How to Connect Overloaded Signals and Slots in Qt 5?

Connecting Overloaded Signals and Slots in Qt 5

When attempting to connect overloaded signals and slots in Qt 5 using the pointer-to-member function syntax, developers may encounter compilation errors. To rectify this issue, one must understand the problem and apply the appropriate solution.

The Problem of Overloaded Signals and Slots

Overloaded signals and slots, such as QSpinBox::valueChanged(int) and QSpinBox::valueChanged(QString), can cause confusion when connecting them. The issue arises when attempting to connect to a signal without explicitly specifying which overload to use.

Solution for Qt 5.7 and Later

From Qt 5.7 onwards, Qt provides helper functions to select the desired signal overload. To specify an overloaded signal, use the qOverload function as follows:

connect(spinBox, qOverload<int>(&QSpinBox::valueChanged),
        slider, &QSlider::setValue);

Solution for Qt 5.6 and Earlier

For versions earlier than Qt 5.7, explicitly cast the signal to the correct type using static_cast:

connect(spinBox, static_cast<void (QSpinBox::*)(int)>(&QSpinBox::valueChanged),
        slider, &QSlider::setValue);

Ugly Workaround

While effective, the static_cast approach is far from elegant. Consider using the following C 11 snippet instead:

template<typename... Args> struct SELECT {
    template<typename C, typename R>
    static constexpr auto OVERLOAD_OF( R (C::*pmf)(Args...) ) -> decltype(pmf) {
        return pmf;
    }
};

connect(spinBox, SELECT<int>::OVERLOAD_OF(&QSpinBox::valueChanged), ...)

Qt 5.7 Helper Functions

Qt 5.7 introduced helper functions to streamline the process. The primary helper is qOverload, with additional options such as qConstOverload and qNonConstOverload.

Documentation Note

In Qt 5.7 and later, the documentation for overloaded signals explicitly provides examples of how to connect to each overload using helper functions. Refer to the documentation for specific guidance.

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