Missing type

This warning category is spelled c{[missing-type]} by qmllint.

Cannot deduce type of alias

What happened?

An alias property points to a property with a C++ type whose QML counterpart was not found. This can be caused by importing a QML module which do not declare its QML dependencies on other modules.

The warning might also indicate that the type of the property referenced by the alias does not have a QML counterpart. The referenced property type might be missing the QML_ELEMENT macro, for example. Refer to Defining QML Types from C++ or Overview - QML and C++ Integration in this case.

Why is this bad?

QML tooling is not able to find the QML counterpart of the C++ type: the compiler can't compile this property alias to C++ and qmllint as well as QML Language Server can't analyze this property alias.

Example

Let our QML module have one C++ class with a property myProperty:

#include <QQuickItem>
#include <QtQml/qqmlregistration.h>
#include <QObject>

class MyCppObject : public QObject
{
    Q_OBJECT
    QML_ELEMENT
public:
    MyCppObject(QObject *parent = nullptr)
        : QObject(parent)
    {}

    Q_PROPERTY(QQuickItem *myProperty READ myProperty WRITE setMyProperty NOTIFY notifyMyProperty)
    QQuickItem *myProperty() { return m_myProperty; }
    void setMyProperty(QQuickItem *item) { emit notifyMyProperty(); m_myProperty = item; }

private:
    QQuickItem *m_myProperty;

signals:
    void notifyMyProperty();
};

with following CMakeLists.txt:

project(mymodule VERSION 0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD_REQUIRED ON)
find_package(Qt6 6.5 REQUIRED COMPONENTS Quick)
qt_standard_project_setup(REQUIRES 6.5)

qt_add_executable(appmymodule
    main.cpp
)

qt_add_qml_module(appmymodule
    URI mymodule
    VERSION 1.0
    QML_FILES Main.qml HelloWorld.qml
    SOURCES mycppobject.cpp mycppobject.h
)

target_link_libraries(appmymodule
 PRIVATE Qt6::Quick
)

The C++ dependency Quick was declared, such that this class can compile and the QQuickItem include can be found. Also, mymodule does not have any dependency on QtQuick.

Now, let's try to use myProperty in an alias in QML. The program will run but QML tooling like the compiler, for example, will complain about the usage of myProperty:

import mymodule

MyCppObject {
    id: root

    property alias myAlias: root.myProperty // not ok: Cannot deduce type of alias [missing-type]
}

The reason for the warning message is that in the QML code, the type QQuickItem of myProperty and its QML counterpart Item are not known, even if you have import QtQuick in your QML file. This is because the same type can be exposed multiple times with different attributes in different modules: mymodule actually has to be precise about the QML type of myProperty.

To fix this warning, add the dependency in the CMakeLists.txt:

qt_add_qml_module(mymodule
    URI mymodule
    ...
    # declarare QML dependency to QtQuick module
    DEPENDENCIES QtQuick
    ...
)

Now, the warning should be gone!

See also Declaring module dependencies.

No type found for property

What happened?

A binding was set on a property whose QML type was not found. This can be caused by a QML module which does not declare its QML dependencies on other modules.

The warning might also indicate that the type of the property does not have a QML counterpart. The property type might be missing the QML_ELEMENT macro, for example. Refer to Defining QML Types from C++ or Overview - QML and C++ Integration in this case.

Why is this bad?

QML tooling is not able to find the QML counterpart of the C++ type: the compiler can't compile this property binding to C++ and qmllint as well as QML Language Server can't analyze this property binding.

Example

Let our QML module have a C++ class with two properties, myProperty and myProperty2:

#include <QQuickItem>
#include <QtQml/qqmlregistration.h>
#include <QObject>

class MyCppObject : public QObject
{
    Q_OBJECT
    QML_ELEMENT
public:
    MyCppObject(QObject *parent = nullptr)
     : QObject(parent)
    {}

    Q_PROPERTY(QQuickItem *myProperty READ myProperty WRITE setMyProperty NOTIFY notifyMyProperty)
    QQuickItem *myProperty() { return m_myProperty; }
    void setMyProperty(QQuickItem *item) { emit notifyMyProperty(); m_myProperty = item; }

    Q_PROPERTY(QQuickItem *myProperty2 READ myProperty2 WRITE setMyProperty2 NOTIFY notifyMyProperty2)
    QQuickItem *myProperty2() { return m_myProperty2; }
    void setMyProperty2(QQuickItem *item) { emit notifyMyProperty2(); m_myProperty2 = item; }

private:
    QQuickItem *m_myProperty;
    QQuickItem *m_myProperty2;

signals:
    void notifyMyProperty();
    void notifyMyProperty2();
};

with following CMakeLists.txt:

project(mymodule VERSION 0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD_REQUIRED ON)
find_package(Qt6 6.5 REQUIRED COMPONENTS Quick)
qt_standard_project_setup(REQUIRES 6.5)

qt_add_executable(appmymodule
    main.cpp
)

qt_add_qml_module(appmymodule
    URI mymodule
    VERSION 1.0
    QML_FILES Main.qml HelloWorld.qml
    SOURCES mycppobject.cpp mycppobject.h
)

target_link_libraries(appmymodule
 PRIVATE Qt6::Quick
)

The C++ dependency Quick was declared, such that this class can compile and the QQuickItem include can be found. Also, mymodule does not have any dependency on QtQuick.

Now, let's try to bind myProperty2 to myProperty in an alias in QML. The program will run but QML tooling like the compiler, for example, will complain about the usage of myProperty:

import mymodule

MyCppObject {
    id: root

    myProperty: myProperty2 // not ok: No type found for property "myProperty". [missing-type]
}

The reason for the warning message is that in the QML code, the type QQuickItem of myProperty and its QML counterpart Item are not known: the dependency 'QtQuick' of mymodule was not declared in the CMakeLists.txt.

To fix this warning, add the dependency in the CMakeLists.txt:

qt_add_qml_module(mymodule
    URI mymodule
    ...
    # declarare QML dependency to QtQuick module
    DEPENDENCIES QtQuick
    ...
)

Now, the warning should be gone!

See also Declaring module dependencies.