
class(Neuron), pointer :: output_neuron
real, intent(in) :: weight
ret%input_neuron => input_neuron
ret%output_neuron => output_neuron
ret%weight = weight
end function new_interval_connection
You want to create an instance of make the constructor for IntervalConnection, not for Connection, as you tried
interface IntervalConnection
module procedure new_interval_connection
end interface Connection
con = IntervalConnection(n1, n2, 5.0)