Question

Is it currently possible to override the structure constructor in Fortran? I have seen proposed examples like this (such as in the Fortran 2003 spec):

module mymod

  type mytype
    integer :: x
    ! Other stuff
  end type

  interface mytype
    module procedure init_mytype
  end interface

contains
  type(mytype) function init_mytype(i)
    integer, intent(in) :: i
    if(i > 0) then
      init_mytype%x = 1
    else
      init_mytype%x = 2
    end if
  end function
end

program test
  use mymod
  type(mytype) :: x
  x = mytype(0)
end program

This basically generates a heap of errors due to redundant variable names (e.g. Error: DERIVED attribute of ‘mytype’ conflicts with PROCEDURE attribute at (1)). A verbatim copy of the fortran 2003 example generates similar errors. I’ve tried this in gfortran 4.4, ifort 10.1 and 11.1 and they all produce the same errors.

My question: is this just an unimplemented feature of fortran 2003? Or am I implementing this incorrectly?

Edit: I’ve come across a bug report and an announced patch to gfortran regarding this issue. However, I’ve tried using a November build of gcc46 with no luck and similar errors.

Edit 2: The above code appears to work using Intel Fortran 12.1.0.

Answer 1

I consulted my copy of the Fortran 2008 standard. That does allow you to define a generic interface with the same name as a derived type. My compiler (Intel Fortran 11.1) won’t compile the code though so I’m left suspecting (without a copy of the 2003 standard to hand) that this is an as-yet-unimplemented feature of the Fortran 2003 standard.

Besides that, there is an error in your program. Your function declaration:

  type(mytype) function init_mytype
    integer, intent(in) :: i

specifies the existence and intent of an argument which is not present in the function specification, which should perhaps be rewritten as:

  type(mytype) function init_mytype(i)