• OOP

gfortran: OOP features [Fortran 2003]

The upcoming 4.5 release of gfortran will contain some experimental support for polymorphic CLASS entities and object oriented features. The purpose of this page is to document which features have been implemented so far, what is still missing and which problems can be expected.

Implemented features

The following features are available on gfortran 4.5 trunk:

• basic CLASS declarations
• polymorphic pointer assignments
• polymorphic dummy arguments
• polymorphic allocation
• SELECT TYPE statements
• intrinsic functions SAME_TYPE_AS and EXTENDS_TYPE_OF
• dynamic dispatch, polymorphic type-bound procedures

Unimplemented features

• unlimited polymorphism: CLASS(*)
• Improved handling of TBPs to make polymorphism work between libraries/programs and complicated inheritance patterns

• parametrized derived types

• Generic procedures with same name as a type ("constructor", PR39427)

• FINALizing subroutines ("destructor")

• ASSOCIATE construct
• allocatable character length

"fortran-dev" branch

The fortran-dev branch tracks the trunk and contains additional fixes.

Fixed for fortran-dev but not in the trunk/in 4.5

• PR41829: Runtime error with dynamic dispatching

Open Problems

There are still several problematic areas. While e.g. scalar CLASS variables work mostly fine, there are a couple of issues with CLASS arrays:

• PR41539: Calling function which takes CLASS: Rank comparison does not work

• PR41600: SELECT TYPE with associate-name => exp: Arrays not supported

• PR41951: ICE in gfc_match_varspec, at fortran/primary.c:1815

Furthermore, the implementation of type-bound procedures is not perfect yet. Although they often work, there are many cases where polymorphic TBP calls fail at runtime:

• PR41829: Runtime error with dynamic dispatching (fixed in the fortran-dev branch)

• PR42385: poylmorphic operators do not work

Apart from these main problems, there are several minor bugs:

• PR41580: SAME_TYPE_AS and EXTENDS_TYPE_OF - add compile-time simplifcation

• PR41599: SELECT TYPE with associate-name => exp: Use (sometimes) a restricted pointer

• PR41753: segfault with -O2 using methods as actual arguments

• PR42051: ICE on array-valued function with CLASS formal argument

• PR42207: Compile-time errors on typed allocation and pointer function result

• PR42353: Bogus Error: Name 'vtype$...' at (1) is an ambiguous reference ...

Internal Representation

The current implementation is such that each class(t) variable consists of a class container (named '.class.t') with the following fields:

• $data: a pointer to a struct of the declared type t

• $vptr: a pointer to a vtab entity

For each derived type t we then construct a vtab entity (called vtab$t), which is of a type vtype$t and currently contains three fields:

• $hash: the unique hash identifier for this type

• $size: the storage size of the type t

• $extends: a pointer to the vtab of the parent type (or NULL if the type is not an extension)

For a correct handling of polymorphic type-bound procedures, the vtypes will have to be extended to contain procedure pointer components for the methods.

Fixed Bugs (29)

• PR41556: Errors in applying operator/assignment to an abstract type

• PR41579: Nesting of SELECT TYPE

• PR41581: Allocation of a CLASS with SOURCE=<class> does not work

• PR41582: Allocation of abstract types requires a type spec or a SOURCE

• PR41583: TYPE IS rejected because of way vtable index is implemented

• PR41585: Reject CLASS(T) as component of TYPE T

• PR41586: Allocatable _scalar_ TYPE/CLASS leak memory

• PR41587: ICE with ALLOCATABLE CLASS components

• PR41608: ICE with CLASS and invalid code

• PR41618: accepts-invalid with CLASS pointer component

• PR41629: gimplification error on valid code

• PR41631: Support CLASS IS () in SELECT TYPE

• PR41648: Type-bound procedures refused

• PR41656: Unresolved GENERIC

• PR41685: ICE: verify_flow_info failed

• PR41706: Calling one TBP as an actual argument of another TBP

• PR41714: ALLOCATE SOURCE= does not properly copy the value from SOURCE

• PR41719: invalid: Intrinsic assignment involving polymorphic variables

• PR41766: SELECT TYPE selector as actual argument with INTENT(INOUT)

• PR41781: bogus undefined label error with SELECT TYPE

• PR41784: ICE in load_derived_extensions

• PR41800: ICE in fold_convert_loc, at fold-const.c:2789

• PR41873: Bogus Error: ABSTRACT INTERFACE must not be referenced...

• PR41937: Error in allocating derived type allocatable components

• PR42053: SELECT TYPE: reject duplicate CLASS IS blocks

• PR42144: deferred TBPs do not work

• PR42167: SELECT TYPE with function return value

• PR42257: Compiler segmentation fault due missing public statement

• PR42335: ICE on CLASS IS (bad_identifier)