How to interface Cpp code through DPI

I've been getting quite a few questions on this topic lately.  Here's one way to do it.  The main issue is that since all DPI is initiated by the simulator, if you use Cpp, then the complete SV <--> DPI <--> Cpp function call must be instantaneous.  This post describes a way to have your Cpp active throughout the simulation.

======file: top.sv =======================
package my_pkg;

import "DPI-C" function int dpi_build_cpp();
import "DPI-C" function void dpi_write_api(int n);
import "DPI-C" function int dpi_read_api();

endpackage // my_pkg
//---------------

module top;
import my_pkg::*;

initial begin
  if (!dpi_build_cpp() )
    $display("Vlog: Cpp did not construct correctly\n");
  else
    $display("Vlog: Cpp ready to go\n");

  dpi_write_api(1234567890);

  #100ns;  // let some time go by

  $display("Vlog: Retriev Cpp member %0d", dpi_read_api() );

  if (!dpi_build_cpp() )
    $display("Vlog: Cpp did not construct correctly\n");
  else
    $display("Vlog: Cpp ready to go\n");

end

endmodule

====file: dpi_cpp_api.cpp ======
#include "dpi_cpp_api.h"

// for vpi_printf
#include "vpi_user.h"

// for printf
#include <stdio.h>

// for cout etc
#include <iostream>

using namespace std;

// Class pointer  must be global - otherwise memory will be cleaned up after DPI call
mycpp_class* mycpp_class_inst = 0;

// #include <iostream>
//----------------C++ -------------------------

mycpp_class::mycpp_class() {
  cout << "Cpp: mycpp_class constructor" << endl;
}

mycpp_class::~mycpp_class() {
}

void mycpp_class::put_cpp(int x_cpp) {
  cout << "Cpp: put_cpp = " << x_cpp << endl;
  mycppint = x_cpp;
}

int mycpp_class::get_cpp() {
  cout << "Cpp: get_cpp\n" << endl;
  return mycppint;
}

//-----------------C/DPI wrapper----------------
extern "C" {

int dpi_build_cpp() {
  // check to see if class has been new-ed yet, if not new it
  if (mycpp_class_inst == 0) {
      mycpp_class_inst = new mycpp_class();
      vpi_printf("DPI-C: Cpp interface to DPI ready to go\n");
      return 1;
  }
  else {
      vpi_printf("DPI-C: Please don't call me twice\n");
      return 0;
  }
}

void dpi_write_api(int xx_c) {
  vpi_printf("DPI-C: dpi_write_api = %0d\n", xx_c);
  mycpp_class_inst -> put_cpp(xx_c);
  }

int dpi_read_api() {
  vpi_printf("DPI-C: dpi_read_api returning value\n");
  return mycpp_class_inst -> get_cpp();
  }

}  // extern C

===File: dpi_cpp_api.h ======
class mycpp_class {
public:
  mycpp_class();    // constructor

  ~mycpp_class();  // destructor

  void put_cpp(int xyz);

  int get_cpp();

 private:
  int mycppint;

};

=====3 Step Script============

  more RUN3
rm -rf  ./INCA_libs *.so

g++ -g -m32 -fPIC -shared -o dpi_cpp_api.so \
-I. -I./c_src -I`ncroot`/tools/inca/include  \
./dpi_cpp_api.cpp || exit

ncvlog -sv -linedebug top.sv  || exit

ncelab top -dpiheader mydpi.h || exit

# -uselic IES for Cpp debug in Simvision
ncsim top -sv_root ./ -sv_lib ./dpi_cpp_api.so -uselic IES

=====irun Script==============

rm -rf INCA_libs
rm *.so

# -uselic IES for Cpp debug in Simvision
irun top.sv -line -dpiheader mydpi.h -g -I.-cpost "dpi_cpp_api.cpp" -end -uselic IES

 =========Simulation Output==============================
ncsim> run
Cpp: mycpp_class constructor
DPI-C: Cpp interface to DPI ready to go
Vlog: Cpp ready to go

DPI-C: dpi_write_api = 1234567890
Cpp: put_cpp = 1234567890
Vlog: Retriev Cpp member DPI-C: dpi_read_api returning value
Cpp: get_cpp

1234567890
DPI-C: Please don't call me twice
Vlog: Cpp did not construct correctly

ncsim: *W,RNQUIE: Simulation is complete.
ncsim> exit