C++ cyclic inclusion issue [duplicate]

Answer 1

When you forward declare, don't put in the #include. Do it like

class Event;
class Logger {
public:
    /*!
     * Constructor
     */
    Logger();
    /*!
     * Destructor
     */
    ~Logger();
    /*!
     * My operator
     */
    Logger& operator<<(const Event& e);
private:
    ...
};

without the #include "event.hpp"

Answer 2

Get rid of the #include "event.hpp" in logger.hpp - the forward declaration of class Event should be enough if all you need is a reference to an Event object in the function prototype:

#ifndef _LOGGER_HPP_
#define _LOGGER_HPP_

// #include "event.hpp"  // <<-- get rid of this line

class Event; // Forward decl

// Class definitions
...

The implementation of class Logger in logger.cpp will likely need to include event.hpp.