c++ - 编译器说每个构造函数和函数都被重新定义并且不会编译。我使用的#IFNDEF 错了吗？ - C++

Question

我正在尝试使用类模板实现我自己的 OrderedList 数据结构。我的 CPP 文件中的每个构造函数和函数在开头“{”时都有错误。错误显示为“重新定义 FUNCTION_NAME FUNTION_NAME previously declared here”。

我让一些同事看过它，但无济于事。这是我的文件和错误:

CPP 文件“MyOrderedList.cpp”

#include "MyOrderedList.h"

template <class E>
MyOrderedList<E>::MyOrderedList() {/*IMPLEMENTATION*/}

template <class E>
MyOrderedList<E>::MyOrderedList(const MyOrderedList<E>& orig) { /*IMPLEMENTATION*/}

template <class E>
void MyOrderedList<E>::operator =(const MyOrderedList<E>& orig){/*IMPLEMENTATION*/}

template <class E>
MyOrderedList<E>::~MyOrderedList() {/*IMPLEMENTATION*/}

template <class E>
void MyOrderedList<E>::insert(E data){/*IMPLEMENTATION*/}

template <class E>
E MyOrderedList<E>::get(int pos) const{/*IMPLEMENTATION*/}

template <class E>
Node<E>* MyOrderedList<E>::getHead() const{/*IMPLEMENTATION*/}

template <class E>
MyOrderedList<E> MyOrderedList<E>::kLargest(int k) const{/*IMPLEMENTATION*/}

template <class E>
MyOrderedList<E> MyOrderedList<E>::operator +(const MyOrderedList<E>& list {/*IMPLEMENTATION*/}

H 文件“MyOrderedList.h”

#ifndef MYORDEREDLIST_H
#define MYORDEREDLIST_H

#include <iostream>
#include <cstdlib>
#include "Node.h"

template <class E>
class MyOrderedList;
template <class E>
std::ostream& operator <<(std::ostream& out, const MyOrderedList<E>& list);

template <class E>
class MyOrderedList {
public:
    MyOrderedList();
    MyOrderedList(const MyOrderedList<E>& orig);
    void operator =(const MyOrderedList<E>& orig);
    virtual ~MyOrderedList();

    bool remove(E data);
    MyOrderedList<E> kLargest(int k) const;
    E get(int pos) const;
    void insert(E data);
    Node<E>* getHead() const;

    MyOrderedList<E> operator +(const MyOrderedList<E>& list);

    friend std::ostream& operator <<(std::ostream& out, const MyOrderedList<E>& list){/*IMPLEMENTATION*/}

private:
    Node<E>* head;
    int size;
};

#include "MyOrderedList.cpp"
#endif  //MYORDEREDLIST_H

Node.h

#ifndef NODE_H
#define NODE_H

#include <iostream>

template <class E>
class Node {
public:
    Node(const E& init_data = NULL, Node<E>* init_link = NULL){data = init_data; link = init_link;}
    Node(const Node<E>& orig);
    virtual ~Node();

    E getData() const{return data;}
    void setData(E newData);

    Node<E>* getLink(){return link;}
    void setLink(Node<E>* nextLink) {link = nextLink;}
private:
    E data;
    Node<E>* link;
};

#endif  /* NODE_H */

编译器错误

MyOrderedList.cpp:12: error: redefinition of `MyOrderedList<E>::MyOrderedList()'
MyOrderedList.cpp:12: error: `MyOrderedList<E>::MyOrderedList()' previously declared here
MyOrderedList.cpp:19: error: redefinition of `MyOrderedList<E>::MyOrderedList(const MyOrderedList<E>&)'
MyOrderedList.cpp:19: error: `MyOrderedList<E>::MyOrderedList(const MyOrderedList<E>&)' previously declared here
MyOrderedList.cpp:42: error: redefinition of `void MyOrderedList<E>::operator=(const MyOrderedList<E>&)'
MyOrderedList.cpp:42: error: `void MyOrderedList<E>::operator=(const MyOrderedList<E>&)' previously declared here
MyOrderedList.cpp:77: error: redefinition of `MyOrderedList<E>::~MyOrderedList()'
MyOrderedList.cpp:77: error: `virtual MyOrderedList<E>::~MyOrderedList()' previously declared here
MyOrderedList.cpp:91: error: redefinition of `void MyOrderedList<E>::insert(E)'
MyOrderedList.cpp:91: error: `void MyOrderedList<E>::insert(E)' previously declared here
MyOrderedList.cpp:119: error: redefinition of `E MyOrderedList<E>::get(int) const'
MyOrderedList.cpp:119: error: `E MyOrderedList<E>::get(int) const' previously declared here
MyOrderedList.cpp:134: error: redefinition of `Node<E>* MyOrderedList<E>::getHead() const'
MyOrderedList.cpp:134: error: `Node<E>* MyOrderedList<E>::getHead() const' previously declared here
MyOrderedList.cpp:140: error: redefinition of `MyOrderedList<E> MyOrderedList<E>::kLargest(int) const'
MyOrderedList.cpp:140: error: `MyOrderedList<E> MyOrderedList<E>::kLargest(int) const' previously declared here
MyOrderedList.cpp:158: error: redefinition of `MyOrderedList<E> MyOrderedList<E>::operator+(const MyOrderedList<E>&)'
MyOrderedList.cpp:158: error: `MyOrderedList<E> MyOrderedList<E>::operator+(const MyOrderedList<E>&)' previously declared here

Answer 1

您的问题是您在 header 中包含函数的定义(间接通过 #include "MyOrderedList.cpp"。因为函数未声明为 inline 编译器在包含 header 的所有翻译单元中生成符号，因此您会得到多重定义的符号。

该问题的一个解决方案是在定义中将函数声明为 inline，以便链接器可以丢弃重复项。将定义保存在单独的文件中并不是什么新鲜事，但也不太常见，一些标准库实现就是这样做的。但是请考虑将 .cpp 重命名为其他名称以避免混淆(大多数人会认为 .cpp 意味着它是要编译的，而不是包含在内)。

另一方面，我建议将其进一步简化，完全删除 .cpp 并只包含所有内容的 .h(仍将所有函数定义标记为 inline)，甚至更进一步，在类定义中定义函数，默认情况下它们是 inline。