Accessing derived-data type array components via pointers

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8

Maybe the problem is in trying to write C++ in any other language?

You can have a read-only look at private values through a callback that implements whatever you need:

module basemod
    use ISO_FORTRAN_ENV, RWP => REAL64

    implicit none
    private

    public :: RWP

    type, public :: mytype_t
        private
        real(RWP), allocatable :: myarray(:)
    contains
        procedure :: access_privates
    end type

    interface mytype_t
        module procedure mytype_t_new
    end interface

    abstract interface
        subroutine i_privates_cb(myarray)
            import
            real(RWP), intent(in) :: myarray(:)
        end subroutine
    end interface
    public :: i_privates_cb

contains
    pure function mytype_t_new(array) result(new)
        type(mytype_t) :: new
        real(RWP), intent(in) :: array(:)
        new%myarray = array
    end function

    subroutine access_privates(self, cb)
        class(mytype_t), intent(in) :: self
        procedure(i_privates_cb) :: cb

        if (.not. allocated(self%myarray)) then
            call cb([real(RWP) ::])
            return
        endif

        call cb(self%myarray)
    end subroutine
end module basemod

module myimpl
    use basemod

    implicit none
    private

    public :: process

contains
    subroutine process()
        type(mytype_t) :: t

        call t%access_privates(cb)

        t = mytype_t([real(RWP) :: (i, integer :: i = 1, 100)])
        call t%access_privates(cb)

    contains
        subroutine cb(array)
            real(RWP), intent(in) :: array(:)

            if (size(array) == 0) then
                print'(/a)','there''s no data'
                return
            endif

            print'(/a)','array is:'
            print'(10(f8.4,:,1x))',array
        end subroutine
    end subroutine
end module myimpl

use myimpl

implicit none

call process()
end

My gfortran version still doesn’t like the declaration of i within the implied-do, but ifx is fine with it:

$ ifx readonly-access.f90 && ./a.out 

there's no data

array is:
  1.0000   2.0000   3.0000   4.0000   5.0000   6.0000   7.0000   8.0000   9.0000  10.0000
 11.0000  12.0000  13.0000  14.0000  15.0000  16.0000  17.0000  18.0000  19.0000  20.0000
 21.0000  22.0000  23.0000  24.0000  25.0000  26.0000  27.0000  28.0000  29.0000  30.0000
 31.0000  32.0000  33.0000  34.0000  35.0000  36.0000  37.0000  38.0000  39.0000  40.0000
 41.0000  42.0000  43.0000  44.0000  45.0000  46.0000  47.0000  48.0000  49.0000  50.0000
 51.0000  52.0000  53.0000  54.0000  55.0000  56.0000  57.0000  58.0000  59.0000  60.0000
 61.0000  62.0000  63.0000  64.0000  65.0000  66.0000  67.0000  68.0000  69.0000  70.0000
 71.0000  72.0000  73.0000  74.0000  75.0000  76.0000  77.0000  78.0000  79.0000  80.0000
 81.0000  82.0000  83.0000  84.0000  85.0000  86.0000  87.0000  88.0000  89.0000  90.0000
 91.0000  92.0000  93.0000  94.0000  95.0000  96.0000  97.0000  98.0000  99.0000 100.0000

And again, no pointers were harmed during the process, :laughing:.