C++: fix crash when accessing empty model from Slint

Fixes #3704

CHANGELOG.md

@@ -34,6 +34,7 @@ All notable changes to this project are documented in this file.
  - Removed the need for C++ exceptions in generated code.
  - Added ability to only build the Slint compiler or use an external compiler.
  - ESP-IDF: wait for vsync before swapping frame buffers.
+ - Fixed crash when accessing empty model from Slint
 
 ### LSP
 

api/cpp/include/slint.h

@@ -341,13 +341,25 @@ using ModelPeer = std::weak_ptr<ModelChangeListener>;
 template<typename M>
 auto access_array_index(const M &model, size_t index)
 {
-    if (const auto v = model->row_data_tracked(index)) {
+    if (!model) {
+        return decltype(*model->row_data_tracked(index)) {};
+    } else if (const auto v = model->row_data_tracked(index)) {
         return *v;
     } else {
         return decltype(*v) {};
     }
 }
 
+template<typename M>
+long int model_length(const M &model) {
+    if (!model) {
+        return 0;
+    } else {
+        model->track_row_count_changes();
+        return model->row_count();
+    }
+}
+
 } // namespace private_api
 
 /// \rst

internal/compiler/generator/cpp.rs

@@ -3035,8 +3035,7 @@ fn compile_builtin_function_call(
             format!("{}.size()", a.next().unwrap())
         }
         BuiltinFunction::ArrayLength => {
-            // note: cast to "long" to avoid signed vs signed comparison warning, because all other integers coming from slint are signed
-            format!("[](const auto &model){{ (*model).track_row_count_changes(); return long((*model).row_count()); }}({})", a.next().unwrap())
+            format!("slint::private_api::model_length({})", a.next().unwrap())
         }
         BuiltinFunction::Rgb => {
             format!("slint::Color::from_argb_uint8(std::clamp(static_cast<float>({a}) * 255., 0., 255.), std::clamp(static_cast<int>({r}), 0, 255), std::clamp(static_cast<int>({g}), 0, 255), std::clamp(static_cast<int>({b}), 0, 255))",

tests/cases/models/array.slint

@@ -24,6 +24,7 @@ const TestCase &instance = *handle;
 assert_eq(instance.get_num_ints(), 5);
 assert_eq(instance.get_n(), 4);
 assert_eq(instance.get_third_int(), 3);
+assert_eq(instance.get_test(), true);
 
 auto model = std::make_shared<slint::VectorModel<int>>(std::vector<int>{1, 2, 3, 4, 5, 6, 7});
 instance.set_ints(model);
@@ -45,6 +46,7 @@ let instance = TestCase::new().unwrap();
 assert_eq!(instance.get_num_ints(), 5);
 assert_eq!(instance.get_n(), 4);
 assert_eq!(instance.get_third_int(), 3);
+assert_eq!(instance.get_test(), true);
 
 let model: std::rc::Rc<slint::VecModel<i32>> = std::rc::Rc::new(vec![1, 2, 3, 4, 5, 6, 7].into());
 instance.set_ints(slint::ModelRc::from(model.clone()));