Forwards declaration question?

This is a discussion on Forwards declaration question? within the C Programming forums, part of the General Programming Boards category; Hello, I have recently looked into forwards declarations and what they do. I understand how to code forward declarations but ...

  1. #1
    Registered User
    Join Date
    Jun 2010
    Posts
    45

    Forwards declaration question?

    Hello,

    I have recently looked into forwards declarations and what they do. I understand how to code forward declarations but I don't understand why we would need them in this particular example.

    Please view the small code sample and I will pose my question at the end:

    Code:
    =================mainc. (does nothing)
    #include <stdio.h>
    #include "PC.h"
    #include "REC.h"
     
    int main()
    {
    int i; 
    }
    
    ==================PC.h
    #ifndef PC_H
    #define PC_H
    
    #include "REC.h"
    typedef struct tag_pc 			
    {unsigned short	gso_flash_address;              
    } pc;                                                 		
    
    #endif // PC_H //
    
    ==================PC.c
    #include <stdio.h>
    #include "PC.h"
    // Methods here...
    
    ==================REC.h
    #ifndef REC_H
    #define REC_H
    
    //struct tag_pc;	// Forwards declaration commented on purpose
    
    typedef struct tag_rec_data
    {unsigned short LKdct__F1;          
    }rec_data;
    
    typedef struct tag_rec                  				
    {unsigned short f__pc_xchoice; 				     
    rec_data *records;
    }rec;                           
    
    int REC_match_link_id(pc  *pObj);           
                        // struct tag_pc     *pObj);           
    
    #endif // REC_H //
    
    ===================REC.c
    #include <stdio.h>
    #include "PC.h"
    #include "REC.h"
    
    int REC_match_link_id(pc     *pObj)           
                          //struct tag_pc      *pObj) 
    {
    // ... other code
    }
    
    ========================


    Okay, the above code gives an error at the "int REC_match_link_id(....)" function declaration and implementation levels. My question is:

    If REC.c includes REC.h and REC.h include PC.h (which is where pc is declared), why is it still not sufficient for the compiler to recognize the "pc *pObj" parameter in the int REC_match_link_id(....) function parameter list?

    I mean isn't the whole PC.h header included in the REC.c compiler's translation unit???? Then, why is it that I have use a forwards declaration for the code above to compile correctly??? I know that if I uncomment the forwards declaration in REC.h and replace the "pc *pObj" parameter to "struct tag_pc" in the function's parameter list everything will compile... but I don't see why it wouldn't compile if I leave the code as above!

    Basically, when we include the PC.h in the REC.h, then, if pc is used anywhere in REC.h/.c it should have visible scope... no?

    All feedback sincerely appreciated!
    Last edited by see the big C; 12-04-2010 at 04:32 PM.

  2. #2
    Banned
    Join Date
    Aug 2010
    Location
    Ontario Canada
    Posts
    9,547
    Rec.h will have to include pc.h so that your pc struct is visible to the int REC_match_link_id() function. Frankly I'd just combine the two headers.

  3. #3
    Registered User
    Join Date
    Jun 2005
    Posts
    6,815
    Quote Originally Posted by see the big C View Post
    If REC.c includes REC.h and REC.h include PC.h (which is where pc is declared), why is it still not sufficient for the compiler to recognize the "pc *pObj" parameter in the int REC_match_link_id(....) function parameter list?
    You need to look more closely at your own code. REC.h does not #include PC.h. PC.h declares the pc type after #include'ing REC.h.

    Therefore, when reading REC.h, the compiler has no information about what the pc type is (typedefs that come later as you - or the compiler - reads the file don't count).

    That means REC.h never has any information about what the type pc is. Since REC.h declares a function that accepts a pointer to pc, but the type pc has not been defined at any point when REC.h is included, a forward declaration is required.
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy or unhelpful in reply to you, or tell you you need to demonstrate more effort before you can expect help, it is likely you deserve it. Suck it up, Buttercup, and read this, this, and this before posting again.

  4. #4
    Registered User
    Join Date
    Jun 2010
    Posts
    45
    Hi CommonTater,

    >Rec.h will have to include pc.h so that your pc struct is visible to the int REC_match_link_id() >function.

    Oh yeah well that's what I thought! try it.. you'll see it still doesn't work!

    Rob

  5. #5
    Registered User
    Join Date
    Jun 2010
    Posts
    45
    Excellent Grumpy,

    I see it now.

    Thanks!

    Sincere regards
    Rob

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. Question about a declaration
    By Boxknife in forum C Programming
    Replies: 3
    Last Post: 06-22-2008, 10:28 PM
  2. Dikumud
    By maxorator in forum C++ Programming
    Replies: 1
    Last Post: 10-01-2005, 06:39 AM
  3. cannot start a parameter declaration
    By Dark Nemesis in forum C++ Programming
    Replies: 6
    Last Post: 09-23-2005, 02:09 PM
  4. Problem with Visual C++ Object-Oriented Programming Book.
    By GameGenie in forum C++ Programming
    Replies: 9
    Last Post: 08-29-2005, 11:21 PM
  5. errors initializing D3D
    By Vacation Guy in forum C++ Programming
    Replies: 3
    Last Post: 08-07-2005, 12:20 PM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21