std::set might work better for systems (?)

tests/zecsy.cpp

@@ -186,3 +186,20 @@ TEST_CASE("Attach multiple components to an entity and verify all are correctly
     REQUIRE_FALSE(e.has<ChoosenOne>());
     REQUIRE_FALSE(e.has<Name>());
 }
+
+TEST_CASE("Create a simple system that processes entities with a specific component and verify it executes correctly")
+{
+    struct Component 
+    {
+        int value = 0;
+    };
+
+    world w;
+    auto e0 = w.make_entity(), e1 = w.make_entity();
+
+    e0.set<Component>(); //or e0.set(Component{})
+    e1.set(Component{20});
+
+    REQUIRE(e0.get<Component>().value == 0);
+    REQUIRE(e1.get<Component>().value == 20);
+}

zecsy.hpp

@@ -7,11 +7,19 @@
 #include <typeindex>
 #include <unordered_map>
 #include <unordered_set>
+#include <set>
 
 namespace zecsy 
 {
     using entity_id = uint64_t;
-    using entities_set = std::unordered_set<entity_id>; 
+
+    /*
+     * unordered_set is also an option. But probably sorting ids may be 
+     * beneficial for queries, because it might increase the chance of reusing 
+     * cpu cache (aka "required components were close to each other"). Definitely
+     * should play around with it 
+     */
+    using entities_set = std::set<entity_id>; 
 
     class entity final
     {