Structs and Classes

All problems in computer science can be solved by another level of indirection. — David Wheeler

Classes are wrapped by a shadow derived-type with methods implemented as type-bound procedures in Fortran and an extension type in Python.


Each class in the input file will create a struct which acts as a shadow class for the C++ class. A pointer to an instance is saved in the shadow class. This pointer is then passed down to the C++ routines to be used as the this instance.

Using the tutorial as an example, a simple class is defined in the C++ header as:

class Class1
    void Method1() {};

And is wrapped in the YAML as:

- decl: class Class1
  - decl: int Method1()


The Fortran interface will create two derived types. The first is used to interact with the C wrapper and uses bind(C). The C wrapper creates a corresponding struct. It contains a pointer to an instance of the class and index used to release the instance. The idtor argument is described in Memory Management.


! helper capsule_data_helper
type, bind(C) :: CLA_SHROUD_capsule_data
    type(C_PTR) :: addr = C_NULL_PTR  ! address of C++ memory
    integer(C_INT) :: idtor = 0       ! index of destructor
end type CLA_SHROUD_capsule_data


// helper capsule_CLA_Class1
struct s_CLA_Class1 {
    void *addr;     /* address of C++ memory */
    int idtor;      /* index of destructor */
typedef struct s_CLA_Class1 CLA_Class1;

The capsule is added to the Fortran shadow class. This derived type can contain type-bound procedures and may not use the bind(C) attribute.

type class1
    type(SHROUD_CLA_capsule_data) :: cxxmem
    procedure :: method1 => class1_method1
end type class1

A function which returns a class, including constructors, is passed a pointer to a F_capsule_data_type. The argument’s members are filled in by the function. The function will return a type(C_PTR) which contains the address of the F_capsule_data_type argument. The interface/prototype for the C wrapper function allows it to be used in expressions similar to the way that strcpy returns its destination argument.

A generic interface with the same name as the class is created to call the constructors for the class. The constructor will initialize the Fortran derived type.

type(class1) var     ! Create Fortran variable.
var = class1()       ! Allocate C++ class instance.

When the constructor is wrapped the destructor should also be wrapper or some other method is provided to release the memory.

Some other type-bound precedures are created to allow the user to get and set the address of the C++ memory directly. This can be used when the address of the instance is created in some other manner (perhaps a C++ module in the application) and it needs to be used in Fortran without being created in Fortran. There is no way to free this memory and must be released outside of Fortran.

type(class1) var
type(C_PTR) addr

addr = var%get_instance()
! addr will not be c_associated
call var%set_instance(caddr)    ! caddr contains address of an instance

Two instances of the class can be compared using the associated method.

type(class1) var1, var2
var1 = get_class(1)    ! A library function to fetch an instance
var2 = get_class(2)
if (var1%associated(var2) then
    print *, "Identical instances"

A full example is at Constructor and Destructor.

Inheritance is implemented using the EXTENDS Fortran keyword. Only single inheritance is supported.

type shape
    type(CLA_SHROUD_capsule_data) :: cxxmem
    procedure :: get_ivar => shape_get_ivar
end type shape

type, extends(shape) :: circle
end type circle


An struct is created for each C++ class.

typedef struct {
    classes::Class1 * obj;
    int idtor;
    // splicer begin class.Class1.C_object
    // splicer end class.Class1.C_object
} PY_Class1;

The idtor argument is used to release memory and described at Memory Management. The splicer allows additional fields to be added by the developer which may be used in function wrappers.

Forward Declaration

A class may be forward declared by omitting declarations. All other fields, such as format and options must be provided on the initial decl of a Class. This will define the type and allow it to be used in following declarations. The class’s declarations can be added later:

- decl: class Class1
     foo: True

- decl: class Class2
  - decl: void accept1(Class1 & arg1)

- decl: class Class1
  - decl: void accept2(Class2 & arg2)

Constructor and Destructor

The constructor and destuctor methods may also be exposed to Fortran.

The class example from the tutorial is:

- decl: class Class1
  - decl: Class1()         +name(new)
      function_suffix: _default
  - decl: Class1(int flag) +name(new)
    function_suffix: _flag
  - decl: ~Class1() +name(delete)

The default name of the constructor is ctor. The name can be specified with the name attribute. If the constructor is overloaded, each constructor must be given the same name attribute. The function_suffix must not be explicitly set to blank since the name is used by the generic interface.

The constructor and destructor will only be wrapped if explicitly added to the YAML file to avoid wrapping private constructors and destructors.

The Fortran wrapped class can be used very similar to its C++ counterpart.

use tutorial_mod
type(class1) obj
integer(C_INT) i

obj = class1_new()
i = obj%method1()
call obj%delete

For wrapping details see Constructor and Destructor.

Member Variables

For each member variable of a C++ class a C and Fortran wrapper function will be created to get or set the value. The Python wrapper will create a descriptor:

class Class1
   int m_flag;
   int m_test;

It is added to the YAML file as:

- decl: class Class1
  - decl: int m_flag +readonly;
  - decl: int m_test +name(test);

The readonly attribute will not write the setter function or descriptor. Python will report:

>>> obj = tutorial.Class1()
>>> obj.m_flag =1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: attribute 'm_flag' of 'tutorial.Class1' objects is not writable

The name attribute will change the name of generated functions and descriptors. This is helpful when using a naming convention like m_test and you do not want m_ to be used in the wrappers.

For wrapping details see Getter and Setter.


Shroud supports both structs and classes. But it treats them much differently. Whereas in C++ a struct and class are essentially the same thing, Shroud treats structs as a C style struct. They do not have associated methods. This allows them to be mapped to a Fortran derived type with the bind(C) attribute and a Python NumPy array.

A struct is defined the same as a class with a declarations field for struct members. In addition, a struct can be defined in a single decl in the YAML file.

- decl: struct Cstruct1 {
          int ifield;
          double dfield;


This is translated directly into a Fortran derived type with the bind(C) attribute.

type, bind(C) :: cstruct1
    integer(C_INT) :: ifield
    real(C_DOUBLE) :: dfield
end type cstruct1

All creation and access of members can be done using Fortran.

type(cstruct1) st(2)

st(1)%ifield = 1_C_INT
st(1)%dfield = 1.5_C_DOUBLE
st(2)%ifield = 2_C_INT
st(2)%dfield = 2.6_C_DOUBLE

The option wrap_struct_as can be set to class to create a shadow type identical to how classes are wrapped. This is useful when wrapping C code which does not support class directly. wrap_struct_as defaults to struct which will create a derived type. wrap_class_as also exists and defaults to class.


Python can treat a struct in several different ways by setting option PY_struct_arg. First, treat it the same as a class. An extension type is created with descriptors for the field methods. Second, as a numpy descriptor. This allows an array of structs to be used easily. Finally, as a tuple of Python types.

When treated as a class, a constructor is created which will create an instance of the class. This is similar to the default constructor for structs in C++ but will also work with a C struct.

import cstruct
a = cstruct.Cstruct1(1, 2.5)
a = cstruct.Cstruct1()

When treated as a NumPy array no memory will be copied since the NumPy array contains a pointer to the C++ memory.

import cstruct
dt = cstruct.Cstruct1_dtype
a = np.array([(1, 1.5), (2, 2.6)], dtype=dt)

The descriptor is created in the wrapper NumPy Struct Descriptor.