Game Maker DLL example with state preserved between calls

citylib.cpp

#include <stdlib.h> 
#define GMEXPORT extern "C" __declspec(dllexport)

struct City {
  int population;
};

City *city;

GMEXPORT double city_init() {
  city = new City;
  city->population = 0;
  return 0;
}

GMEXPORT double city_set_population(double pop) {
  city->population = pop;
  return 0;
}

GMEXPORT double city_get_population() {
  return city->population;
}

GMEXPORT double city_destroy() {
  delete city;
  return 0;
}

object0_create.gml

_city_init = external_define("citylib.dll", "city_init", dll_cdecl, ty_real, 0);
_city_set_population = external_define("citylib.dll", "city_set_population", dll_cdecl, ty_real, 1, ty_real);
_city_get_population = external_define("citylib.dll", "city_get_population", dll_cdecl, ty_real, 0);
_city_destroy = external_define("citylib.dll", "city_destroy", dll_cdecl, ty_real, 0);

show_message("Creating city...");
external_call(_city_init);
show_message("City created, setting population to 10000...");
external_call(_city_set_population, 10000);
show_message("Population set, getting population...");
var pop = external_call(_city_get_population);
show_message("...the population is " + string(pop) + ". Destroying city...");
external_call(_city_destroy);
show_message("City destroyed.");