Question

我不能为我的生活弄清楚为什么我会收到这些错误，特别是因为我有警卫。

这些是我的错误（请忽略我为计算机命名的内容）：

1＆gt; main.obj：错误LNK2005：“class std :: basic_ostream＆gt;＆gt;＆amp; __cdecl operator＆lt;＆lt;（class std :: basic_ostream＆gt;＆amp;，class＆gt; Date＆amp;）”（？？6 @ YAAAV？$ basic_ostream @ DU？$ char_traits @ D @ std @@@ std @@ AAV01 @ AAVDate @@@ Z）已经＆gt;在loan.obj中定义

1＆gt; main.obj：错误LNK2005：“class std :: basic_ostream＆gt;＆gt;＆amp; __cdecl operator＆lt;＆lt;（class std :: basic_ostream＆gt;＆amp;，class＆gt; Loan＆amp;）”（？？6 @ YAAAV？$ basic_ostream @ DU？$ char_traits @ D @ std @@@ std @@ AAV01 @ AAVLoan @@@ Z）已经＆gt;在loan.obj中定义

1＆gt; C：\ Users \ SweetAssSarah \ Documents \ Visual Studio＆gt; 2012 \ Projects \ ConsoleApplication4 \ Debug \ a1.exe：致命错误LNK1169：找到一个或多个乘法＆gt;已定义的符号

这是我的4个文件： main.cpp中：

#ifndef _main_cpp
#define _main_cpp

#include<iostream>
#include "loan.h"
#include "date.h"

using namespace std;

void main(){
    const int MAX_SIZE = 80;
    char response[MAX_SIZE];

    Loan sarah("Sarah", "123 Awesomeville ", Date (01,February,2010));

    cout << sarah.getName() << " address: " << sarah.getAddress() << endl;
    cout << "Date: " << sarah.getDate() << endl;

    //keep console open until user types a key and enter
    cout <<"\n\n" << "Press ENTER to continue";
    cin.getline(response, MAX_SIZE);

    return;
}    
#endif

loan.cpp：

#ifndef _loan_cpp
#define _loan_cpp

#include <iostream>
#include "loan.h"
#include "date.h"

using namespace std;

Loan::Loan(char * aName, char * anAddress, Date aDate){
    name = aName;
    address = anAddress;
    date = aDate;
    cout <<"CONSTRUCTING: " << name << "\n"; 
}    
Loan::~Loan(){
    cout << "DESTRUCTING: " << name << endl;
} 
char * Loan::getName() {return  name;} 
char * Loan::getAddress(){return address;} 
Date Loan::getDate(){return date;}

void Loan:: printOn(ostream & ostr) {
    cout << name << " address: " << address << endl;
}    
#endif

loan.h：

#ifndef _loan_h
#define _loan_h

#include <math.h> //for the pow() function to do exponentiation
#include <iostream>
#include "date.h"

using namespace std;

class Loan{
public:
    Loan(char *, char *, Date );//constructor
    ~Loan();

    char * getName();
    char * getAddress();
    Date getDate();

    void printOn(ostream & ostr);
private: 
    char * name; 
    char * address;
    Date date; //requires class Date to have a default constructor

};
ostream & operator<<(ostream & ostr, Loan & aLoan) { 
    aLoan.printOn(ostr); 
    return ostr; 
}
#endif

date.h：

#ifndef _date_h
#define _date_h
#include <iostream>

enum Month {January=1, February, March, April, May, June, July, August,
September, October, November, December};

using namespace std;

class Date{
public:
//  Date() {};
    Date(int aDay = 1, Month aMonth = May, int aYear = 2005){
        day = aDay;
        month = aMonth;
        year = aYear;
}    
void printOn(ostream & o){
    o << day << "/" << month << "/" << year;
}
private:
    int day;
    Month month;
    int year;
};    
ostream & operator<<(ostream & ostr, Date & d) { 
    d.printOn(ostr); 
    return ostr; 
} 
#endif

请帮忙！

Answer 1

如评论中所述，＆lt;＆lt;运算符需要在cpp文件中内联或定义。在头文件中定义函数时，它将在包含标题的每个cpp文件中进行编译。如果在多个cpp文件中包含标头，则会出现编译器错误，因为相同的代码将被编译为多个.obj文件。链接器不知道要使用哪个.obj文件并抛出错误。

解决方案1 - 拆分为.h和.cpp

loan.h

ostream & operator<<(ostream & ostr, Loan & aLoan);

loan.cpp

ostream & operator<<(ostream & ostr, Loan & aLoan) { 
    aLoan.printOn(ostr); 
    return ostr; 
}

解决方案2 - 内联

loan.h

inline ostream & operator<<(ostream & ostr, Loan & aLoan) { 
    aLoan.printOn(ostr); 
    return ostr; 
}

第二个解决方案将使编译器通过在调用发生的代码中的每个位置内联函数代码来解析函数调用。这会导致编译代码中出现冗余，对于大型函数应该避免使用。

解决方案3 - 静态

loan.h

static ostream & operator<<(ostream & ostr, Loan & aLoan) { 
    aLoan.printOn(ostr); 
    return ostr; 
}

通过将函数声明为静态，它具有内部链接。您仍然决定编译器是否将其内联到编译器。

已经定义了Obj？链接错误？

1 个答案: