Properly link flecs library

engine/libs/flecs/examples/cpp/queries/hierarchy/src/main.cpp

@@ -0,0 +1,77 @@
+#include <hierarchy.h>
+#include <iostream>
+
+struct Position {
+    double x, y;
+};
+
+// Tags for local/world position
+struct Local { };
+struct World { };
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // Create a hierarchy. For an explanation see the entities/hierarchy example
+    flecs::entity sun = ecs.entity("Sun")
+        .add<Position, World>()
+        .set<Position, Local>({1, 1});
+
+        ecs.entity("Mercury")
+            .child_of(sun)
+            .add<Position, World>()
+            .set<Position, Local>({1, 1});
+
+        ecs.entity("Venus")
+            .child_of(sun)
+            .add<Position, World>()
+            .set<Position, Local>({2, 2});
+
+        flecs::entity earth = ecs.entity("Earth")
+            .child_of(sun)
+            .add<Position, World>()
+            .set<Position, Local>({3, 3});
+
+            ecs.entity("Moon")
+                .child_of(earth)
+                .add<Position, World>()
+                .set<Position, Local>({0.1, 0.1});
+
+    // Create a hierarchical query to compute the global position from the
+    // local position and the parent position.
+    flecs::query<const Position, const Position, Position> q = 
+        ecs.query_builder<const Position, const Position, Position>()
+            // Modify terms from template to make sure the query selects the
+            // local, world and parent position components.
+            .term_at(1).second<Local>()
+            .term_at(2).second<World>()
+            .term_at(3).second<World>()
+
+            // Extend the 2nd query argument to select it from the parent
+            .term_at(2)
+                // Get from the parent, in breadth-first order (cascade)
+                .parent().cascade()
+                // Make term component optional so we also match the root (sun)
+                .optional()
+            // Finalize the query
+            .build();
+
+    // Do the transform
+    q.iter([](flecs::iter& it, 
+        const Position *p, const Position *p_parent, Position *p_out) 
+    {
+        for (auto i : it) {
+            p_out[i].x = p[i].x;
+            p_out[i].y = p[i].y;
+            if (p_parent) {
+                p_out[i].x += p_parent->x;
+                p_out[i].y += p_parent->y;
+            }
+        }
+    });
+
+    // Print world positions
+    ecs.each([](flecs::entity e, flecs::pair<Position, World> p) {
+        std::cout << e.name() << ": {" << p->x << ", " << p->y << "}\n";
+    });
+}