illegal member initialization

[Nakruf]

Error C2614 'Ares::ObjectManagement::Entity': illegal member initialization: 'm_manager' is not a base or member

		class Entity
		{
		private:
			Manager *m_manager;
			bool m_alive{true};

		public:
			bool isAlive()const { return m_alive; }
			Entity(Manager &manager) : m_manager(&manager) {}
		};

[ne555]

class Manager; //forward declaration
class Entity{
   //...
};

[Nakruf]

Can't I have a #include headers.h included?

[Peter87]

Yes, you can, as long as they don't include each other.

[Nakruf]

Can't I use pragma once to solve the issue of having them include each other?

[mbozzi]

Where it is supported, #pragma once usually prevents the same file from being included twice. This avoids multiple definition errors, but it does not eliminate the case where a definition in file A depends on the contents of file B, and vise-versa.
A.hpp

# pragma once
# include "B.hpp"
struct Manager: Entity {};

B.hpp

# pragma once
# include "A.hpp"
struct Entity { Manager* m; };

This case can be resolved using a forward declaration:
A.hpp

# pragma once
# include "B.hpp"
struct Manager: Entity {}; 

B.hpp

# pragma once
struct Manager; 
struct Entity { Manager* p };

Usually # pragma once is equivalent to the include-guard trick, which is more portable:
A.hpp

# if ! defined YOUR_PROJECT_A_HPP_INCLUDED
# define YOUR_PROJECT_A_HPP_INCLUDED
// header contents here
# endif