When is declaration of a class used in C++?

Question

Since class definition is internal link, we must define a class before we use it. I can't figure out a situation to use class declaration, like class A;. Is there?

Answer 1

That is a forward declaration, which is a method for dealing with circular dependencies in header files.

For example if two classes each hold a pointer to another, then you need to forward declare the class that is defined further down in the translation unit.

Answer 2

It is for forward declarations; imagine two classes having a pointer or mention to each other's instances:

 class A;
 class B;

 class A {
   class B*ptr;
   int x;
   /// etc
 };

 class B : A {
   std::string s;
   std::vector<A> v;
   //// etc
 };

It is also useful for readability; you might want to list all the classes first in some header.

And you could just forward declare a class and simply use its pointers without defining that class.

^{BTW, C99 has similar forward declarations for struct, union; and it is also useful to forward declare functions.}

Answer 3

You are asking about need for forward declaration.

Imagine following hypothetical example:

class Man
{ 
  Woman mySpouse;
};

class Woman 
{
  Man mySpouse;
};

we must definite a class before we use it.

Yes. So you can see for above example, this is impossible chicken-egg situation (aka circular dependency). This is why you need forward declaration and pointers, as shown below:

class Man;    // Forward declaration
class Woman;  // Forward declaration
class Man
{ 
  Woman * pMySpouse;
};

class Woman 
{
  Man * pMySpouse;
};

Thanks to forward declaration, this should work OK.

See answers to this question : When can I use a forward declaration?