What Qt type do I need to use to pass a lambda as a function argument?

Question

Now that Qt5 supports connecting signals to lambda functions, I would like to be able to pass a lambda as an argument to another function. I've got a function that looks something like this:

void SomeFunc(Functor f)
{
    connect(obj, &MyObject::someSignal, f);
}

However, the compiler complains when I do that:

"Functor" has not been declared

Changing Functor to QtPrivate::Functor yields:

QtPrivate::Functor is not a type

Basically, all I want to do is pass the argument that QObject::connect is going to get to my function. What type do I need to use?

Answer 1

There are two options:

template<typename Functor>
void SomeFunc(Functor f)
{
    connect(obj, &MyObject::someSignal, f);
}

or

void SomeFunc(std::function<void(/*ArgumentTypes...*/)> f)
{
    connect(obj, &MyObject::someSignal, f);
}

The first option simply forwards any argument to the connect, the second uses polymorphic function pointer from C++11 standard library. It's template argument must correspond to the signature of the signal. Qt signals are void, the /ArgumentTypes.../ need to be replaced by the list of arguments of the function. So if the signal is declared as

void someSignal(int, QString);

the function will be

std::function<void(int, QString)>