Thread: 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 .........

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.

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

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.