slint-ui/slint · error

Invalid type in public properties

Error message

Invalid type in public properties

What it means

In the C++ generator, public API is emitted only for property types that have a C++ representation. `p.ty.cpp_type()` returned None for a property included in the public API, so the generator panics. Only types mapped to C++ (int, float, bool, string, color, brushes, structs, models, etc.) may appear in a public component/global API.

Solutions

1. Extend `Type::cpp_type()` in the compiler to map the missing type to a C++ type
2. Remove the property from the public API or change its declared type to a C++-supported one
3. Reproduce with the offending .slint file, identify the property type, and add a mapping or a compile-time rejection with a friendly diagnostic instead of an expect

Example fix

// before
let cpp_property_type = p.ty.cpp_type().expect("Invalid type in public properties");
// after
let cpp_property_type = p.ty.cpp_type().unwrap_or_else(||
    panic!("property '{}' of type {:?} cannot be exposed to C++", prop_name, p.ty));

fn is_cpp_expressible(ty: &Type) -> bool {
    !matches!(ty, Type::Invalid | Type::Void) && ty.cpp_type().is_some()
}
// assert every public property passes is_cpp_expressible before codegen

fn assert_public_property_cpp_support(name: &str, ty: &Type) {
    assert!(ty.cpp_type().is_some(), "public property '{name}' has no C++ type mapping ({ty:?})");
}

When it happens

Trigger: A property or callback declared public whose Slint type has no `cpp_type()` mapping — e.g. an exotic internal type, a pure-layout type, or a newly added language type not yet mapped for C++ — is processed by `generate_public_api_for_properties` / the live-preview declaration path.

Common situations: Slint development: adding a new builtin type to the compiler and forgetting to extend `cpp_type()`; declaring public properties with types that are only valid internally; version drift between compiler types and the C++ generator.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Invalid type in public properties
• default value requested for unknown struct field
• Expression::ObjectAccess: Cannot find a key in an object
• model two-way bindings only exist on sub-components
• ShowPopupWindow is invoked on a sub-component

AI-assisted analysis of slint-ui/slint@3a7e700487 (2026-09-16). Data as JSON: /api/errors/317fa5ce0468698f. Report an issue: GitHub.

Appendix: source

Thrown at internal/compiler/generator/cpp.rs:3771

            ];
            declarations.push((
                Access::Public,
                Declaration::Function(Function {
                    name: accessor_names::cpp_accessor_name(name, AccessorKind::Invoker),
                    signature: format!(
                        "({}) const -> {ret}",
                        param_types
                            .iter()
                            .enumerate()
                            .map(|(i, ty)| format!("{ty} arg_{i}"))
                            .join(", "),
                    ),
                    statements: Some(call_code),
                    ..Default::default()
                }),
            ));
        } else {
            let cpp_property_type = p.ty.cpp_type().expect("Invalid type in public properties");
            let prop_getter: Vec<String> = vec![
                "slint::private_api::assert_main_thread();".into(),
                "[[maybe_unused]] auto self = this;".into(),
                format!("return {}.get();", access),
            ];
            declarations.push((
                Access::Public,
                Declaration::Function(Function {
                    name: accessor_names::cpp_accessor_name(name, AccessorKind::Getter),
                    signature: format!("() const -> {cpp_property_type}"),
                    statements: Some(prop_getter),
                    ..Default::default()
                }),
            ));

            if !p.read_only() {
                let prop_setter: Vec<String> = vec![
                    "slint::private_api::assert_main_thread();".into(),

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