entities/fwd_declare_component

• C

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  #include <fwd_declare_component.h>
  #include <stdio.h>
   
  // This example shows how to use utility macro's to forward declare and use a 
  // component. The ECS_COMPONENT automatically declares a variable that holds a
  // component id, but this variable is scoped to the function only. When trying
  // to access the component from another function, this typically causes errors
  // that look like this:
  //   "FLECS_IDComponentNameID" is undefined
  //
  // Forward declaring a component will make the component id available from other
  // functions, which fixes this error.
   
  // The component type
  typedef struct {
      double x, y;
  } Position;
   
  // The forward declaration of the component id variable. This variable will have
  // the name FLECS_IDPositionID, to ensure its name won't conflict with the type.
  ECS_COMPONENT_DECLARE(Position);
   
  // When you want forward declare a component from a header, make sure to use the
  // extern keyword to prevent multiple definitions of the same variable:
  //
  // In the header:
  //   extern ECS_COMPONENT_DECLARE(Position);
  //
  // In *one* of the source files:
  //  ECS_COMPONENT_DECLARE(Position);
   
   
  // To forward declare entities created with ECS_ENTITY, ECS_TAG or ECS_PREFAB,
  // use ECS_DECLARE.
  ECS_DECLARE(Wizard);
   
  // Regular entity handles can also be forward declared
  ecs_entity_t Platoon_1;
   
  ecs_entity_t create_npc(ecs_world_t *world) {
      ecs_entity_t result = ecs_new(world);
      
      // Without the forward declaration, this would have thrown a compiler error
      ecs_set(world, result, Position, {10, 20});
      ecs_add(world, result, Wizard);
   
      // An entity that's not declared using the utility macro's can be added with
      // ecs_add_id. This has the same effect, with as only difference that this
      // function doesn't look for a variable named FLECS_EPlatoon_1
      ecs_add_id(world, result, Platoon_1);
   
      return result;
  }
   
  int main(int argc, char *argv[]) {
      ecs_world_t *world = ecs_init_w_args(argc, argv);
   
      // Register the component and assign the forward declared variable.
      ECS_COMPONENT_DEFINE(world, Position);
   
      // Same for the tag
      ECS_TAG_DEFINE(world, Wizard);
   
      // A forward declared entity can be assigned as any variable
      Platoon_1 = ecs_new(world);
   
      // Create new entity with Position
      ecs_entity_t e = create_npc(world);
      const Position *p = ecs_get(world, e, Position);
      printf("{%f, %f}\n", p->x, p->y);
   
      // To access the component id variable directly, use the ecs_id macro:
      printf("Id of Position is %u\n", 
          (uint32_t)ecs_id(Position)); // the cast avoids formatting warnings
   
      return ecs_fini(world);
   
      // Output:
      //  {10.000000, 20.000000}
      //  Id of Position is 497
  }