Unclear design when connecting a functor object with operator()

[TheWisp]

There is no way to distinguish between a functor and a lambda.

A functor may be treated the same way as a lambda and get copied into the signal. As a result, the copy gets notified instead of the functor object. This is fine if all the members are pointer or references or the operator only causes global side-effects. However, if we rely on the data in the functor, the result is wrong.

struct stuff{
    int x = 0;
    void operator()(int y) { x+=y; }
};

stuff s;
sig.connect(s);
sig(10);
cout << s.x; //prints 0 instead of 10

On the other hand, if we treat the functor as a regular object, it may go out of bounds without disconnecting, dangling the callback. In case of a lambda, there is actually no way to save the connection into the structure for an RAII cleanup.

[TheWisp]

l-value functors or lambdas are now stored as references, whereas r-value ones are by value.