From 1735af21a4c7e0a78ac9d0b5fc193b61680b7067 Mon Sep 17 00:00:00 2001
From: Sander Mertens <mertenalexander@meta.com>
Date: Wed, 11 Dec 2024 09:50:19 -0800
Subject: [PATCH] Fix crash when iterating empty type

Summary:
This diff fixes a crash when iterating the type of an entity without components, like here:

```
flecs::entity e = world.entity();

for (auto id : e.type()) { // crash
}
```

Differential Revision: D67102905
---
 distr/flecs.h                     | 17 ++++++++++----
 include/flecs/addons/cpp/type.hpp | 17 ++++++++++----
 test/cpp/project.json             |  4 +++-
 test/cpp/src/Entity.cpp           | 38 +++++++++++++++++++++++++++++++
 test/cpp/src/main.cpp             | 12 +++++++++-
 5 files changed, 78 insertions(+), 10 deletions(-)

diff --git a/distr/flecs.h b/distr/flecs.h
index d8f1b7e771..86906bd22a 100644
--- a/distr/flecs.h
+++ b/distr/flecs.h
@@ -27821,12 +27821,20 @@ struct type {
         return flecs::id(world_, type_->array[index]);
     }
 
-    flecs::id_t* begin() const {
-        return type_->array;
+    const flecs::id_t* begin() const {
+        if (type_ && type_->count) {
+            return type_->array;
+        } else {
+            return &empty_;
+        }
     }
 
-    flecs::id_t* end() const {
-        return &type_->array[type_->count];
+    const flecs::id_t* end() const {
+        if (type_ && type_->count) {
+            return &type_->array[type_->count];
+        } else {
+            return &empty_;
+        }
     }
 
     /** Implicit conversion to type_t */
@@ -27836,6 +27844,7 @@ struct type {
 private:
     world_t *world_;
     const type_t *type_;
+    flecs::id_t empty_;
 };
 
 /** #} */
diff --git a/include/flecs/addons/cpp/type.hpp b/include/flecs/addons/cpp/type.hpp
index 8a903fc4cb..ceef4aa14e 100644
--- a/include/flecs/addons/cpp/type.hpp
+++ b/include/flecs/addons/cpp/type.hpp
@@ -56,12 +56,20 @@ struct type {
         return flecs::id(world_, type_->array[index]);
     }
 
-    flecs::id_t* begin() const {
-        return type_->array;
+    const flecs::id_t* begin() const {
+        if (type_ && type_->count) {
+            return type_->array;
+        } else {
+            return &empty_;
+        }
     }
 
-    flecs::id_t* end() const {
-        return &type_->array[type_->count];
+    const flecs::id_t* end() const {
+        if (type_ && type_->count) {
+            return &type_->array[type_->count];
+        } else {
+            return &empty_;
+        }
     }
 
     /** Implicit conversion to type_t */
@@ -71,6 +79,7 @@ struct type {
 private:
     world_t *world_;
     const type_t *type_;
+    flecs::id_t empty_;
 };
 
 /** #} */
diff --git a/test/cpp/project.json b/test/cpp/project.json
index 73579eb99e..7391beeb15 100644
--- a/test/cpp/project.json
+++ b/test/cpp/project.json
@@ -296,7 +296,9 @@
                 "get_mut_pair_second_invalid_type",
                 "ensure_pair_second_invalid_type",
                 "set_pair_second_invalid_type",
-                "get_ref_pair_second_invalid_type"
+                "get_ref_pair_second_invalid_type",
+                "iter_type",
+                "iter_empty_type"
             ]
         }, {
             "id": "Pairs",
diff --git a/test/cpp/src/Entity.cpp b/test/cpp/src/Entity.cpp
index 7b03b7c628..979c9c809c 100644
--- a/test/cpp/src/Entity.cpp
+++ b/test/cpp/src/Entity.cpp
@@ -4863,3 +4863,41 @@ void Entity_get_ref_pair_second_invalid_type(void) {
     test_expect_abort();
     world.entity().get_ref_second<Position>(v);
 }
+
+void Entity_iter_type(void) {
+    flecs::world world;
+
+    flecs::entity e = world.entity().add<Position>().add<Velocity>();
+
+    int32_t count = 0;
+    bool pos_found = false;
+    bool velocity_found = false;
+
+    for (auto id : e.type()) {
+        count ++;
+        if (id == world.id<Position>()) {
+            pos_found = true;
+        }
+        if (id == world.id<Velocity>()) {
+            velocity_found = true;
+        }
+    }
+
+    test_int(count, 2);
+    test_assert(pos_found == true);
+    test_assert(velocity_found == true);
+}
+
+void Entity_iter_empty_type(void) {
+    flecs::world world;
+
+    flecs::entity e = world.entity();
+
+    int32_t count = 0;
+
+    for (auto id : e.type()) {
+        count ++;
+    }
+
+    test_int(count, 0);
+}
diff --git a/test/cpp/src/main.cpp b/test/cpp/src/main.cpp
index 7a5c45ac9d..b75c920aa7 100644
--- a/test/cpp/src/main.cpp
+++ b/test/cpp/src/main.cpp
@@ -289,6 +289,8 @@ void Entity_get_mut_pair_second_invalid_type(void);
 void Entity_ensure_pair_second_invalid_type(void);
 void Entity_set_pair_second_invalid_type(void);
 void Entity_get_ref_pair_second_invalid_type(void);
+void Entity_iter_type(void);
+void Entity_iter_empty_type(void);
 
 // Testsuite 'Pairs'
 void Pairs_add_component_pair(void);
@@ -2520,6 +2522,14 @@ bake_test_case Entity_testcases[] = {
     {
         "get_ref_pair_second_invalid_type",
         Entity_get_ref_pair_second_invalid_type
+    },
+    {
+        "iter_type",
+        Entity_iter_type
+    },
+    {
+        "iter_empty_type",
+        Entity_iter_empty_type
     }
 };
 
@@ -6882,7 +6892,7 @@ static bake_test_suite suites[] = {
         "Entity",
         NULL,
         NULL,
-        276,
+        278,
         Entity_testcases
     },
     {