C++ Class Object inside another class - c++

Question

It keeps me getting this error in Event.h:
field ‘group’ has incomplete type ‘Group’
For context, I want to have a class Group which has an owner (from class Person) and it consists of a vector of people (class Person):
Group.h
class Person;
#include "Person.h"
Class Group
{
private:
std::string name;
std::vector<Person> people;
int size = 0;
Person owner;
public:
Group(Person owner);
~Group();
}
In the Person class, I want to have just a vector of lists (class List, not important for this specific error). Note that in the Person class I have a constructor Person(int id);
In the Event class, I want to have a group of people invited that can be saved as a Group class:
Event.h
class Group;
#include "Group.h"
class Event
{
private:
std::string tittle;
std::string description;
bool locked;
bool checked;
Group group;
public:
Event(std::string tittle);
~Event();
}
Why can't I have a Person owner on my group?
Edit:
I don't know why, but now it works. I guarded everything with just #pragma once and maybe I changed something in the way I compiled. Thanks for all answers :)

Answer 1

You are defining something out of order. Perhaps the #ifdef guards.
This compiles just fine:
class Person {};
class Group
{
private:
std::string name;
std::vector<Person> people;
int size = 0;
Person owner;
public:
Group( Person owr );
~Group();
};
class Event
{
private:
std::string tittle;
std::string description;
bool locked;
bool checked;
Group group;
public:
Event(std::string tittle);
~Event();
};
Godbolt: https://godbolt.org/z/f785vK1dq

Answer 2

The following works fine for me (Online Demo):
Person.h
#ifndef Person_H
#define Person_H
class Person
{
};
#endif
Group.h
#ifndef Group_H
#define Group_H
#include "Person.h"
#include <string>
#include <vector>
class Group
{
private:
std::string name;
std::vector<Person> people;
int size = 0;
Person owner;
public:
Group(Person owner) : owner(owner) {}
};
#endif
Event.h
#ifndef Event_H
#define Event_H
#include "Group.h"
#include <string>
class Event
{
private:
std::string tittle;
std::string description;
bool locked = false;
bool checked = false;
Group group;
public:
Event(std::string tittle) : tittle(tittle), group(Person{}) {}
};
#endif
main.cpp
#include <iostream>
#include "Event.h"
int main()
{
Event evt("title");
return 0;
}