c++ - When can I use a forward declaration? -

I am looking for definition when allowing me to announce a class forwarding in another category header file :

Do I allow it for a base class, a class organized as a member, to pass a member function from the context, etc.?

Keep yourself in the compiler's position: When you declare a type of forward, all the compilers Knows that this type is present; It does not know anything about its size, members, or methods, that is why it is called incomplete type , therefore, you do not use the type to declare a member, or base class Because the compiler will need to know the type of layout.

Assume the following forwarded declaration.

  square X;  
  What can you do with an incomplete type:   
  

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incomplete type Announce a member to become an indicator or a reference to:

  class Foo {X * pt; X & PT; };  
  
  
  
  
   
); X F2 ();              
  zero f3 (X *, X & amp;) {} X & amp; F4 () {} X * F5 () {}  
 
  
  What you can not do with an incomplete type:  
 
 
 
 Use it as a base class 
 
  class Foo: X {} // compiler error!  
 
 
 
 Use this to declare a member: 
 
  square feet {X m; // Compiler Error! };  
 
   
 
  define  actions or methods that use this type  
  zero f1 (xx) {} // compiler error! X F2 () {} // compiler error!             ; Zero method () {m-> Some method (); // Compiler Error! Int i = m- & gt; Some fields; // Compiler Error! }}; When it comes to a template, there is no absolute rule: whether you can use an incomplete type as a template parameter, depending on which  
 for example,  std :: vector & lt; T & gt;  Its parameters require a complete type, while  boost :: container:: vector & lt; T & gt;  Sometimes, if you use some member work then only one full type is required; for example. 
 
 A well-documented template should point out all the requirements of their parameters in their documents, whether they are of full form or not.