add opaque C++ => Fortran example

Author: scivision

src/cxx/CMakeLists.txt

@@ -6,6 +6,10 @@ target_include_directories(struct_cxx PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/../c)
 add_library(error_cxx OBJECT error_lib.cxx)
 
 # -- C++ calling Fortran
+add_executable(cpp_opaque opaque_main.cxx)
+target_link_libraries(cpp_opaque PRIVATE opaque_fortran)
+add_test(NAME C++_Fortran_opaque COMMAND cpp_opaque)
+
 add_executable(cpp_poly_fcn poly_fcn_main.cpp)
 target_link_libraries(cpp_poly_fcn PRIVATE poly_fcn_fortran)
 add_test(NAME C++_Fortran_poly_fcn COMMAND cpp_poly_fcn)

src/cxx/opaque_main.cxx

@@ -0,0 +1,19 @@
+// passing a Fortran-only type to/from C++ where only Fortran operates on opaque type
+#include <iostream>
+
+extern "C" void init_opaque_C(void**);
+
+extern "C" void use_opaque_C(void**);
+
+int main(){
+
+  void* myf;
+
+  init_opaque_C(&myf);
+
+  use_opaque_C(&myf);
+
+  std::cout << "OK: opaque" << std::endl;
+
+  return EXIT_SUCCESS;
+}

src/fortran/CMakeLists.txt

@@ -1,5 +1,7 @@
 add_library(bool_fortran bool_lib.f90)
 
+add_library(opaque_fortran opaque_lib.f90)
+
 add_library(vector_fortran OBJECT vector_lib.f90)
 
 add_library(error_fortran OBJECT error_lib.f90)

src/fortran/opaque_lib.f90

@@ -0,0 +1,60 @@
+module opaque
+
+use, intrinsic :: iso_c_binding, only : C_PTR, c_loc, c_f_pointer
+
+implicit none (type, external)
+
+type :: mytype
+  integer :: secret
+end type mytype
+
+contains
+
+subroutine init_opaque(my)
+
+  type(mytype), intent(out) :: my
+
+  my%secret = 42
+
+end subroutine init_opaque
+
+
+subroutine use_opaque(my)
+
+  type(mytype), intent(in) :: my
+
+  if (my%secret /= 42) error stop "opaque not initialized"
+
+end subroutine use_opaque
+
+!> C shims
+
+subroutine init_opaque_C(myC) bind(C, name="init_opaque_C")
+
+  type(C_PTR), intent(out) :: myC
+
+  type(mytype), pointer :: my
+
+  allocate(my)
+  !! Note: allocate is necessary or heap error results
+  myC = c_loc(my)
+
+  call init_opaque(my)
+
+  ! print *, "Fortran:init_opaque_C: my%secret=", my%secret
+
+end subroutine init_opaque_C
+
+
+subroutine use_opaque_C(myC) bind(C, name="use_opaque_C")
+
+  type(C_PTR), intent(in) :: myC
+
+  type(mytype), pointer :: my
+
+  call c_f_pointer(myC, my)
+  call use_opaque(my)
+
+end subroutine use_opaque_C
+
+end module opaque