1

Consider this class from the WinAPI:

typedef struct tagRECT
{
    LONG    left;
    LONG    top;
    LONG    right;
    LONG    bottom;
} RECT, *PRECT, NEAR *NPRECT, FAR *LPRECT;

I am enhancing it in a class named Rect which allows you to multiply/add/subtract/compare two Rects, along with other features. The only real reason I need my Rect class to know about RECT is because the class features a conversion operator that allows a Rect to be passed as a RECT, and to be assigned a RECT.

But, in the file Rect.h, I do not want to include <Windows.h>, I only want to include <Windows.h> in the source file so that I may keep my inclusion tree small.

I know that structures can be forward declared like so: struct MyStruct; But, the actual name of the structure is tagRECT and it has an object list, so I am kind of confused as to how to forward declare it. Here is a portion of my class:

// Forward declare RECT here.

class Rect {
    public:
        int X, Y, Width, Height;

        Rect(void);
        Rect(int x, int y, int w, int h);
        Rect(const RECT& rc);

        //! RECT to Rect assignment.
        Rect& operator = (const RECT& other);

        //! Rect to RECT conversion.
        operator RECT() const;

        /* ------------ Comparison Operators ------------ */

        Rect& operator <  (const Rect& other);
        Rect& operator >  (const Rect& other);
        Rect& operator <= (const Rect& other);
        Rect& operator >= (const Rect& other);
        Rect& operator == (const Rect& other);
        Rect& operator != (const Rect& other);
};

Would this be valid?

// Forward declaration
struct RECT;

My thought is no, since RECT is just an alias of tagRECT. I mean, I know the header file would still be valid if I did this, but when I create the source file Rect.cpp and include <Windows.h> there, I fear that is where I am going to experience problems.

How could I forward declare RECT?

halfer
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Brandon Miller
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    Just a note. I did the same thing, I also wanted an enhanced Version of the RECT structure. At the end my structures derived from RECT because then no casting is needed when using winapi function expecting RECT as Parameter. I found this as a better solution and something you might take into account – roohan Nov 16 '12 at 01:40
  • @user1017443 That's a good idea. I had originally considered it, but I decided against it because I wanted 'Width' and 'Height' instead of 'cx' and 'cy'. Which, that probably isn't a great excuse seeing as how that way requires more writing on the class-users' part. – Brandon Miller Nov 16 '12 at 01:56
  • But either way that would require including '' in the file. – Brandon Miller Nov 16 '12 at 01:56
  • You can still define member functions called Width for example and these just return the member of the RECT structure. And yes deriving is easier from a user perspective. – roohan Nov 16 '12 at 02:56

2 Answers2

3

You can multiply declare a typedef name, and also forward declare the struct name at the same time:

typedef struct tagRECT RECT;

https://ideone.com/7K7st7

Note that you can't call a function returning incomplete type, so the conversion operator RECT() const cannot be called if tagRECT is only forward declared.

Potatoswatter
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3

You doesn't need to know the function definition before actually dereferencing the type.

So you can forward declare in your header file (because you will not make any dereferencing here) then include Windows.h in your source file.

[edit] Didn't seen that it was a typedef. However, the other answer is wrong : there is a way to (kind of) forward declare a typedef.

Community
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Maël Nison
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    Thanks, didn't know that. It's certainly not a good practice, but in this case it's M$'s fault not OP's. It's not really a forward declaration of the typedef, but multiple declarations which have to be compatible. – Potatoswatter Nov 16 '12 at 01:32
  • Would this be any different if something was declared `extern "C"` or not? I'm wondering if any of the windows headers are declared that way, and what effect that may have. – Kevin Anderson Nov 16 '12 at 03:47
  • @Kevin Presumably the C interfaces are, but `extern "C"` is a linkage modifier and types and typedefs don't have linkage so it shouldn't matter. – Potatoswatter Nov 16 '12 at 07:27
  • @Potatoswatter - I was wondering if a struct in C with the same members was a different size than it is in C++ and how that may affect things in other cases, since "struct" and "class" are very very close in C++, but obviously only one of them exists in C. – Kevin Anderson Nov 16 '12 at 12:50