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 4.5: some experimental support for polymorphic entities (CLASS variables) and object oriented features

• 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(*)), bug fixes.

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(*)
• Bug fixes (especially related to allocatable character length scalar variables)

Unimplemented features

• parametrized derived types

• FINALizing subroutines ("destructor", PR37336)

• ASSOCIATE construct (some fine points, cf. PR38936)

• derived type I/O (PR48298), early draft patch

• allocatable character length arrays and as derived-type components (PR45170)

Open Problems

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

PRs related to CLASS arrays (13):

• Issues listed in the patch submission email for polymorphic arrays

• PR43412: BT_CLASS does not does not set array spec

• PR46991: polymorphic assumed-size actual arguments

• PR47506: [Fortran 90+] Assumed-size array checks (polymorphic and component)

• PR47680: ICE with polymorphic array elements as dummy

• PR51610: Class container does not properly handle POINTER and TARGET

• PR51632: Bogus argument checking for generated _def_init parameter and _copy procedure with CAF

• PR52143: ICE with polymorphic function result in gfc_class_vptr_get

• PR55427: ICE with class-array-pointer result

• PR55824: ICE with ALLOCATE and SOURCE= TRANSPOSE/RESHAPE

• PR55825: Bogus rank error with CLASS pointer assignment using structure constructors

• PR56136: ICE on lhs-reallocation of an object with overloaded (elemental) assignment operator

• PR56691: Allocatable array: wrong offset when passing to CLASS dummy

• PR56765: compilation errors/ICE with polymorphic array

Also the implementation of SELECT TYPE still has a few bugs (7):

• 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] ))

• PR55850: SELECT TYPE issues

• PR55901: type is (character(len=*)) misinterpreted as array

• PR55731: [4.7 Regression] ICE on (wrongly) referencing polymorphic array in select type

Bugs concerning type-bound generics & operators (5):

• 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

• PR49074: Ambiguous generics not diagnosed (Defined assignment w/ CLASS arrays: Incomplete error message)

• PR53694: GENERIC type-bound procs should be available without part-ref syntax

Miscellaneous (compile-time) bugs (13):

• PR45076: gfortran.dg/dynamic_dispatch_6.f03 ICEs with -fprofile-use

• PR47399: ICE with TBP of a PARAMETER

• PR47805: Overridding hidden (private) TPB is rejected

• PR49213: gfortran rejects structure constructor expression

• PR49592: Non-polymorphic ALLOCATE with polymorphic SOURCE= rejected

• PR49630: ICE on obsolescent deferred-length type bound character function

• PR51864: ALLOCATE with polymorphic array constructor as SOURCE=

• PR51961: ALLOCATE with MOLD= rejects if source-expr has a different rank

• PR52010: Intrinsic assignment of a CLASS to a TYPE

• PR52227: TARGET attribute mishandled in polymorphic types

• PR53718: [4.7 regression] gfortran generates asm label twice in the same output file

• PR53951: Later TARGET statement ignored for CLASS

• PR55960: ICE in replace_comp, at fortran/expr.c:4356

Accepts-invalid, ICE-on-invalid and diagnostic bugs (12):

• PR37222: Checks when overriding type-bound procedures are incomplete

• PR42188: F03:C612. The leftmost part-name shall be the name of a data object.

• PR43207: invalid pointer assignment => type%parent

• 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

• PR53655: CLASS-related warnings: "copy" defined but not used; "default initializer" warnings

• PR54578: -Wunused-function warnings for copy functions

• PR54880: ICE in gfc_create_module_variable, at fortran/trans-decl.c:4013

• PR55172: [4.7 Regression] ICE on invalid: gfc_variable_attr(): Bad array reference

• PR55959: ICE in in gfc_simplify_expr, at fortran/expr.c:1920

• PR56261: seg fault call procedure pointer on polymorphic array

• PR56266: ICE on invalid in gfc_match_varspec

Runtime problems (wrong code, missed optimization) (11):

• PR46321: Polymorphic deallocation

• 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)

• PR51160: Memory leak with abstract type

• 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

• PR55057: wrong result with abstract type

• PR56218: Segfault with allocatable intent(out) derived type argument having allocatable component

Extended features of Fortran 2008 (8):

• PR43366: [F08] Intrinsic assign to polymorphic variable

• PR46371: [Coarray] SELECT TYPE: scalar coarray variable is rejected

• PR51947: [F08] Polymorphic coarrays: Bogus errors for dummy arguments

• PR52994: [F08] internal compiler error: in gfc_trans_assignment_1, at fortran/trans-expr.c:6881

• PR54756: [F08] Should reject CLASS, intent(out) in PURE procedures

• PR55887: [F08] ICE with CLASS and data-target pointer association in (default) initialization

• PR55905: [F08] ICE for polymorphic dummy argument with an allocatable coarray component

• PR56496: [F08] ICE with TYPE(*) coarray and SELECT TYPE

Non-standard extensions (1):

• PR52531: Compilation fails with polymorphic dummy argument and OpenMP

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: [planned for GCC 4.8 and later] a procedure-pointer to a wrapper function with assumed-rank argument, which handles finalization, including the deallocation and finalization of the allocatable components. (Will be implemented for 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.

Fixed Bugs

Fixed in 4.5 (34)

• 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)

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

• PR43169: gfortran rejects pure procedure with select type construct

• PR43199: ICE when reading module file: find_array_spec(): Component not found

• PR43256: TBP with missing optional arg

• PR43291: [4.5 regression] Type mismatch in argument; passed CLASS(t1) to CLASS(t2)

Fixed in 4.6 (84)

• PR40117: [F2008] Type-bound procedure: allow list after PROCEDURE

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

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

• PR41829: Runtime error with dynamic dispatching

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

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

• PR42385: poylmorphic operators do not work

• PR43388: [F2008] ALLOCATE with MOLD=

• PR43696: Bogus error: Passed-object dummy argument must not be POINTER

• PR43895: internal compiler error: verify_ssa failed

• PR43896: ICE in gfc_conv_variable, at fortran/trans-expr.c:551

• PR43986: gfortran.dg/dynamic_dispatch_4.f03 doesn't work on Linux/ia64

• PR43945: Derived type with GENERIC: resolved to the wrong specific TBP

• PR43990: ICE in output_constructor_regular_field, at varasm.c:49

• PR44044: SELECT TYPE with class-valued function

• PR44047: SELECT TYPE with associate-name and allocatable selector

• PR44064: ICE with file containing two modules and one program

• PR44065: Undefined reference to vtab$...

• PR44211: ICE with TBP of pointer component of derived type array

• PR44212: ICE when defining a pointer component before defining the class and calling a TBP then

• PR44434: ICE in in gfc_add_component_ref

• PR44541: wrong code for polymorphic variable with INTENT(OUT)/Alloc w/ MOLD

• PR44549: [F2008] Type-bound procedure: bogus error from list after PROCEDURE

• PR44558: ICE on invalid code: called TBP subroutine as TBP function

• PR44565: [4.6 Regression] ICE in gimplify_expr with array-valued generic TBP

• PR44596: Dynamic dispatch uses broken types

• PR44614: Wrongly importing a symbol into an interface without IMPORT

• PR44616: ICE if CLASS(foo) is used before its definition

• PR44649: F2008: storage_size intrinsic (also working for polymorphic types)

• PR44696: ASSOCIATED fails on polymorphic variables

• PR44868: Error recovery: ICE after invalid TBP call

• PR44869: generic TBPs not initialized properly

• PR44912: Segmentation fault on TBP

• PR44917: Detect ambiguous specifics in a generic TBP interface

• PR44925: C_LOC with CLASS pointer

• PR44926: Ambiguous interface not detected

• PR44936: Generic TBP not resolved correctly at compile time

• PR44962: ICE with specification expression SIZE(<CLASS>)

• PR45004: Segfault with allocatable scalars and move_alloc

• PR45271: Polymorphic code breaks when changing order of USE statements

• PR45384: double free with SELECT TYPE

• PR45420: poylmorphic TBP calls in a CLASS DEFAULT clause

• PR45438: [4.6 Regression] ICE with -fcheck=pointer

• PR45439: SELECT TYPE bogus complaint about INTENT

• PR45451: Inconsistent status of ALLOCATABLE components inside CLASS variables

• PR45456: [4.6 Regression] Bogus pointer initialization error on pointer-valued TBP

• PR45494: Wrong dummy argument not rejected

• PR45674: Undefined references for extended types

• PR45746: ICE in fold_convert_loc: pointer to allocatable array with select type

• PR45827: [4.6 Regression] mio_component_ref(): Component not found

• PR45848: ICE on invalid code in fortran/symbol.c:2410

• PR45933: [4.6 regression] ICE in gfc_add_component_ref, at fortran/class.c:77

• PR45961: [4.6 Regression] Problem with polymorphic type-bound operators

• PR46161: Invalid: Passing non-polymorphic to allocatable polymorphic dummy

• PR46174: ALLOCATE with SOURCE: Deep copy missing

• PR46196: gfortran compiles invalid generic TBP: dummy arguments are type compatible

• PR46313: class container naming collisions

• PR46330: [4.6 Regression] ICE after revision 166368

• PR46344: [4.6 Regression] ICE with allocatable CLASS components

• PR46370: [Coarray] ALLOCATE: Error allocating CLASS coarrays

• PR46408: Segfault when running gfortran.dg/class_allocate_6.f03

• PR46413: ICE when printing a polymorphic type

• PR46448: [4.6 Regression] symbol '__copy_...' is already defined

• PR46581: [4.6 Regression] segfault in SELECT TYPE with associate-name

• PR46809: ICE with -fcheck=pointer for CLASS IS

• PR46838: Initialization of polymorphic allocatable components

• PR46849: MODULE PROCEDURE resolution does not work in BLOCK or SELECT TYPE

• PR46971: [4.6 Regression] ICE on long class names

• PR47024: STORAGE_SIZE (for polymorphic types): Segfault at run time

• PR47082: [4.6 Regression] ICE in gfc_conv_component_ref

• PR47180: EXTENDS_TYPE_OF returns the wrong result for disassociated polymorphic pointers

• PR47189: calling STORAGE_SIZE on a NULL-initialized class pointer

• PR47194: EXTENDS_TYPE_OF still returns the wrong result if the polymorphic variable is unallocated

• PR47204: [4.6 Regression] ICE: SIGSEGV in gfc_free_namespace (symbol.c:3323) on invalid code

• PR47455: [4.6 Regression] internal compiler error: in fold_convert_loc, at fold-const.c:2028

• PR47463: ICE in gfc_add_component_ref

• PR47565: [4.6 Regression] Segfault with TBP

• PR47572: Invalid: Allocatable polymorphic with init expression

• PR47637: Memory leak involving INTENT(OUT) CLASS argument w/ allocatable components

• PR47728: ICE on invalid CLASS declaration

• PR47730: ICE on invalid source in connection with SELECT TYPE

• PR47745: Segfault with CLASS(*) and derived type dummy arguments

• PR47767: SELECT TYPE fails to execute correct TYPE IS block

• PR48059: [4.6 Regression] ICE in in gfc_conv_component_ref: character function of extended type

Fixed in 4.7 (56)

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

• PR43214: TBP with non-scalar PASS

• PR43969: ALLOCATED() with polymorphic arrays

• PR44568: ICE with TBP of polymorphic derived type array

• PR46262: tree check: expected function_type or method_type, have pointer_type

• PR46328: type-bound operator call with non-trivial polymorphic operand

• PR46356: Erroneous procedure/intent error and ICE for class dummy argument

• PR46990: gfortran rejects passing a CLASS variable to TYPE

• PR47546: Internal error - free_pi_tree(): Unresolved fixup

• PR47601: Internal Error: mio_component_ref(): Component not found

• PR47978: Invalid INTENT in overriding TBP not detected

• PR48095: Invalid assignment to procedure pointer component not rejected

• PR48291: [4.6/4.7 Regression] internal compiler error, new_symbol(): Symbol name too long

• PR48351: Realloc on assignment fails if parent component is CLASS

• PR48699: [4.6/4.7 Regression] MOVE_ALLOC inside SELECT TYPE

• PR48705: ICE with generic TBP

• PR48810: [4.6/4.7 Regression] TPB rejected because module procedure is private

• PR48887: SELECT TYPE: Associate name shall not be a pointer/allocatable

• PR48946: Deferred Overloaded Assignment

• PR49074: Defined assignment w/ CLASS arrays: Incomplete error message

• PR49112: [4.6/4.7 Regression] Missing type-bound procedure, "duplicate save" warnings and internal compiler error

• PR49417: [4.6/4.7 Regression] ICE on invalid CLASS component declaration

• PR49562: [4.6/4.7 Regression] ICE-on-invalid: assigning value to type-bound function

• PR49638: length parameter is ignored when overriding type bound character functions with constant length

• PR50225: The allocation status for polymorphic allocatable function results is not set properly

• PR50227: [4.7 Regression] ICE-on-valid with allocatable class variable

• PR50625: [4.6/4.7 Regression] ALLOCATABLE attribute lost for module CLASS variables

• PR50640: [4.7 Regression] FAIL: gfortran.dg/select_type_12.f03 -O (internal compiler error)

• PR50919: Don't use vtable for NON_OVERRIDABLE TBP

• PR50960: vtables not marked as constant

• PR51052: Internal compiler error in gfc_conv_component

• PR51056: [4.7 Regression] Bogus "Unused module variable '__vtab_domain_Domain_container'"

• PR51293: ICE on ANY with overloaded == operator

• PR51334: ICE with type-bound operator: tree check: expected record_type or union_type or qual_union_type, have function_type in gfc_conv_component_ref, at fortran/trans-expr.c:556

• PR51378: Structure constructor wrongly rejects parent components if only child has PRIVATE comps

• PR51383: arrays of extended types break when associated

• PR51514: Wrong code when passing a CLASS to a TYPE

• PR51529: gfortran.dg/class_to_type_1.f03 is miscompiled: Uninitialized variable used

• PR51605: SELECT TYPE with TARGET (internal compiler error gfc_trans_block_construct, at fortran/trans-stmt.c:984)

• PR51634: ICE with polymorphic operators

• PR51809: [4.7 Regression] ICE (segfault) depending on USE statements order

• PR51733: No allocate on assign for class objects with allocatable components

• PR51754: ICE on valid with class arrays

• PR51870: ICE with ALLOCATE and SOURCE-expr function returning BT_CLASS (cf. also PR51943 and PR51946)

• PR51913: [4.6/4.7 Regression] bug when submitting a class pointer to a subroutine

• PR51948: [4.6/4.7 Regression] Rejects valid: Function result value in MOVE_ALLOC, nested in SELECT TYPE

• PR51970: gimplification failed for polymorphic MOVE_ALLOC

• PR51972: ALLOCATE misses memset/calloc, causing segfault

• PR51977: MOVE_ALLOC: Bogus "must have the same rank 0/1" for same-rank arrays

• PR51995: Polymorphic class fails at runtime

• PR52013: Polymorphism and coarrays: Support as class container

• PR52016: Polymorphism and elemental: missing diagnostic

• PR52024: GENERIC operator cannot be resolved

• PR52029: _copy should be PURE

• PR52102: Wrong result with ALLOCATE w/ SOURCE= and nonpoly. array constructor source-expr

• PR52044: Invalid memory access with ALLOCATE, default initializer and polymorphic array components

Backported (1):

• PR53255: With TYPE, wrong type-bound operator used: of parent instead of overridden one

Fixed in 4.8 (32)

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

• PR42769: ICE in resolve_typebound_procedure

• PR45836: Type Bound Procedure Error - Type Mismatch

• PR45900: Static TBP resolved incorrectly

• PR46952: Spurious "recursive call" error with type bound procedure

• PR47710: Improve ambiguity check for GENERIC TBP w/ PASS and NOPASS

• PR49591: Multiple identical specific procedures in type-bound operator not detected

• PR50981: Issues with polymorphism and ELEMENTAL

• PR52270: Polymorphic vars: wrong intent(in) check, passing nonptr variable to intent(in) ptr dummy

• PR52552: ICE when trying to allocate non-allocatable object giving a dynamic type

• PR52968: Call to type-bound procedure wrongly rejected

• PR53191: C614 (F2003) or C618 (F2008) not implemented for class expressions

• PR53255: With TYPE, wrong type-bound operator used: of parent instead of overridden one

• PR53328: Ambiguous check for type-bound GENERIC shall ignore PASSed arguments

• PR53643: ICE (segfault) with INTENT(OUT) CLASS array

• PR53876: [4.8 Regression] ICE with class array

• PR54134: ICE overriding derived type bound function with allocatable character as result

• PR54195: [4.8 Regression] IMPORT fails with GENERIC TBP: "is already present in the interface"

• PR54243: ICE (segfault) in gfc_type_compatible for invalid BT_CLASS

• PR54244: ICE in gfc_add_component_ref, at fortran/class.c:210

• PR54594: Type-bound ASSIGNMENTs (elemental + array version) rejected as ambiguous

• PR54667: gimplification failure with c_f_pointer

• PR54778: an ICE on invalid OO code

• PR54784: [4.7/4.8 Regression] wrong code in polymorphic allocation with SOURCE

• PR54832: [4.8 Regression] Type-bound operator not picked up with RESULT variable

• PR54881: [4.8 Regression] ICE in fold_convert_loc, at fold-const.c:2016

• PR54917: [4.7/4.8 Regression] TRANSFER on polymorphic variable causes ICE

• PR54990: [4.8 Regression] ICE in tree_operand_check on SELECT TYPE

• PR54992: [4.8 Regression] Wrong offset in the array offset calculation when using nonclass%class(index)%nonclass

• PR55037: [4.8 Regression] ICE with local allocatable variable of abstract type

• PR55199: Equivalenced variable has wrong type when used with generic member function

• PR55297: [4.8 Regression] type-bound operator clashes with abstract interface

Fixed in 4.9 (2)

• PR56284: ICE with alternate return in type-bound procedure

• PR56500: "IMPLICIT CLASS(...)" wrongly rejected

Object-oriented Fortran codes

• PSBLAS

• Galacticus

• ForTrilinos

Literature

• D. Rouson et al., Scientific Software Design: The Object-Oriented Way (2012)

• Arjen Markus, Modern Fortran in Practice (2012)

• S. Arabas et al., Object-oriented implementations of the MPDATA advection equation solver in C++, Python and Fortran (arXiv, 2013)