Binding C++ and Fortran - c++

Question

I want to combine C++ and Fortran together. My Fortran code will use a C++ function and C++ function changes variables of Fortran and sends them back. The C++ function is built with other C++ codes, e.g. the C++ function will use some sub-function in other .cpp file. I make the Fortran code with ifort and I added that C++ function as one object file, test.o in my Fortran makefile. I also put every needed C++ .o file(support test.o) in makefile. It shows the error
#6633, "The type of the actual argument differs from the type of the dummy argument".
Here is the code.
Fortran code
use, intrinsic :: ISO_C_BINDING, only: C_INT, C_DOUBLE
implicit double precision(a-h,o-z),integer(i-n)
Interface
integer (C_INT) function SolveBIE_(x, y, aa, m) BIND(C, NAME='SolveBIE_')
use, intrinsic :: ISO_C_BINDING
implicit none
type (C_PTR), value :: x
type (C_PTR), value :: y
type (C_PTR), value :: aa
integer (C_INT), value :: m
end function SolveBIE_
end Interface
integer (C_INT) :: m
real (C_DOUBLE), ALLOCATABLE, DIMENSION(:,:), target :: x
real (C_DOUBLE), ALLOCATABLE, DIMENSION(:,:), target :: y
real (C_DOUBLE), ALLOCATABLE, DIMENSION(:,:), target :: aa
ALLOCATE(x(0:MAXLEN,MAXINTERFACES))
ALLOCATE(y(0:MAXLEN,MAXINTERFACES))
ALLOCATE(aa(0:MAXLEN,MAXINTERFACES))
My Fortran code run
mm = SolveBIE_(x(1,1),y(1,1),aa(1,1),m)
Using the C++ code and where the error is from, on x, y, aa
I use x(1,1) instead of x, because if using x, then there is another error
#6634,"the shape matching rules of actual arguments and dummy arguments have been violated"`
I don't understand why it should be x(1,1). Why is this working, not x?
My C++ code
#ifdef __cplusplus
extern "C" {
#endif
int solveBIE_(double *ini_bdry_x, double *ini_bdry_y, double *ini_bdry_um, int *fM)
{
double(*bdry_node)[2] = new double[M1][2];
for (int k = 0; k < M; k++) {
bdry_node[k+1][0] = ini_bdry_x[k+1];
bdry_node[k+1][1] = ini_bdry_y[k+1];
bdry_theta[k+1] = Atan(ini_bdry_x[k+1], ini_bdry_y[k+1]);}
... some functions in other .cpp file

Answer 1

The way your interface is written, you have to construct a C_PTR to array x and pass that as the first argument:
use, intrinsic :: ISO_C_BINDING, only: C_INT, C_DOUBLE, C_PTR, C_LOC
! ...
type(C_PTR) PTRx
! ...
PTRx = C_LOC(x(LBOUND(x,1),LBOUND(x,2)))
! ...
mm = solveBIE_(PTRx, PTRy, PTRaa, m)
As shown above, you would have to fix the next two arguments as well. But you need to rewrite the interface for argument fM because as matters stand, Fortran will pass an integer by value whereas C++ is expecting a pointer. Given that, I would rewrite the interface completely, using the names given for the arguments in the C++ function and passing everything by reference. Names for dummy arguments are potentially visible in Fortran, so it's useful for them to be meaningful. In the following I assume that fM points to a scalar in the callee:
Interface
function SolveBIE_(ini_bdry_x, ini_bdry_y, ini_bdry_um, fM) &
BIND(C, NAME='SolveBIE_')
import
implicit none
integer(C_INT) SolveBIE_
real(C_DOUBLE) :: ini_bdry_x(*)
real(C_DOUBLE) :: ini_bdry_y(*)
real(C_DOUBLE) :: ini_bdry_um(*)
integer (C_INT) :: fM
end function SolveBIE_
end Interface
Then later on you can invoke it more or less normally as
mm = SolveBIE_(x,y,aa,m)
Note that x(1,1) was wrong because LBOUND(x,1) = 0, not 1!