C++: Forward class definition required to compile some code

Question

I am programming a small game in C++ as an exercise, and I've come across a conundrum. The basic idea is that a Level has a vector of Entity, and each Entity instance knows which level it belongs to. Pretty straightforward so far.

I was getting some nasty compile errors in my Entity class as it couldn't figure out what Level was. A simple forward class declaration right before the declaration of Entity easily fixed that. However, I am already including "Level.h" in "Entity.h". And Level and Entity are both declared and defined in the same namespace.

Note: Entity.h also includes Level.h.

#include "Level.h"

namespace GameNS {

  // Required, otherwise the compiler complains
  class Level;

  class Entity
  {
  public:
  ...

Shouldn't the compiler already know what Level is, by the time it reaches Entity?

Answer 1

No, that's not how the C++ compiler works. What it does is it basically passes through the files one translation unit at a time. A TU is, loosely put, an implementation file with all its includes.

If you have include guards in Level.h and Entity.h (you probably do) and they include each other, one of them, depending on the order, will be left out by the compiler, and one of the names won't be declared.

As a rule of thumb, use forward-declarations when you can, and includes where you must.