C++_ front-facing statement

Why should I have a forward statement?

eg

-Defines a class A, which uses object B of Class B, and then defines a Class B, which also contains an object A of Class A, as follows:

// a.h   " B.h "  class A  {  ....   Private :      b b;  };   // b.h   " a.h "  class B  {  ....   Private :      A;  

A compilation, there is a problem with each other , the solution is to declare Class B in the a.h file, and then use a pointer to B.

// a.h    // #include "b.h"   class B;    class A   {   ....    Private :   b b;   };    // b.h    " a.h "   class B  {   ....    Private :   A;   };

Benefits of the predecessor statement:

-When we use the predecessor Declaration of Class B in Class A, when we modify Class B, we only need to recompile class B, without recompiling A.H's

-Of course, when you really use Class B, you must include the B.h

-Therefore, when you define a header file, you can rely on the predecessor declaration to not rely on the definition, and, where appropriate, should use a type of pre-declaration to reduce the header file import.

eg

-When a pointer member of a class is defined in a header file, the pointer is a pointer to another type class account that is defined in the project account* M_account;

-then add the preceding declaration before the class declaration of the header file: class account;

-OK, without having to include the header file where the account is located

Conditions of use of the predecessor declaration:

-assuming class xxx is in Xxx.h, the following three scenarios can be used with the predecessor declaration without the inclusion of xxx.h

-1. A function declaration with a parameter or return type of XXX;

-2. Class static members of type XXXT;

-3. Class member variable declared as xxx* or xxx&;

-Need to include xxx.h in several cases

-1. If there is a non-static member in your class that defines type XXX

-2. Your class inherits XXX's words

Attention:

-Just about to understand, there are a lot of unclear places

-To be Continued ...

Reference:

-http://blog.csdn.net/moxiaomomo/article/details/8083199

-http://qimo601.iteye.com/blog/1406992

C++_ front-facing statement