Passing string from Fortran to c++ [duplicate] - c++

Question

This question already has answers here:
passing char arrays from c++ to fortran
(3 answers)
Closed 5 years ago.
I am following the following example (http://fortranwiki.org/fortran/show/Fortran+and+Cpp+objects) to adapt a C++ code to Fortran, first I am trying to learn Fortran and making connection between Fortran and C++.
I modified the code where I would like to pass a string from Fortran to C++
#include <iostream>
#include <string>
using namespace std;
class CWLiDAR {
string name;
public:
CWLiDAR(string);
};
CWLiDAR::CWLiDAR(string name)
{
this->name = name;
}
/*
*/
/* C wrapper interfaces to C++ routines */
#ifdef __cplusplus
extern "C" {
#endif
CWLiDAR* CWLiDAR__new(string name)
{
return new CWLiDAR(name);
}
void CWLiDAR__delete(CWLiDAR* This)
{
delete This;
}
#ifdef __cplusplus
}
#endif
The Fortran wrapper
module CWLiDAR_module
use, intrinsic :: ISO_C_Binding!, only: C_int, C_ptr, C_NULL_ptr
implicit none
private
type CWLiDAR_type
private
type(C_ptr) :: object = C_NULL_ptr
end type CWLiDAR_type
interface
function C_CWLiDAR__new (name, name_len) result(this) bind(C, name = "CWLiDAR__new")
import
type(C_ptr) :: this
character(name_len, kind=C_CHAR), intent(IN) :: name
integer :: name_len
end function C_CWLiDAR__new
subroutine C_CWLiDAR__delete (this) bind(C, name = "CWLiDAR__delete")
import
type(C_ptr), value :: this
end subroutine C_CWLiDAR__delete
end interface
interface new
module procedure CWLiDAR__new
end interface new
interface delete
module procedure CWLiDAR__delete
end interface delete
public :: new, delete, CWLiDAR_type
contains
! Fortran wrapper routines to interface C wrappers
subroutine CWLiDAR__new(this, name, name_len)
type(CWLiDAR_type), intent(out) :: this
character(*) :: name
integer :: name_len
this%object = C_CWLiDAR__new(name, name_len)
end subroutine CWLiDAR__new
subroutine CWLiDAR__delete(this)
type(CWLiDAR_type), intent(inout) :: this
call C_CWLiDAR__delete(this%object)
this%object = C_NULL_ptr
end subroutine CWLiDAR__delete
end module CWLiDAR_module
The main
program main
use cwlidar_module
type(CWLiDAR_type) :: lidar
call new(lidar, "Test", 4)
call delete(lidar)
end program main
How should I modify the Fortran wrapper to pass a string from Fortran to C++?

Answer 1

First of all, you cannot use std::string at the inteface between Fortran and C. std::string is some C++ implementation defined object that has no guaranteed memory layout for interoperability with Fortran.
You must use plain C strings instead. So, CVLiDAR__new() must accept an argument of type char*, and the fortran interface must declare this argument as
character(kind = c_char) :: name(*)
The Fortran wrapper routine must take some Fortran character dummy argument, and proceed to copy its contents into a suitably allocated character(kind = c_char) array, which is then passed to the C function.
At this point, your C interface function may proceed to convert it again into a C++ std::string. This conversion is optional, as C++ can handle C strings just as well as C can. Adding the conversion would allow the rest of the code be more pure C++, though.
Do not forget to correctly terminate the C string with a zero character when you assemble it on the Fortran side!
These argument conversions are indeed a PITA, and if you have any number of interface functions that you need to call from Fortran, it may be advisable to write a generator script that produces the Fortran wrappers. An example of such a script can be found within the Climate Data Interface library (CDI, source code can be found at https://code.mpimet.mpg.de/projects/cdi/files) in the file cdi-1.8.1/interfaces/f2003/bindGen.rb, its output is published in the same tar-ball under cdi-1.8.1/src/mo_cdi.f90. This script may be complete overkill for your case, but it works quite fine for CDI and its output may inspire/help you getting ideas of how to do your conversions right.

Answer 2

You CANNOT pass a C++ string (actually std::string) from Fortran. It is simply not compatible with C or Fortran, it is an internal C++ type. You have to use the C wrapper and pass C compatible character arrays.