未定义对班级成员的引用 [英] Undefined reference to class member

问题描述

我制作了一个类定义文件Key.h及其代码Key.cpp.
我给Record命名为typedef Key Record.当我在main.cpp中创建Record对象作为代码Record data;时.我收到一条错误消息:未定义对Key :: Key(int)的引用".
Key.cpp类中还有另一个错误，指出未在此范围内声明x".我在这里放置了这些文件的代码，并在注释中也写了关于错误的内容.
Key.h

I have made a class definition file Key.h and its code Key.cpp.
I have given alias to Record as code typedef Key Record. When I create an object of Record in main.cpp as code Record data;. I get an error saying "Undefined reference to Key::Key(int)".
There''s another error in class Key.cpp saying "x was not declared in this scope". I have put up the code of these files here and have also written about error in the comments.
Key.h

class Key
{
int key;
public:
static int comparisons;
Key(int x=0);
int the_key()const;
};
bool operator == (const Key &x,const Key &y);
bool operator >= (const Key &x,const Key &y);
bool operator <= (const Key &x, const Key &y);
bool operator > (const Key &x, const Key &y);
bool operator < (const Key &x, const Key &y);
bool operator != (const Key &x, const Key &y);

Key.cpp

Key.cpp

#include"Key.h"
int Key::comparisons=0;
int Key::the_key()const
{
    return x;  //Something wrong here I think?
}
bool operator == (const Key &x, const Key &y)
{
    Key::comparisons++;
    return (x.the_key()==y.the_key());
}

main.cpp

main.cpp

#include <iostream>
#include "List.h"
#include "Key.h"
using namespace std;
typedef Key Record;
int main()
{
    int n;
    List<int> the_list;
    Record data; // Underfined refernece to Key::Key(int)
    cout<<"Enter the number of records to be stored. "<<endl;
    cin>>n;
    for(int i=0;i<n;i>    {
    the_list.insert(i,i);//(position, List_entry &x)
    }
    cout<<the_list.size();
    cout<<"Record value: ";
    return 0;
}</int></iostream>

Main.cpp:http://pastebin.com/R8RQyGVs
Key.h http://pastebin.com/DrAH0MPq
Key.cpp http://pastebin.com/xjTvgxxA
非常感谢您的帮助！

Main.cpp: http://pastebin.com/R8RQyGVs
Key.h http://pastebin.com/DrAH0MPq
Key.cpp http://pastebin.com/xjTvgxxA
Thanks a lot for help!!

推荐答案

应该不是

should''nt that be

int Key::the_key()const
{
    return key; 
}

对于其余的，我不知道"List"是什么...，所以我无法编译其余的代码.

for the rest, I don''t know what "List" is ... so I cannot compile the rest of the code.

我可以在您描述的代码中准确地看到两个错误:

实际上，x不在Key::the_key的范围内:

I can see exactly two errors in the code you describe:

Indeed, x is not in the scope of Key::the_key:

int Key::the_key()const
{
    //no declaration of x anywhere is scope
    //did you mean key (private field)?
    return x; 
}

错误消息"Undefined reference to Key::Key(int)"与以下内容有关:

The error message "Undefined reference to Key::Key(int)" is related to the following:

class Key
{
public:
   Key(int x=0);
//...
};

构造函数键已声明，但我在任何地方都找不到构造函数的主体.

不幸的是，当您定义类时，使用的C ++编译器不会报告此类问题.它仅在使用该类时报告该问题；然后调用没有主体的构造函数或其他函数.换句话说，当静态地知道它们从未被调用时，编译器允许在没有主体的情况下声明(非虚拟)函数.

如您所愿，您需要在Key.cpp中定义一个主体，或在Key.h中内联它:

Constructor Key is declared, but I did not find the body of the constructor anywhere.

Unfortunately, C++ compiler you use does not report such problem when the class is defined; it only reports the problem when the class is used; and the constructor or other function without a body is called. In other words, the compiler allows declarations of (non-virtual) functions without a body when it is statically known they are never called.

As you probably intended, you need to either define a body in you Key.cpp or inline it in Key.h:

class Key
{
public:
   Key(int x=0) { key = x; }
//...
};

顺便说一句，您已经使用名称x作为要分配给key的参数的事实，这向我暗示，这也是您遇到"x was not declared in this scope"问题的原因.也许，您以某种方式无法理解功能参数范围仅限于此功能(或构造函数)?如果是这样，您如何想象它会起作用?

而已.谢谢.

By the way, the fact you''ve use the name x for a parameter to be assigned to key hints me that this is also a reason for your problem with "x was not declared in this scope". Maybe, you somehow fail to understand that a function paramer scope is limited to this function (or constructor)? If so, how would you imagine it could work?

That''s it. Thank you.

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