C++ 对象的前向声明

Question

我正在尝试弄清楚如何在新对象存储其他新对象的地方转发声明对象，例如：

一个队列，包含节点，这些节点包含包含 2 个列表或最好是 2 个队列的数据项。

这是我的 Data.h

#ifndef DATA_H_
#define DATA_H_

class List;
class Data {
public:
    Data();
    Data(int aID);
    virtual ~Data();
    virtual int getID();
    virtual void setID(int aID);
private:
    int ID;
    List *listOne;
    List *listTwo;
};

#endif /* DATA_H_ */

还有我的Data.c

#include "Data.h"
#include "List.h"

Data::Data() {
    listOne = new List();
    listTwo = new List();
    Data::ID = 0;
}

Data::Data(int aID) {
    Data::listOne = new List();
    Data::listTwo = new List();
    Data::ID = aID;
}

Data::~Data() {
    delete(listOne);
    delete(listTwo);
}

int Data::getID() {
    return Data::ID;
}

void Data::setID(int aID) {
    Data::ID = ID;
}

错误信息：

..\Data.cpp: In constructor 'Data::Data()':
..\Data.cpp:12:23: error: invalid use of incomplete type 'class List'
  listOne = new List();
                       ^
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp:13:22: error: invalid use of incomplete type 'class List'
  listTwo = new List();
                      ^
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp: In constructor 'Data::Data(int)':
..\Data.cpp:18:29: error: invalid use of incomplete type 'class List'
  Data::listOne = new List();
                             ^
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp:19:28: error: invalid use of incomplete type 'class List'
  Data::listTwo = new List();
                            ^
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp: In destructor 'virtual Data::~Data()':
..\Data.cpp:24:18: warning: possible problem detected in invocation of delete operator: [-Wdelete-incomplete]
  delete(listOne);
                  ^
..\Data.cpp:24:18: warning: invalid use of incomplete type 'class List'
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp:24:18: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined
  delete(listOne);
                  ^
..\Data.cpp:25:17: warning: possible problem detected in invocation of delete operator: [-Wdelete-incomplete]
  delete(listTwo);
                 ^
..\Data.cpp:25:17: warning: invalid use of incomplete type 'class List'
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp:25:17: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined
  delete(listTwo);

现在，如果我删除 Data.h 中的前向声明并让 Data.h 包含 List.h，我将得到：并导致循环继承周期

..\Data.h:25:2: error: 'List' does not name a type
  List *cpubursts;
  ^
..\Data.h:26:2: error: 'List' does not name a type
  List *iobursts;

我将如何修复这种设置？

编辑：List.h

#ifndef LIST_H_
#define LIST_H_

#include "Node.h"

namespace std {

class List {
private:
    Node *top;
public:
    List();
    virtual ~List();
    virtual Node* getTop();
    virtual void add(Node * linkedNode);
    virtual void remove(Node * linkedNode);
};

} /* namespace std */

#endif /* LIST_H_ */

Node.h

#ifndef NODE_H_
#define NODE_H_

#include "Data.h"

namespace std {

class Node {
public:
    Node();
    Node(Data * aItem);
    virtual ~Node();
    virtual Node* getNext();
    virtual void setNext(Node * linkedNode);
    virtual Data* getData();
    virtual void setData(Data* item);
private:
    Node *next;
    Data *item;
};

} /* namespace std */

#endif /* NODE_H_ */

Answer 1

创建 object 时，编译器必须知道 class，因此您必须在 Data.cpp.[=10] 中包含定义它的 header =]

请注意，不建议使用原始的新建和删除，使用直接实例或使用智能指针。您可以使用带有前向声明的 std::unique，但是当编译器即将发出 std::unique_ptr 的析构函数的代码时，请确保完整的 class 定义是已知的。

Answer 2

您在 std 命名空间内定义 List，并在其外部转发声明。当你这样做时，你说的是 "there's a class called List in the global namespace"，当然没有。

你需要表达的是"there's a class called List inside the std namespace"，可以这样操作：

namespace std {
    class List;
}

class Data {
    /* your class */
};

我想问你为什么要在命名空间 std 中定义你自己的 class，但这不是重点。

C++ 对象的前向声明

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