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gfortran: OOP features [Fortran 2003/2008]

OOP here refers to "Object-Oriented Programming", which was introduced to Fortran in the 2003 standard. The purpose of this page is to document which features have been implemented in gfortran so far, what is still missing and which problems can be expected.

Summary:

gfortran 4.4: type extension, abstract types, non-polymorphic type-bound procedures
gfortran 4.5: experimental support for polymorphic entities (CLASS variables)
gfortran 4.6: almost complete implementation of scalar polymorphism (GSoC 2010 project of Janus Weil)
gfortran 4.7: polymorphic arrays, constructors, bug fixes and minor enhancements
gfortran 4.8: unlimited polymorphism (CLASS(*)) and various bug fixes
gfortran 4.9: finalization & polymorphic deallocation
gfortran 5: bug-fixing
gfortran 6: bug-fixing
gfortran 7: derived-type I/O
gfortran 8: parametrized derived types

Contents

gfortran: OOP features [Fortran 2003/2008]

Implemented features

The following features are available in gfortran 4.5:

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 (only partially working)
allocatable scalars with allocatable components (partially working, fully in 4.6)

These additional features are available in 4.6:

improved handling of TBPs to make polymorphism work between libraries/programs and complicated inheritance patterns
ALLOCATE with MOLD
STORAGE_SIZE intrinsic function
proper handling of generic TBPs
ASSOCIATE construct (except some special cases)
(re)allocate on assignment (non-polymorphic)
allocatable character length scalar variables (deferred-length type parameter)

These additional features are available in 4.7:

polymorphic arrays
generic procedures with same name as a type ("constructor")
lots of bug fixes
enhanced diagnostics
performance improvements

These additional features are available in 4.8:

unlimited polymorphism: CLASS(*)
allocatable character length arrays (PR45170)
bug fixes (especially related to allocatable character length scalar variables)

These additional features are available in 4.9:

proper polymorphic deallocation
finalization ("destructor"), note: finalization is currently only done for a subset of the situations in which it should occur
deferred-length character components

These additional features are available in version 7:

user-defined derived-type I/O (PR48298)
Intrinsic assignment to polymorphic variables (F08) (PR43366)

These additional features are available in version 8:

parametrized derived types

Unimplemented features

Remaining finalization tasks (PR37336)
ASSOCIATE construct (some fine points, cf. PR38936)

Internal Representation

The current implementation is such that each class(t) variable consists of a class container (named __class_ X) with two fields (PR53971):

_data: a pointer to a struct of the declared type t
_vptr: a pointer to a vtab entity (see below)

For each derived type t we then construct a vtab entity (called __vtab_ X), which is of a type __vtype_ X and contains the following fields:

_hash: a unique hash identifier for this type
_size: the storage size of the type t (in bytes)
_extends: a pointer to the vtab of the parent type (or NULL if the type is not an extension)
_def_init: a pointer to a default-initialized variable of type t
_copy: a procedure pointer to a deep-copy function used for ALLOCATE with polymorphic SOURCE=
_final: a procedure pointer to a wrapper function with assumed-rank argument, which handles finalization, including the deallocation and finalization of the allocatable components (cf. PR46321 and PR37336)
a number of procedure pointer components for the implementation of polymorphic type-bound procedure calls

In the naming of class containers, vtabs and vtypes, the X is usually being replaced by a combination of module name and type name. However, if the resulting string is too long, we insert a hashed string instead, which is a hex representation of the type's _hash value.

Object-oriented Fortran codes

Here are some examples of codes that make use of object-oriented features from Fortran 2003 (certain not the most comprehensive list, though) ...

Literature

Open Problems

[Here is a list of all OOP-related bugs.]

PRs related to CLASS arrays (10):

Issues listed in the patch submission email for polymorphic arrays
PR46991: polymorphic assumed-size actual arguments
PR47506: [Fortran 90+] Assumed-size array checks (polymorphic and component)
PR51610: Class container does not properly handle POINTER and TARGET
PR55824: ICE with ALLOCATE and SOURCE= TRANSPOSE/RESHAPE
PR55825: Bogus rank error with CLASS pointer assignment using structure constructors
PR56691: Allocatable array: wrong offset when passing to CLASS dummy
PR56765: compilation errors/ICE with polymorphic array
PR57284: ICE with find_array_spec for polymorphic arrays
PR58043: Incorrect behaviour of polymorphic array
PR58331: Bogus rank checking with explicit-/assumed-size arrays and CLASS

Also the implementation of SELECT TYPE still has a few bugs (6):

PR41599: SELECT TYPE with associate-name => exp: Use (sometimes) a restricted pointer
PR52265: TREE dump confusing with nested SELECT TYPE
PR54322: Wrong TARGET-attribute handling with CLASS IS/TYPE IS
PR55849: Implement temporary support for SELECT TYPE(name => array ( [vector-subscript] ))
PR57116: ICE for pointer assignment inside SELECT TYPE
PR58906: SELECT TYPE with CLASS IS generates ICE

Bugs concerning type-bound generics & operators (6):

PR41951: Not diagnosing ambiguous operators (TB vs. INTERFACE)
PR46897: type-bound defined ASSIGNMENT(=) not used for derived type component in intrinsic assign
PR48786: Generic ambiguity check too strict for polymorphic dummies
PR53694: GENERIC type-bound procs should be available without part-ref syntax
PR57696: Defined assignment for components not used when those are ALLOCATABLE
PR58620: Defined assignment not called for TYPE when the type's extension is used

Miscellaneous (compile-time) bugs (8):

PR47805: Overridding hidden (private) TPB is rejected
PR49213: gfortran rejects structure constructor expression
PR49592: Non-polymorphic ALLOCATE with polymorphic SOURCE= rejected
PR51961: ALLOCATE with MOLD= rejects if source-expr has a different rank
PR52227: TARGET attribute mishandled in polymorphic types
PR53951: Later TARGET statement ignored for CLASS
PR58557: Issues with CLASS/TYPE functions in array constructors: reject valid, memory leaks, invalid free
PR58857: CLASS wrongly rejected in BLOCK DATA

Accepts-invalid, ICE-on-invalid and diagnostic bugs (5):

PR37222: Checks when overriding type-bound procedures are incomplete
PR50252: Error message on "call x%y" (x not declared) can be more informative
PR51208: ALLOCATE with SOURCE= or MOLD=: Diagnose if variable occurs twice
PR54880: ICE in gfc_create_module_variable, at fortran/trans-decl.c:4013
PR59103: Reject deallocate/intent(out) for polymorphic var in PURE procedures (IR F08/0033)

Runtime problems (wrong code, missed optimization) (9):

PR46783: TRANSFER with polymorphic MOLD=
PR47505: Intrinsics which should operate on polymorphic objects (BT_CLASS)
PR49475: [debugging] Add DWARF info for Fortran's OOP features (extension, member functions)
PR51284: CLASS and VALUE attribute: No copy to temporary done
PR53800: Wrong copy-in/copy-out when passing CLASS array to assumed-shape TYPE
PR54035: TBP wrongly binds to a generic name if the specific name is the same
PR54618: wrong-code with CLASS(...), INTENT(OUT) -- and OPTIONAL or ALLOCATABLE
PR57590: wrong code with class variables of different shapes
PR58644: Missing .data ref in passing a CLASS array as actual argument to a TYPE

Extended features of Fortran 2008 (4):

PR46371: [Coarray] SELECT TYPE: scalar coarray variable is rejected
PR52994: [F08] internal compiler error: in gfc_trans_assignment_1, at fortran/trans-expr.c:6881
PR56496: [F08] ICE with TYPE(*) coarray and SELECT TYPE
PR57710: [F08] _vptr not set for allocatable CLASS component inside BLOCK

Fixed Bugs

Fixed in 4.5.x (36)

Fixed in 4.6.x (103)

Fixed in 4.7.x (58)

Fixed in 4.8.x (49)

Fixed in 4.9.x (46)

Fixed in 5.x (18)

Fixed in 6.x (8)

Fixed in 7.x (27)

Fixed in 8.x (2)