Dynamic Class

[Lee__Ho]

In this simple program, when I compile the program it will give me an error in regard to "the allocated BoundaryConditionLTU is already allocated". I know the problem but I don't know how to fix it. Please let me know how to fix this. Thank you

    :
    :
    ! ClassBoundaryCond is an abstract class. Note: ClassDirichlet_BC, ClassNeumann_BC, and ClassMix_B are the inheirtance of the ClassBoundaryCond (Extended type of the ClassBoundaryCond).
    class(ClassBoundaryCond),allocatable,Dimension(:) :: BoundaryConditionLTU

    subroutine initiateBoundaryCondition()
    implicit none

    integer :: i
    ! Date : 06/24/2019
    ! Arguments
    ! Body
    do i=1,6
        if (leftBC(i) == 0) then
            allocate(ClassDirichlet_BC::BoundaryConditionLTU(i))
        else if (leftBC(i) == 1) then
            allocate(ClassNeumann_BC::BoundaryConditionLTU(i))
        else if (leftBC(i) == 2) then
            allocate(ClassMix_BC::BoundaryConditionLTU(i))
        end if
    end do
    end subroutine initiateBoundaryCondition
 

[Lee__Ho]

Note ClassDirichlet_BC, ClassNeumann_BC, and ClassMix_BC are exended type of ClassBoundaryCond. Actually they are the inheritance of ClassBoundaryCond.

[Ferdinand_T_]

It appears you intend to populate an array "BoundaryConditionLTU" with boundary conditions of different dynamic types, depending on the input "leftBC".

There are two problems in your code:
(1) An array can only be allocated once and as a whole (array elements cannot be allocated individually)
(2) All array elements must be of equal type (one cannot mix different dynamic types in a single array)

The standard solution is to create a container type, e.g.

type :: ClassBoundaryCondContainer
    class(ClassBoundaryCond), allocatable :: bc
end type

and then to work with an array of containers instead:

type(ClassBoundaryCondContainer), dimension(:), allocatable :: BoundaryConditionLTU

Now these containers can be individually allocated and accessed via BoundaryConditionLTU(i)%bc, and your code would only need to change minimally:

! Make room for 6 BC's
allocate(BoundaryConditionLTU(6))

! Allocate individual BC's depending on leftBC
do i=1,6
    if (leftBC(i) == 0) then
        allocate(ClassDirichlet_BC::BoundaryConditionLTU(i)%bc)
    else if (leftBC(i) == 1) then
        allocate(ClassNeumann_BC::BoundaryConditionLTU(i)%bc)
    else if (leftBC(i) == 2) then
        allocate(ClassMix_BC::BoundaryConditionLTU(i)%bc)
    end if
end do

I hope that was useful?
Kind Regards, Ferdinand