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Smart-Pointers Example

The user_object_smart_pointer.f90 file demonstrates the use of Smart-Pointers, including

  • A module that defines user_object_t and user_object_ptr_t types,
  • A submodule defining a constructor funciton and a free final subroutine,
  • A main program with a block construct that causes finalization of the user_object when the program reaches the end block statement.

This example exhibits several important subtleties:

  1. Smart-Pointers automate object finalization, eliminating the need for allocatable objects.
  2. The main program source-allocates a raw user_object pointer and then passes the pointer to a user_object_ptr_t() constructor.
  3. The user_object_ptr_t() constructor nullifies the received pointer to encourage the intended practice in which all pointers associated with the object are reference-counted pointers.
  4. All assignments in the main program and its internal subroutine perform shallow copies, thereby creating new references to user_object without copying the object.

With gfortran 13.0.1 20230321 or later in your PATH, run the example as follows:

fpm run --example user_object_smart_pointer

or with the NAG Fortran compiler in your PATH, use

fpm run --example user_object_smart_pointer --compiler nagfor --flag -fpp

either of which should produce the following output:

 Allocating user_object pointer.
 Defining smart_pointer_1.
 Reference count =  1
 Copying smart_pointer_1 into smart_pointer_2.
 Reference count =  2
 Copying smart_pointer_2 into smart_pointer_3.
 Reference count =  3
 smart_pointer_3 going out of scope.
 Reference count =  2
 smart_pointer_1 and smart_pointer_2 going out of scope
 free(): user_object deallocated