example: custom identifier (and some fixes for the purpose)

test/example/custom_identifier.cpp

@@ -0,0 +1,57 @@
+#include <type_traits>
+#include <gtest/gtest.h>
+#include <entt/entity/entity.hpp>
+#include <entt/entity/registry.hpp>
+
+struct entity_id {
+    using entity_type = typename entt::entt_traits<entt::entity>::entity_type;
+
+    entity_id(entity_type value = entt::null)
+        : entt{value}
+    {}
+
+    entity_id(const entity_id &other)
+        : entt{other.entt}
+    {}
+
+    operator entity_type() const {
+        return entt;
+    }
+
+private:
+    entity_type entt;
+};
+
+template<>
+struct entt::entt_traits<entity_id>: entt::entt_traits<entt::entity> {};
+
+TEST(Example, CustomIdentifier) {
+    entt::basic_registry<entity_id> registry{};
+    entity_id entity{};
+
+    ASSERT_FALSE(registry.valid(entity));
+    ASSERT_TRUE(entity == entt::null);
+
+    entity = registry.create();
+
+    ASSERT_TRUE(registry.valid(entity));
+    ASSERT_TRUE(entity != entt::null);
+
+    ASSERT_FALSE((registry.has<int, char>(entity)));
+    ASSERT_EQ(registry.try_get<int>(entity), nullptr);
+
+    registry.emplace<int>(entity, 42);
+
+    ASSERT_TRUE((registry.any<int, char>(entity)));
+    ASSERT_EQ(registry.get<int>(entity), 42);
+
+    registry.destroy(entity);
+
+    ASSERT_FALSE(registry.valid(entity));
+    ASSERT_TRUE(entity != entt::null);
+
+    entity = registry.create();
+
+    ASSERT_TRUE(registry.valid(entity));
+    ASSERT_TRUE(entity != entt::null);
+}