Example: Custom value types

Since an unlimited polymorphic argument is used when storing hash table values, no modification is required to set custom derived types as values.

However, when retrieving values it is convenient to define a custom getter routine to correctly obtain the derived type value from the table.

In this demo, a custom getter is defined for a string_t type. The getter must obtain a raw polymorphic allocatable from the hash table, and determine the type using the select type construct. The getter should therefore implement some form of error handling for the case when the requested key is not of the expected derived type.

!> Example program demonstrating how to store and retrieve
!>  custom derived types using `fhash`
program fhash_demo
  use fhash, only: fhash_tbl_t, key=>fhash_key, fhash_key_t
  implicit none

  type string_t
    character(:), allocatable :: s
  end type string_t

  type(fhash_tbl_t) :: tbl
  character(:), allocatable :: key_str
  type(string_t) :: str1, str2

  print *, '# fhash demo program: derived-type-demo'

  str1%s = 'Hello fhash'
  key_str = 'my_key'

  print *, 'Storing value "',str1%s,'" with key: "',key_str,'"'
  call tbl%set(key(key_str), value=str1)

  print *, 'Retrieving value with key: "',key_str,'"'
  call fhash_get_string(tbl,key(key_str),str2)
  print *, '   value = "',str2%s,'"'

  print *

  contains

  !> Define custom getter for string_t type
  subroutine fhash_get_string(tbl,k,string)
    type(fhash_tbl_t), intent(in) :: tbl
    class(fhash_key_t), intent(in) :: k
    type(string_t), intent(out) :: string

    integer :: stat
    class(*), allocatable :: data

    call tbl%get_raw(k,data,stat)

    if (stat /= 0) print *, 'error ', stat! Error handling: key not found

    select type(d=>data)
    type is (string_t)
      string = d
    class default
      ! Error handling: found wrong type
      print *, 'error'
    end select

  end subroutine fhash_get_string

end program fhash_demo

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