error: invalid use of incomplete type ‘struct Item’

Question

I have been having a lot of trouble with my headers and making sure everything is declared correctly. First off my files:

//Main.cpp
#include "Item.h"
#include "Warehouse.h"
using namespace std;

int main() {
    ...
}

//Item.h
#ifndef ITEM_H
#define ITEM_H
#include <string>
using namespace std;

class Item {
    ...
};
#endif  /* ITEM_H */

//Item.cpp
#include "Item.h"

//Warehouse.h
#define WAREHOUSE_H
#ifndef ITEM_H
#define ITEM_H
using namespace std;

class Item;
class Warehouse {
    ...
private:
    Item* array; //problem starts with this
};
#endif  /* WAREHOUSE_H */

//Warehouse.cpp
#include "Warehouse.h"
#include "Item.h"

Warehouse::Warehouse() {
    array = new Item[arraySize]; //and this is where I get the error
}

I am pretty sure the problem has to do with my header in Warehouse.h but every combination I try does not work. Sorry if not enough of the code is posted but I figure the problem is with the includes and declarations.

Thanks ahead of time.

edit: to clarify this is not in one file. I just wrote it like this to simplify things. Each one of the above is a separate file.

You're missing semicolons at the end of all your classes. Also, it's *really really* not a good idea to put `using namespace` inside a header. — Cameron, Mar 31 '14 at 21:54
Think of what happens when two different header files have the same include guards. — juanchopanza, Mar 31 '14 at 22:08
@juanchopanza: I feel like this question would benefit from an answer in [this style](http://stackoverflow.com/a/5320021/103167) Feel free to cut+paste if that saves you time. — Ben Voigt, Mar 31 '14 at 22:16

score 3 · Answer 1 · answered Mar 31 '14 at 22:17

Your include guards in the header file Warehouse.h are not correct.

Instead of

//Warehouse.h
#define WAREHOUSE_H
#ifndef ITEM_H
#define ITEM_H
using namespace std;

// ...

#endif  /* WAREHOUSE_H */

you want

//Warehouse.h
#ifndef WAREHOUSE_H
#define WAREHOUSE_H
using namespace std;

// ...

#endif  /* WAREHOUSE_H */

With the current version the class definition in item.h is never included in Warehouse.cpp because the mixed-up include guards in Warehouse.h prevent item.h to be read due to the order of

//Warehouse.cpp
#include "Warehouse.h"
#include "Item.h"    //Warehouse.cpp
#include "Warehouse.h"
#include "Item.h"

Then the compiler does not know the definition of Item at that point, hence the error.

Another thing: Do not form the habit of using namespace std in header files. This will lead to issues at some point.

Aside: `using namespace std;` is not cool, especially in header files. — juanchopanza, Mar 31 '14 at 23:41

Vlad from Moscow · Answer 2 · 2014-03-31T22:16:27.807

0

Problem is not in this declaration

private:
    Item* array; //problem starts with this

You may define a pointer to an incomplete type.

I think the problem is in a statement where you try to allocate an object for this pointer using operator new or to dereference the pointer.

Also I do not see any reason why you do not want to include header Item.h in header Warehouse.h instead of using elaborated name

class Item;

edited Mar 31 '14 at 22:16

answered Mar 31 '14 at 22:09

Vlad from Moscow

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error: invalid use of incomplete type ‘struct Item’

2 Answers2