C / C++ / MFC

26-Oct-07 5:48

I am using MFC,when I declare a X Object int class Y and a Y Object in class X,then compile Error,I know ,i have to use extern in file.h but I don't put it anywhere,please post a small code about extern to help me.Thanks everyone

Mark Salsbery26-Oct-07 5:56

26-Oct-07 5:56

class Y;  //<-- forward declaration

class X
{
   Y y;  <font color="Red">//<-- sizeof(Y) unknown here!</font>
};

class Y
{
   X x;
};

This shows the order the compiler needs to see the declarations, regardless of
whether you use header files or not.

This will only work with object pointers, since the compiler doesn't know the size of a Y
object when it encounters the X class.

The only solution, then, is something like:

class Y;  //<-- forward declaration

class X
{
   Y *py;
};

class Y
{
   X x;
};

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

nhathoang26-Oct-07 7:21

26-Oct-07 7:21

Oh thanks about your question,but I'm sorry,I want to ask about include<file.h> in MFC,you can try,
If you create two class in MFC,they are X and Y,I suppose in class you have a Object X,exam X1,and in X is Y1 belong Y:
class X
{
Y Y1
}
class Y
{
X X1
}
At MFC,These two classes stay two distributed places,so At class Y you must to call include<x.h> and at class X is <include y.h="">,this is a cause to compile error,Please help me

Mark Salsbery26-Oct-07 7:53

26-Oct-07 7:53

Please reread my response.

It doesn't matter if you use MFC or not, or if you use header files or not.

The compiler is going to parse the code from the top-down. To resolve the
circular reference problem, you have to have similar code to the code I posted
and it needs to be in that order.

Again, this will not work:

class X
{
Y Y1 <-- the compiler does NOT know what a Y object is here!!!
}
class Y
{
X X1
}

You could try this, but it won't work either:

class Y;
class X
{
Y Y1 <-- the compiler does NOT know what SIZE a Y object is here!!!
}
class Y
{
X X1
}

So, you must use a pointer. The compiler knows the size of a pointer:

class Y;
class X
{
Y *pY1
}
class Y
{
X X1
}

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

nhathoang26-Oct-07 8:37

26-Oct-07 8:37

Oh no,when you use MFC (Dialoge box) ,you create two class (class X and class Y)which different place,you can't spacify which class will compile before

Mark Salsbery26-Oct-07 8:53

26-Oct-07 8:53

Oh yes, you have complete control over the order of the code.

You always have to put your code in the order you want it compiled.
That doesn't change for "#include"d header files - they get compiled
right where you place the #include.

You're really missing my point here. My point is that you can't include
a type in a class that the compiler doesn't know anything about. You can,
however, use an empty class declaration (e.g. "class SomeClass;") to let the
compiler know that a class of that name exists but its size is unknown.

You are NOT going to be able to have a "circular reference" like you're showing
without using a pointer to an object of the unknown type in at least one of the
classes in question.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

David Crow26-Oct-07 9:21

David Crow

26-Oct-07 9:21

Come on, Mark, quite beating around the bush and tell us exactly what is going on. Do you have to make it so blasted difficult?

Mark Salsbery26-Oct-07 9:36

26-Oct-07 9:36

Heh. I'm trying! My head is clogged and I'm hopped up on cold medicine.

That's my story and I'm sticking to it Smile | :)

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

nhathoang26-Oct-07 10:05

26-Oct-07 10:05

sorry Mark,My foreign language is too bad,so I use many word which not true,I am sorry,

Fernando A. Gomez F.26-Oct-07 18:08

Fernando A. Gomez F.

26-Oct-07 18:08

What was first, the egg or the chicken?

That's the problem that Mark is trying to explain.

Hope is the negation of reality - Raistlin Majere

nhathoang26-Oct-07 21:55

26-Oct-07 21:55

dear Mark,I sorry about this question,My English language is very bad,so I don't good at using words,I thanks about your your answers,and I hope you can skip for me
good look to you

Mark Salsbery27-Oct-07 7:02

27-Oct-07 7:02

No problem Smile | :)

Did you fix your issue?

I don't know how else to explain it - you have to include your .h files and/or
add the appropriate lines of code in the correct order for the compiler.

The sample code in my replies looks really simple, but it should have everything you
need to resolve your circular reference. MFC is just C++ - all the same rules apply.

Cheers,
Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

nhathoang27-Oct-07 8:09