Qt Signals and Slots Connecting overloaded signals/slots
Example
While being better in many regards, the new connection syntax in Qt5 has one big weakness: Connecting overloaded signals and slots. In order to let the compiler resolve the overloads we need to use static_casts to member function pointers, or (starting in Qt 5.7) qOverload and friends:
#include <QObject>
class MyObject : public QObject
{
Q_OBJECT
public:
explicit MyObject(QObject *parent = nullptr) : QObject(parent) {}
public slots:
void slot(const QString &string) {}
void slot(const int integer) {}
signals:
void signal(const QString &string) {}
void signal(const int integer) {}
};
int main(int argc, char **argv)
{
QCoreApplication app(argc, argv);
// using pointers to make connect calls just a little simpler
MyObject *a = new MyObject;
MyObject *b = new MyObject;
// COMPILE ERROR! the compiler does not know which overloads to pick :(
QObject::connect(a, &MyObject::signal, b, &MyObject::slot);
// this works, now the compiler knows which overload to pick, it is very ugly and hard to remember though...
QObject::connect(
a,
static_cast<void(MyObject::*)(int)>(&MyObject::signal),
b,
static_cast<void(MyObject::*)(int)>(&MyObject::slot));
// ...so starting in Qt 5.7 we can use qOverload and friends:
// this requires C++14 enabled:
QObject::connect(
a,
qOverload<int>(&MyObject::signal),
b,
qOverload<int>(&MyObject::slot));
// this is slightly longer, but works in C++11:
QObject::connect(
a,
QOverload<int>::of(&MyObject::signal),
b,
QOverload<int>::of(&MyObject::slot));
// there are also qConstOverload/qNonConstOverload and QConstOverload/QNonConstOverload, the names should be self-explanatory
}
