Use of undefined type [duplicate]

In State::State, you are using Core before it is actually defined. You can fix this easily in your example by moving the constructor implementation out of the class definition:

class State{
public:
    State(Core* core);
};

class Core{
   // This stays the same...
};

State::State(Core* core)
{
   core->setState();
}

It’s much more common in practice to have the implementation of these functions in a separate implementation (.cpp) files, in which case the forward declarations would work as you’ve expected.

In that case:

// State.h
class Core;

class State{
public:
    State(Core* core);
};

And

// Core.h
#include "State.h"
#include <iostream> //This is probably a good reason to further separate
                    //Core.h into Core.h and Core.cpp

class Core{
public:
    Core(){
        State state(this);
    }

    void setState(){
        std::cout << "setting state" << std::endl;
    }
};

And the implementation file:

// State.cpp
#include "State.h"
#include "Core.h"

State::State(Core* core)
{
   core->setState();
}