Question

I have such code with 2 structures:

#include <list>

using namespace std;

struct Connection;
struct User;    

typedef list<Connection> Connections;
typedef list<User> Users;

struct User {
    Connections::iterator connection;
};

struct Connection {
    Users::iterator user;
};

But when I try to compile it, the compiler (C++ Builder XE) return me such error – “Undefined structure ‘Connection’“.

Can anyone help me with my problem?

@ereOn,
struct Connection;
struct User;
struct Connection {
Users::iterator user;
};
typedef list Connections;
typedef list Users;

struct User {
    Connections::iterator connection;
};

Undefined structure ‘User’

Answer 1

You’re using incomplete type as type argument to std::list which invokes undefined bevahior according to the C++ Standard.

§17​.4.3.6/2 says,

In particular, the effects are undefined in the following cases:
— if an incomplete type (3.9) is used as a template argument when instantiating a template component.

So one solution would be using pointer of incomplete type.

struct Connection;
struct User;    

typedef list<Connection*> Connections; //modified line
typedef list<User*> Users;             //modified line

struct User {
    Connections::iterator connection;
};

struct Connection {
    Users::iterator user;
};

This will work because pointer to an incomplete type is a complete type, the compiler can know the size of Connection* which is equal to sizeof(Connection*) even if Connection hasn’t been defined yet.