Circular dependency struct, error redefinition of struct when using forward declaration

Question

The following code compiles in C, using ARMCC under Keil, However fails to compile in C++, using G++ under eclipse. The original code had some const keywords but that seems to be causing another, less important issue, so I removed them for now.

struct MENU
{
    struct MENU *   NextMenu;
    struct MENU *   PrevMenu;
    void            (* InitFunction)(void);
};

typedef struct MENU MENU_T;

MENU_T MENU_A; // <- this forward declaration is needed for circular reference between structs
MENU_T MENU_B;
MENU_T MENU_C;

MENU_T
MENU_A = // <- However redefinition error occurs here
{
    .NextMenu = &MENU_B,
    .PrevMenu = &MENU_C,
    .InitFunction = 0,
};

MENU_T
MENU_B =
{
    .NextMenu = &MENU_C,
    .PrevMenu = &MENU_A,
    .InitFunction = 0,
};

MENU_T
MENU_C =
{
    .NextMenu = &MENU_A,
    .PrevMenu = &MENU_B,
    .InitFunction = 0,
};

As far as I can tell, the first line is a declaration and not a definition, so the "error redefinition" should not occur...

Answer 1

With MENU_T MENU_A; , you do not "forward declare" a variable named MENU_A , you rather define it. Hence, when you later write MENU_T MENU_A = { ... } , you get an error indicating that you redefine a variable with the same name MENU_A .

For just declaring a variable (without defining it), use keyword extern :

extern MENU_T MENU_A; // declaration of MENU_A, not a definition
extern MENU_T MENU_B;
extern MENU_T MENU_C;

MENU_T MENU_A = 
{
    .NextMenu = &MENU_B,
    .PrevMenu = &MENU_C,
    .InitFunction = 0,
};

...