C++ struct and typdef

Question

I am not sure about the title because I don't know what it is really about.

I was trying to understand the code in the link below.

Color Based Particle Filter

I generally understand what the program does, but I couldn't figure out "state.h". What does this code do? Especially the "typedef", "State_" and "pp" parts look stranger to me.

I will put some of the code here for clarity reasons.

struct StateData;
struct State_;
typedef State_ (*State)(StateData&);
struct State_
{
    State_( State pp ) : p( pp ) { }
    operator State()
    {
        return p;
    }
    State p;
};

State_ state_start(StateData& d);
State_ state_selecting(StateData& d);
State_ state_initializing(StateData& d);
State_ state_tracking(StateData& d);

Any kind of help would be greatly appreciated.

Answer 1

Concisely: State is an alias for function pointer.State_ is a wrapper class that has a State member, and is implicitly convertible into that State.

The reason why State_ wrapper is needed, is because there is no way to express a function that returns a pointer to a function of the same type. The wrapper gets rid of the self-reference.

---

Line by line:

struct StateData;

A forward declaration of a class StateData.

struct State_;

A forward declaration of a class State_.

typedef State_ (*State)(StateData&);

This one is a bit tricky. It defines State as a type alias for a function pointer that can point to a function that returns a State_ and takes a StateData& as an argument. The functions declared at the end of the snippet can be pointed by a function pointer of this type.

In my opinion, the chosen name is very confusing considering that there is already a State_ class. While I'm usually against hungarian notation, I would recommend to always apply a suffix or prefix to denote a function pointer, say state_fun or state_handler or state_callback,

struct State_
{

This starts the definition of State_ calss.

    State_( State pp ) : p( pp ) { }

This defines a constructor for the class. The argument is of the function pointer type that was defined earlier. It initializes the member that will be declared shortly.

    operator State()
    {
        return p;
    }

A member function. More specifically, a user defined conversion into the function pointer type.

    State p;

Declares the member that was initialized in the constructor.

};

State_ state_start(StateData& d);
State_ state_selecting(StateData& d);
State_ state_initializing(StateData& d);
State_ state_tracking(StateData& d);

Free functions, that can be pointed by a State.

Answer 2

State_ is a structure type.

State is a pointer to function, accepting a parameter of type StateData& and returning State_.

typedef a b; defines a type b which is exactly the same as a.

p is a field of class State_, pp is a parameter of the constructor. p(pp) is a special syntax for constructor only, initializing p to the value of pp