Can't include struct in another file

Question

I'm making a multiple file project in c++. I have this code:

lista.h

struct elem
{
    account info;
    elem* next;
};

typedef elem* lista;

The error is showed here, were "lista* a" is declared.

login.h:

struct account
{
    string user = "";
    int hash_pass = 0;
};

struct list
{
   lista* a;
   int size;
};

login.cc:

#include "login.h"
#include "lista.h"
....

lista.cc

#include "login.h"
#include "lista.h"
....

In lista.cc and login.cc i have included login.h and lista.h but in login.h doesnt recognize lista as name of a type.

Please show a [MCVE] and show the all error messages tha compiles displays. — Jabberwocky, Aug 20 '18 at 08:55

fractals · Accepted Answer · 2018-08-20T09:21:09.293

Circular dependency! Assuming string type is well-defined somewhere else in the header files (maybe std::string?), this is because you included the files in the wrong order.

#include "login.h"
#include "lista.h" 
....

This is basically equivalent to:

struct account
{
    string user = "";
    int hash_pass = 0;
};

struct list
{
   lista* a;
   int size;
};

struct elem
{
    account info;
    elem* next;
};

typedef elem* lista;
....

As you can see, lista appears even before the typedef, which is why you're getting an error.

Obviously you do not want to care about in which order you are including the header files, so the right solution here would be to include lista.h in login.h with proper header guards. But that is not enough in this case: there is a circular dependency here, as lista.h needs struct account from login.h and login.h needs lista from lista.h. Therefore, we add a forward declaration as well. See this link for more info. Your final code would then be:

lista.h:

#ifndef LISTA_H_
#define LISTA_H_
struct account; // forward declaration

struct elem
{
    account* info; // notice that `account` now has to be a pointer
    elem* next;
};

typedef elem* lista;
#endif

login.h :

#ifndef LOGIN_H_
#define LOGIN_H_
#include "lista.h"

struct account
{
    string user = "";
    int hash_pass = 0;
};

struct list
{
   lista* a;
   int size;
};
#endif

score 1 · Answer 2 · answered Aug 20 '18 at 08:55

1

If you want to use something declared on A.h inside B.h, you need to include A.h in B.h. Hence, need to include lista.h in login.h.

answered Aug 20 '18 at 08:55

Ricardo Costeira

3,171
2
23
23

1

The OP will also need include guards in his header files. – Martin Bonner supports Monica Aug 20 '18 at 09:00
1

... and remove the now redundant `#include lista.h` from login.cc – acraig5075 Aug 20 '18 at 09:02
@acraig5075 Only if `login.cc` does not use anything from `lista.h` itself. If it does, you should keep the include, because who knows what will happen with `login.h` in the future - it might no longer need/include `lista.h` eventually. – Max Langhof Aug 20 '18 at 09:05

score 0 · Answer 3 · answered Aug 20 '18 at 09:01

0

Include lista.h in login.h as you your login header needs access to lista :)

answered Aug 20 '18 at 09:01

Prodigle

1,757
12
23

score 0 · Answer 4 · answered Aug 20 '18 at 09:02

Your problem boils down to this:

struct elem
{
  account info;   // <<< account is not known here
  elem* next;     // elem is not known here
};

typedef elem* lista;   

struct account
{
  std::string user = "";
  int hash_pass = 0;
};

struct list
{
  lista* a;
  int size;
};    

typedef elem* lista;

If you correct the order of declarations it compiles fine:

struct account
{
  std::string user = "";
  int hash_pass = 0;
};

struct elem
{
  account info;
  elem* next;
};

typedef elem* lista;

struct list
{
  lista* a;
  int size;
};

Can't include struct in another file

4 Answers4