#ifndef FOO_H
#define FOO_H

#include "bar.h"

class Foo
{
       public:
           Foo(Bar *bar)
           {
               _bar = bar;
               _bar->setFoo(this);
           }
           virtual ~Foo() {}

       private:
           Bar* _bar;
};

#endif // FOO_H

#ifndef BAR_H
#define BAR_H

#include "foo.h"

class Bar
{
       public:
           virtual ~Bar() {}

           void setFoo(Foo *foo) { _foo = foo; }

       private:
           Foo* _foo;
};

#endif // BAR_H

#pragma once

class Foo
{
public:
    virtual bool isTrue();
}

#include "foo.h"

bool Foo::isTrue() {...}

class Foo
{
public:
    virtual bool isTrue();
}

bool Foo::isTrue() {...}

while (true)
{
   processInput();
   update();
   render();
}