A few more little features like facts and the ability to store a system for later running.

scratchpad/myecstest.cpp

@@ -7,15 +7,6 @@
 #include <any>
 
 using namespace fmt;
-
-struct Position {
-  double x, y;
-};
-
-struct Velocity {
-  double x, y;
-};
-
 typedef unsigned long Entity;
 
 typedef std::unordered_map<Entity, std::any> EntityMap;
@@ -23,6 +14,7 @@ typedef std::unordered_map<Entity, std::any> EntityMap;
 struct World {
   unsigned long entity_count = 0;
   std::unordered_map<std::type_index, EntityMap> $components;
+  std::unordered_map<std::type_index, std::any> $facts;
 
   Entity entity() {
     return ++entity_count;
@@ -33,6 +25,18 @@ struct World {
     return $components[std::type_index(typeid(Comp))];
   }
 
+  template <typename Comp>
+  void set(Comp val) {
+    $facts[std::type_index(typeid(Comp))] = val;
+  }
+
+  template <typename Comp>
+  Comp &get() {
+    // use .at to get std::out_of_range if fact not set
+    std::any &res = $facts.at(std::type_index(typeid(Comp)));
+    return std::any_cast<Comp&>(res);
+  }
+
   template <typename Comp>
   void assign(Entity ent, Comp val) {
     EntityMap &map = entity_map_for<Comp>();
@@ -42,7 +46,8 @@ struct World {
   template <typename Comp>
   Comp &component(Entity ent) {
     EntityMap &map = entity_map_for<Comp>();
-    std::any &res = map[ent];
+    // use .at for bounds checking
+    std::any &res = map.at(ent);
     return std::any_cast<Comp&>(res);
   }
 
@@ -74,6 +79,25 @@ struct World {
       }
     }
   }
+
+  template<typename CompA, typename CompB>
+  std::function<void()> runner(std::function<void(const Entity&, CompA&, CompB&)> cb) {
+    return [&]{
+      system<CompA, CompB>(cb);
+    };
+  }
+};
+
+struct Position {
+  double x, y;
+};
+
+struct Velocity {
+  double x, y;
+};
+
+struct Gravity {
+  double level;
 };
 
 int main() {
@@ -110,9 +134,13 @@ int main() {
       println("entity={}, vel.x, vel.y, pos.x={}, pos.y={}", ent, vel.x, vel.y, pos.x, pos.y);
   });
 
+  println("--- Creating facts (singletons)");
+  me.set<Gravity>({0.9});
+
   println("--- Query only entities with Position and Velocity:");
   me.system<Position, Velocity>([&](const auto &ent, auto &pos, auto &vel) {
-      println("entity={}, vel.x, vel.y, pos.x={}, pos.y={}", ent, vel.x, vel.y, pos.x, pos.y);
+      Gravity &grav = me.get<Gravity>();
+      println("grav={}, entity={}, vel.x, vel.y, pos.x={}, pos.y={}", grav.level, ent, vel.x, vel.y, pos.x, pos.y);
   });
 
   // now remove Velocity
@@ -123,5 +151,12 @@ int main() {
       println("entity={}, vel.x, vel.y, pos.x={}, pos.y={}", ent, vel.x, vel.y, pos.x, pos.y);
   });
 
+  println("--- Create a stored system you can save for later.");
+  auto movementSystem = me.runner<Position, Velocity>([&](const auto &ent, auto &pos, auto &vel) {
+      println("entity={}, vel.x, vel.y, pos.x={}, pos.y={}", ent, vel.x, vel.y, pos.x, pos.y);
+  });
+
+  movementSystem();
+
   return 0;
 }