Properly link flecs library

engine/libs/flecs/examples/cpp/rules/setting_variables/src/main.cpp

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+#include <setting_variables.h>
+#include <iostream>
+
+// This example extends the component_inheritance example, and shows how
+// we can use a single rule to match units from different players and platoons
+// by setting query variables before we iterate.
+//
+// The units in this example belong to a platoon, with the platoons belonging
+// to a player.
+
+// Unit datamodel
+struct Unit { };
+struct CombatUnit : Unit { };
+struct MeleeUnit : CombatUnit { };
+struct RangedUnit : CombatUnit { };
+struct Warrior : MeleeUnit { };
+struct Wizard : RangedUnit { };
+struct Marksman : RangedUnit { };
+
+// Player/Platoon tags
+struct Player { };
+struct Platoon { };
+
+static const int PlayerCount = 100;
+static const int PlatoonsPerPlayer = 3;
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // See component_inheritance example
+    ecs.component<CombatUnit>().is_a<Unit>();
+    ecs.component<MeleeUnit>().is_a<CombatUnit>();
+    ecs.component<RangedUnit>().is_a<CombatUnit>();
+
+    ecs.component<Warrior>().is_a<MeleeUnit>();
+    ecs.component<Wizard>().is_a<RangedUnit>();
+    ecs.component<Marksman>().is_a<RangedUnit>();
+
+    // Populate store with players and platoons
+    for (int p = 0; p < PlayerCount; p ++) {
+        flecs::entity player;
+
+        // Give first player a name so we can look it up later
+        if (p == 0) {
+            player = ecs.entity("MyPlayer");
+        } else {
+            player = ecs.entity();
+        }
+
+        // Add player tag so we can query for all players if we want to
+        player.add<Player>();
+
+        for (int pl = 0; pl < PlatoonsPerPlayer; pl ++) {
+            flecs::entity platoon = ecs.entity().add<Player>(player);
+
+            // Add platoon tag so we can query for all platoons if we want to
+            platoon.add<Platoon>();
+
+            // Add warriors, wizards and marksmen to the platoon
+            ecs.entity().add<Warrior>().add<Platoon>(platoon);
+            ecs.entity().add<Marksman>().add<Platoon>(platoon);
+            ecs.entity().add<Wizard>().add<Platoon>(platoon);
+        }
+    }
+
+    // Create a rule to find all RangedUnits for a platoon/player. The 
+    // equivalent query in the query DSL would look like this:
+    //   (Platoon, $Platoon), Player($Platoon, $Player)
+    //
+    // The way to read how this query is evaluated is:
+    // - find all entities with (Platoon, *), store * in _Platoon
+    // - check if _Platoon has (Player, *), store * in _Player
+    flecs::rule<RangedUnit> r = ecs.rule_builder<RangedUnit>()
+        .with<Platoon>().second("$Platoon")
+        .with<Player>("$Player").src("$Platoon")
+        .build();
+
+    // If we would iterate this rule it would return all ranged units for all
+    // platoons & for all players. We can limit the results to just a single
+    // platoon or a single player setting a variable beforehand. In this example
+    // we'll just find all platoons & ranged units for a single player.
+
+    int player_var = r.find_var("Player");
+    int platoon_var = r.find_var("Platoon");
+
+    // Iterate rule, limit the results to units of MyPlayer
+    r.iter().set_var(player_var, ecs.lookup("MyPlayer")) 
+        .each([&](flecs::iter& it, size_t index, RangedUnit) {
+            flecs::entity unit = it.entity(index);
+            std::cout << "Unit " << unit.path() << " of class " 
+                      << it.id(1).str() << " in platoon "
+                      << it.get_var(platoon_var).path() << " for player "
+                      << it.get_var(player_var).path() << "\n";
+        });
+
+    r.destruct();
+
+    // Output:
+    //  Unit ::499 of class Wizard in platoon ::496 for player ::MyPlayer
+    //  Unit ::503 of class Wizard in platoon ::500 for player ::MyPlayer
+    //  Unit ::507 of class Wizard in platoon ::504 for player ::MyPlayer
+    //  Unit ::498 of class Marksman in platoon ::496 for player ::MyPlayer
+    //  Unit ::502 of class Marksman in platoon ::500 for player ::MyPlayer
+    //  Unit ::506 of class Marksman in platoon ::504 for player ::MyPlayer
+
+    // Try removing the set_var call, this will cause the iterator to return
+    // all units in all platoons for all players.
+}