How to Resolve Circular Reference in Header File

Question

I have the following header file (simplified for illustration here):

#include <vector>
class Box;
class Item {
  int row;
  int column;
  Box *box;
public:
  Item(Box *b) : box(b) {}
  void thiswontcompile() { box->dosomething(); }
  void foo();
};
class Box {
  std::vector<Item*> items;
public:
  Box() {}
  void addsquare(Item *sq) { items.push_back(sq); }
  void bar() { for (int i=0; i<items.size(); i++) items[i]->foo(); }
  void dosomething();
};

The compiler objects to the line with thiswontcompile(), complaining about an invalid use of the incomplete type 'class Box'. I know I can correct the error by moving this definition to the implementation file, but is there any way to keep all of this coding in the header file?

Answer 1

You can move the definition of thiswontcompile to the end of the header file, you just need to make the function inline.

#include <vector>
class Box;
class Item {
  int row;
  int column;
  Box *box;
public:
  Item(Box *b) : box(b) {}
  void thiswontcompile();
  void foo();
};
class Box {
  std::vector<Item*> items;
public:
  Box() {}
  void addsquare(Item *sq) { items.push_back(sq); }
  void bar() { for (int i=0; i<items.size(); i++) items[i]->foo(); }
  void dosomething();
};

inline void Item::thiswontcompile() { box->dosomething(); }