forward declaration in C++

ducttape · 05-08-2009, 05:03 AM

myProgram.cpp:

Code:

class childClass
{
          parentClass parent;  //reference to class that created this child
          childClass(parentClass p)
          {
                    parent = p;
          }
};

class parentClass
{
          childClass child(this);
};

This would work fine in Java, which I'm more accustomed to, but I need to do it in C++. The problem being that C++ compilers read classes sequentially and this code gives the error "error C2061: syntax error : identifier 'parentClass'" because it does not know what parentClass is yet. All the advice on the internet says to create a forward declaration for parentClass BEFORE the definition for childClass, and then keeping the definition for parentClass after childClass, such:

Code:

class parentClass;

class childClass
{
          parentClass parent;  //reference to class that created this child
          childClass(parentClass p)
          {
                    parent = p;
          }
};

class parentClass
{
          childClass child(this);
};

But this code then gives the error: "error C2079: 'myProgram:

arent' uses undefined class 'parentClass'"

I've tried swapping around the classes, too, so that parentClass comes before childClass with a forward declaration for childClass. This gives the same error. I've also tried putting the classes in seperate *.cpp files. I must admit I don't really know how to do this, but the way I did it resulted in the same error (...uses undefined class...).

What do you think? I'm a decent Java and C# programmer but C++ is just taking the .........

JVene · 05-08-2009, 05:15 AM

If you take the function body out of the class, and move it into a CPP (or at least after the complete declaration of all the other classes ) you won't have a problem.

cpjust · 05-08-2009, 06:39 AM

You need to change this member variable:

Code:

parentClass parent;

to

Code:

parentClass* parent;

because a forward declaration doesn't tell the compiler anything about the size of the parentClass object. So you're stuck with using a pointer instead.

anon · 05-08-2009, 08:20 AM

Code:

parentClass parent;  //reference to class that created this child

The comment is wrong. This is not a reference and parent won't have anything to do with the (class) instance that created this child (in C++).

Use a pointer as mentioned above and the forward declaration will be enough.

Brafil · 05-08-2009, 09:12 AM

You should use pointers or references anyway. Copying a whole class instance in a constructor? Ouch.

sarav957 · 10-10-2009, 11:56 AM

Quote:

Originally Posted by Brafil

You should use pointers or references anyway. Copying a whole class instance in a constructor? Ouch.

HI,
u r write in the internet most of the even dont understand the basics of c++.well in the forward declration is an unfilled class so u cant create an object but you can create an object pointer or references.

cpjust · 10-10-2009, 03:00 PM

Quote:

Originally Posted by sarav957

HI,
u r write in the internet most of the even dont understand the basics of c++.well in the forward declration is an unfilled class so u cant create an object but you can create an object pointer or references.

You might want to check the expiration date on threads before posting. Five month old threads are pretty stale. Learning to type properly would help too.

05-08-2009, 05:03 AM	#1
ducttape Registered User Join Date: May 2009 Posts: 1	forward declaration in C++ myProgram.cpp: Code: class childClass { parentClass parent; //reference to class that created this child childClass(parentClass p) { parent = p; } }; class parentClass { childClass child(this); }; This would work fine in Java, which I'm more accustomed to, but I need to do it in C++. The problem being that C++ compilers read classes sequentially and this code gives the error "error C2061: syntax error : identifier 'parentClass'" because it does not know what parentClass is yet. All the advice on the internet says to create a forward declaration for parentClass BEFORE the definition for childClass, and then keeping the definition for parentClass after childClass, such: Code: class parentClass; class childClass { parentClass parent; //reference to class that created this child childClass(parentClass p) { parent = p; } }; class parentClass { childClass child(this); }; But this code then gives the error: "error C2079: 'myProgram:arent' uses undefined class 'parentClass'" I've tried swapping around the classes, too, so that parentClass comes before childClass with a forward declaration for childClass. This gives the same error. I've also tried putting the classes in seperate *.cpp files. I must admit I don't really know how to do this, but the way I did it resulted in the same error (...uses undefined class...). What do you think? I'm a decent Java and C# programmer but C++ is just taking the .........
ducttape is offline

05-08-2009, 05:15 AM	#2
JVene Registered User Join Date: May 2007 Posts: 147	If you take the function body out of the class, and move it into a CPP (or at least after the complete declaration of all the other classes ) you won't have a problem.
JVene is offline

05-08-2009, 06:39 AM	#3
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,122	You need to change this member variable: Code: parentClass parent; to Code: parentClass* parent; because a forward declaration doesn't tell the compiler anything about the size of the parentClass object. So you're stuck with using a pointer instead. __________________ "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008
cpjust is offline

05-08-2009, 09:12 AM	#5
Brafil Making mistakes Join Date: Dec 2008 Posts: 347	You should use pointers or references anyway. Copying a whole class instance in a constructor? Ouch.
Brafil is offline

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
how do you resolve this error?	-EquinoX-	C Programming	32	11-05-2008 04:35 PM
Quantum Random Bit Generator	shawnt	C++ Programming	62	06-18-2008 10:17 AM
failure to import external C libraries in C++ project	nocturna_gr	C++ Programming	3	12-02-2007 03:49 PM
We Got _DEBUG Errors	Tonto	Windows Programming	5	12-22-2006 05:45 PM
Dikumud	maxorator	C++ Programming	1	10-01-2005 06:39 AM