C + + Learning path: Forward statement

Introduction:

In a special case, when two classes need to reference each other to form a "ring" reference, it is not possible to define the use first. We need to use a forward statement.

However, the class of the forward declaration cannot be instantiated.

  1   #pragma  once 2   3  #include  " b.h    4  class   A   {  6   public  :    A ();   ~a ();   10   B B_ ; 11 }; 

#pragma once"A.h"class  b{public:    B ();     ~B ();    A A_;};

Two classes that contain a compilation hint error are:

1 1>------Started Build: Project: Test, configuration: Debug Win32------2 1>B.cpp3 1>g:\cpp\ Code \TEST\TEST\A.H (Ten): Error C2146: syntax error: Missing ";" (in front of the identifier "B_")4 1>g:\cpp\ Code \TEST\TEST\A.H (Ten): Error C4430: Missing type specifier-assumed to beint。 Note: C + + does not support the defaultint5 1>A.cpp6 1>g:\cpp\ Code \test\test\b.h (9): Error C2146: syntax error: Missing ";" (in front of the identifier "A_")7 1>g:\cpp\ Code \test\test\b.h (9): Error C4430: Missing type specifier-assumed to beint。 Note: C + + does not support the defaultint8 1>Generating Code ...9========== Generation: Success0One, failure1A, Latest0One, skip0A ==========

So what is a forward statement?

#pragma once"A.h"class  //forward declaration class  b{  public:    B ();     ~B ();    //Non-instancing };

This is still wrong, because the forward declaration is not instantiated.

Compile hints:

1>------Started Build: Project: Test, configuration: Debug Win32------1>B.cpp1>g:\cpp\ Code \TEST\TEST\A.H (Ten): Error C2146: syntax error: Missing ";" (in front of the identifier "B_")1>g:\cpp\ Code \TEST\TEST\A.H (Ten): Error C4430: Missing type specifier-assumed to beint。 Note: C + + does not support the defaultint1>A.cpp1>g:\cpp\ Code \test\test\b.h ( One): Error C2079: "B::a_" using undefinedclass"A"1>Generating Code ...========== Generation: Success0One, failure1A, Latest0One, skip0A ==========

Very simple processing, only need to use the pointer. It is also possible to pass a trial reference.

1 #pragmaOnce2#include"A.h"3 4 classA;5 classB6 {7  Public:8 B ();9~B ();Ten  OneAA_; A};

Nested classes: Demo

1#include <iostream>2 using namespacestd;3 4 5 classOuter6 {7     classInner8     {9      Public:Ten         voidFun () One         { Acout <<"inner:fun ...."<<Endl; -         } -     }; the  -  Public: - Inner obj_; -     voidFun () +     { -cout <<"outer:fun ..."<<Endl; + Obj_.fun (); A     } at  - }; -  - intMainvoid) - { - Outer o; in O.fun (); -}

# # From a scope perspective, nested classes are hidden in the perimeter class, and the class name can only be used in a perimeter class. If you use the class name in the scope of the perimeter class, you need to add a name qualifier;

# #嵌套类中的成员函数可以再它的类体外定义;

# #嵌套类的成员函数对外围类的的成员没有访问权, and vice versa.

# #嵌套类仅仅是语法上的嵌入

1#include <iostream>2 using namespacestd;3 4 5 classOuter6 {7     classInner8     {9      Public:Ten         voidFun (); One     }; A  -  Public: - Inner obj_; the     voidFun () -     { -cout <<"outer:fun ..."<<Endl; - Obj_.fun (); +     } -  + }; A voidOuter::inner::fun () at { -cout <<"inner:fun ...."<<Endl; - } -  - intMainvoid) - { in Outer o; - O.fun (); to}

Out-of-class declarations only need to remember to add multiple class names.

C + + Learning path: Forward statement