Class member of an undeclared class

Question

How to make this bit of code work, if these two classes are declared in the same ".h" file? How to make it work if they are in the separate ones?

Simple question, but googling didn't help me.

class Container
{
    Piece p;
public:
    Container() :p(this) {};
};

class Piece
{
    Container* cont;
public:
    Piece(Container * c) :cont(c) {};
};

score 2 · Accepted Answer · answered Jul 16 '17 at 12:25

2

Forward declare Container and then define it later on:

class Container;

class Piece
{
    Container* cont;
public:
    Piece(Container * c) :cont(c) {};
};

class Container
{
    Piece p;
public:
    Container() :p(this) {};
};

answered Jul 16 '17 at 12:25

Hatted Rooster

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Why can't I first declare Piece, and after that Container? Is there a way to do it that is not dependent on the order of declaration? – Shocky2 Jul 16 '17 at 12:31
@shocky2 Because `Piece` holds a pointer to `Container`, pointers are allowed to have an incomplete type, but `Container` has a `Piece` object, objects aren't allowed to be incomplete. An incomplete type being a type that has been forward declared. – Hatted Rooster Jul 16 '17 at 12:32
@CisForCookies Just assumed OP meant the other way around. – Hatted Rooster Jul 16 '17 at 12:34
Wha if they are in separate files? – Shocky2 Jul 16 '17 at 12:34
@Shocky2 Files don't change anything, it's still inclusion order. – Hatted Rooster Jul 16 '17 at 12:34
@Shocky2 - Then `Container.h` better include `Piece.h` – StoryTeller - Unslander Monica Jul 16 '17 at 12:34
1

@RickAstley Wasn't directed to you :) I meant to explain to OP that what you did is actually declaring Piece first – CIsForCookies Jul 16 '17 at 12:37

9T9 · Answer 2 · 2017-08-04T06:01:31.637

1

Forward declaration is what you are looking for.

class Container;

class Piece
{
    Container* cont;
public:
    Piece(Container * c) :cont(c) {};
};
class Container
{
    Piece p;
public:
    Container() :p(this) {};
};

If you google you would find more information. See the one below.

When can I use a forward declaration?

edited Aug 04 '17 at 06:01

answered Jul 16 '17 at 13:10

9T9

698
2
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1

Thanks. Didn't know the name for it. – Shocky2 Jul 16 '17 at 13:30

Class member of an undeclared class

2 Answers2