Question

Compiler: ifort version 12.1.5

I’m writing some Fortran code and I’d like to make use of some F2003 OOP features, but I’m hitting some stumbling blocks. Paring down the example, I wish to have two derived types A and B, each of which have a pointer to instances of the other. In Fortran, circular dependencies between modules are explicitly disallowed, so these two types would have to reside in the same module. This compiles:

module testModule
implicit none

type A
 type(B),pointer :: b1
end type A

type B
 type(A),pointer :: a1
end type B

contains
[some possibly type-bound procedures]
end module

Now, I want to implement some constructors for these types, and try this code:

module testModule
implicit none

type A
 type(B),pointer :: b1
end type A

interface A
 module procedure A_ctor
end interface

type B
 type(A),pointer :: a1
end type B

interface B
 module procedure B_ctor
end interface

contains
function A_ctor()
 type(A),target :: A_ctor
end function
function B_ctor()
 type(B),target :: B_ctor
end function
end module

Now, this doesn’t compile, throwing an error

This is not a derived type name. [B]

on line 5 above. Why does adding the interfaces throw an error? How does one handle circular dependencies in derived types in Fortran, as one would use a forward class declaration in C++?

Answer 1

Your code is correct Fortran 2003. You are handling the circular type dependency in the correct way. The compiler is in error.

I have experienced and seen others report issues with ifort 12.1.5 and programmer overrides of structure constructors (where there is a generic interface to a function that has a derived type result, with the interface having the same name as the derived type – as you have here). A workaround for this situation to rename the generic interface (and subsequent references) for the constructor function to a name different from the derived type.