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! Source - Page 44 of https://personalpages.manchester.ac.uk/staff/david.d.apsley/lectures/fortran/fortranB.pdf
module Defs
implicit none
private
public point, point2d
type, abstract :: point
contains
procedure(func), deferred :: radius
end type point
abstract interface
real function func( this )
import point
class(point) this
end function func
end interface
type, extends(point) :: point2d
real x, y
contains
procedure :: radius => r2d
end type point2d
contains
real function r2d( this )
class(point2d) this
r2d = sqrt( this%x ** 2 + this%y ** 2 )
end function r2d
end module Defs
program main
use Defs
implicit none
real :: res
class(point), pointer :: ptr
type(point2d), target :: p2d = point2d( 3, 4 )
ptr => p2d
res = ptr%radius()
if (res /= 5.0) error stop
end program main
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