Calling C++ from Fortran

Foo.cpp

#include "Foo.hpp"
#include <iostream>
using namespace std;


Foo::Foo(int _a, int _b): a(_a), b(_b){
    cout << "C++ side, constructor" << endl;
}

Foo::~Foo(){
    cout << "C++ side, destructor" << endl;
}

int Foo::bar(int c) const{
    return a + c;
}

double Foo::baz(double d) const{
    return d + b;
}

void foo_speaker(string s){
    Foo f(4, 2);
    cout << s << " Foo(4, 2).bar(3) is: " <<  f.bar(3) << endl;
}

foo.h

#ifdef __cplusplus // Are we compiling this with a C++ compiler ?
extern "C" {
    class Foo;
    typedef Foo FOO;
#else
    // From the C side, we use an opaque pointer.
    typedef struct FOO FOO;
#endif

// Constructor
FOO* create_foo(int a, int b);

// Destructor
void delete_foo(FOO* foo);

// The const qualificators maps from the member function to pointers to the
// class instances.
int foo_bar(const FOO* foo, int c);
double foo_baz(const FOO* foo, double d);

void foo_speaker(const char* s);

#ifdef __cplusplus
}
#endif

Foo.hpp

#include <string>

class Foo {
public:
    Foo(int a, int b);
    ~Foo();

    int bar(int c) const;
    double baz(double d) const;
private:
    int a;
    int b;
};

void foo_speaker(std::string s);

foo_capi.cpp

#include "foo.h"
#include "Foo.hpp"

#include <iostream>
using namespace std;

FOO* create_foo(int a, int b){
    cout << "C API, create_foo" << endl;
    return new Foo(a, b);
}

void delete_foo(FOO* foo){
    cout << "C API, delete_foo" << endl;
    delete foo;
}

int foo_bar(const FOO* foo, int c){
    return foo->bar(c);
}

double foo_baz(const FOO* foo, double d){
    return foo->baz(d);
}

void foo_speaker(const char* s) {
    foo_speaker(string(s));
}

foo_cdef.f90

! C functions declaration
interface
    function create_foo_c(a, b) bind(C, name="create_foo")
        use iso_c_binding
        implicit none
        type(c_ptr) :: create_foo_c
        integer(c_int), value :: a
        integer(c_int), value :: b
    end function

    subroutine delete_foo_c(foo) bind(C, name="delete_foo")
        use iso_c_binding
        implicit none
        type(c_ptr), value :: foo
    end subroutine

    function foo_bar_c(foo, c) bind(C, name="foo_bar")
        use iso_c_binding
        implicit none
        integer(c_int) :: foo_bar_c
        ! The const qualification is translated into an intent(in)
        type(c_ptr), intent(in), value :: foo
        integer(c_int), value :: c
    end function

    function foo_baz_c(foo, c) bind(C, name="foo_baz")
        use iso_c_binding
        implicit none
        real(c_double) :: foo_baz_c
        type(c_ptr), intent(in), value :: foo
        real(c_double), value :: c
    end function

    ! void functions maps to subroutines
    subroutine foo_speaker_c(str) bind(C, name="foo_speaker")
        use iso_c_binding
        implicit none
        character(len=1, kind=C_CHAR), intent(in) :: str(*)
    end subroutine
end interface

foo_mod.f90

module libfoo
    use iso_c_binding

    private
    public :: foo, foo_speaker

    ! Yes, include is a keyword in Fortran !
    include "foo_cdef.f90"

    ! We'll use a Fortan type to represent a C++ class here, in an opaque maner
    type foo
        private
        type(c_ptr) :: ptr ! pointer to the Foo class
    contains
        ! We can bind some functions to this type, allowing for a cleaner syntax.
#ifdef __GNUC__
        procedure :: delete => delete_foo_polymorph ! Destructor for gfortran
#else
        final :: delete_foo ! Destructor
#endif
        ! Function member
        procedure :: bar => foo_bar
        procedure :: baz => foo_baz
    end type

    ! This function will act as the constructor for foo type
    interface foo
        procedure create_foo
    end interface

contains ! Implementation of the functions. We just wrap the C function here.
    function create_foo(a, b)
        implicit none
        type(foo) :: create_foo
        integer, intent(in) :: a, b
        create_foo%ptr = create_foo_c(a, b)
    end function

    subroutine delete_foo(this)
        implicit none
        type(foo) :: this
        call delete_foo_c(this%ptr)
    end subroutine

    ! Bounds procedure needs to take a polymorphic (class) argument
    subroutine delete_foo_polymorph(this)
        implicit none
        class(foo) :: this
        call delete_foo_c(this%ptr)
    end subroutine

    integer function foo_bar(this, c)
        implicit none
        class(foo), intent(in) :: this
        integer, intent(in) :: c
        foo_bar = foo_bar_c(this%ptr, c)
    end function

    double precision function foo_baz(this, c)
        implicit none
        class(foo), intent(in) :: this
        double precision, intent(in) :: c
        foo_baz = foo_baz_c(this%ptr, c)
    end function

    subroutine foo_speaker(str)
        implicit none
        character(len=*), intent(in) :: str
        character(len=1, kind=C_CHAR) :: c_str(len_trim(str) + 1)
        integer :: N, i

        ! Converting Fortran string to C string
        N = len_trim(str)
        do i = 1, N
            c_str(i) = str(i:i)
        end do
        c_str(N + 1) = C_NULL_CHAR

        call foo_speaker_c(c_str)
    end subroutine
end module

Makefile

FC = gfortran
CXX = g++

UNAME := $(shell uname -s)

FCFLAGS = -Wall -Wextra
CCFLAGS = -Wall -Wextra
ifeq ($(UNAME_S),Darwin)
	LDFLAGS = -lstdc++
else
	LDFLAGS = -lc++
endif

all: test.x
test.o : foo_mod.o

%.x : %.o foo_mod.o foo_capi.o Foo.o
	${FC} $^ -o $@ ${LDFLAGS}

%.o : %.f90
	${FC} ${FCFLAGS} -c $< -o $@

%.o : %.cpp
	${CXX} ${CCFLAGS} -c $^ -o $@

.PHONY : clean

clean :
	${RM} -rf *.o *.mod test.x

test.f90

program test
    use libfoo
    implicit none
    type(foo) :: f

    ! Create an object of type foo
    f = foo(3, 4)

    ! Call bound procedures (member functions)
    write(*,*) f%bar(60), " should be ", 63
    write(*,*) f%baz(10d0), " should be ", 14.0d0

    call foo_speaker("From Fortran!")

    ! The destructor should be called automatically here, but this is not yet
    ! implemented in gfortran. So let's do it manually.
#ifdef __GNUC__
    call f%delete
#endif
end program

Many thanks! Very instructive!! Just a minor thing... in Makefile line 4 is UNAME_S := $(shell uname -s), but in line 8 you compare with UNAME and not UNAME_S

Thank you very much! It's really cool to see such an instructive piece of code for Fortran/C++ binding. It would be very interesting, if you could show us the way, how to implement binding for two classes (possibly one of them abstract), where one of them inherits from the other one, what do you think?

@MartinBeseda I am sorry I don't know how to do this.

Fortran has its own concept of inheritance with extends, which you may be able to use here:

class Parent {
     virtual void foo() = 0;
}

class Child {
     void foo() override {}
     void non_virtual() {}
}

type :: Parent
    type(c_ptr), private :: parent_handle
contains
    procedure :: foo
end type

type, extends(Parent) :: Child
    type(c_ptr), private :: child_handle
contains
    procedure :: non_virtual
end

Then, if you take care of assigning the right pointers to parent_handle and child_handle, this should work. You will also need to take care to only free the pointers once, either through parent_handle or child_handle

Very useful, thanks!