难以理解循环声明

Question

嗨，我试图创建一个工厂方法，返回类A的派生类，我无法理解循环声明，我希望你能帮我解决这个问题。

谢谢。

AChildOne.cpp

---

#include "AChildOne.h" 

AChildOne.h

---

#ifndef ACHILDONE_H 
#define ACHILDONE_H 
#include "A.h" 

class A_CHILD_ONE : public A { 
}; 

#endif 

A.cpp

---

#include "A.h" 

void A::a(){ 
    Factory::fact(); 
}; 

A.H

---

#ifndef A_H 
#define A_H 

#include "Factory.h" 

class A { 
    public: 
     static void a(); 
}; 

#endif 

Factory.cpp

---

#include "Factory.h" 
A *Factory::fact(){ 
    return new A_CHILD_ONE; 
} 

Factory.h

---

#ifndef FACTORY_H 
#define FACTORY_H 

#include "A.h" 
#include "AChildOne.h" 

class Factory { 
    public: 
    static A *fact(); 
}; 

#endif 

编译错误

g++ A.cpp Factory.cpp AChildOne.cpp -o test 
In file included from Factory.h:5:0, 
       from A.h:4, 
       from A.cpp:1: 
AChildOne.h:5:30: error: expected class-name before ‘{’ token 
class A_CHILD_ONE : public A { 
          ^
In file included from A.h:4:0, 
       from A.cpp:1: 
Factory.h:9:10: error: ‘A’ does not name a type 
    static A *fact(); 
     ^
A.cpp: In static member function ‘static void A::a()’: 
A.cpp:4:2: error: ‘fact’ is not a member of ‘Factory’ 
    Factory::fact(); 
^
In file included from A.h:4:0, 
       from AChildOne.h:3, 
       from AChildOne.cpp:1: 
Factory.h:9:10: error: ‘A’ does not name a type 
    static A *fact(); 
     ^

Answer 1

在Factory.h尝试包括A.h;在A.h中，您尝试包括Factory.h。

将Factory.h代入A.cpp并从A.h中移除应该有所帮助。

Factory声明依赖于A接口。 A声明不依赖于Factory声明，但A定义。

另外，Factory.h不需要知道约AChildOne.h，但Factory.cpp呢。所以移动#include AChildOne.h到Factory.cpp。