Question

我正在尝试创建基本的登录页面，但是当我尝试构建程序时，出现语法错误。

这是我得到的错误：

Error   1   error LNK2019: unresolved external symbol "struct UserRecord __cdecl ReadData(class 
std::basic_ifstream<char,struct std::char_traits<char> > &,struct UserRecord)" (?ReadData@@YA?
AUUserRecord@@AAV?$basic_ifstream@DU?$char_traits@D@std@@@std@@U1@@Z) referenced in function _main  
c:\Users\Emerson\documents\visual studio 
2012\Projects\ConsoleApplication11\ConsoleApplication11\small_eas12d_p6.obj ConsoleApplication11

当程序从我可以得到的函数调用main函数时，我得到两次相同的错误。但是我无法找到解决问题的方法。

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

const int Max_Persons = 50, Max_Attempts = 3;

struct UserRecord
{
   string userID;
   string password;
   int PIN;

};

typedef struct UserRecord AccountInfo;

void PrintHeading();
void Login_Success();
void Kicked();
void Login_Failed(int);
void FileCheck(ifstream &);
UserRecord ReadData(ifstream &, AccountInfo);
bool checkData(bool, AccountInfo, int);




int main()
{
    int i = 0;
    bool passorfail = false;
    int attempts = 0;

    ifstream infile;



    AccountInfo Accounts[Max_Persons];

    FileCheck(infile);

    Accounts[Max_Persons] = ReadData(infile, Accounts[Max_Persons]);

    PrintHeading();

    do
    {
        passorfail = checkData(passorfail, Accounts[Max_Persons], attempts);

    } while (!passorfail || attempts < 3);

    if (attempts <= 3)
        Kicked();

    if(passorfail)
        Login_Success();

    infile.close();

    return 0;
}

AccountInfo ReadData(ifstream & infile, AccountInfo Accounts[Max_Persons])
{
    int UserNum;
    string str;


    getline(infile, str, '\n' );

     UserNum = atoi(str.c_str());


    for(int i = 0; i < UserNum; i++)
    {
        infile >> Accounts[i].userID;

        infile >> Accounts[i].password;

        infile >> Accounts[i].PIN;

    }



    return Accounts[Max_Persons];
}



void FileCheck(ifstream & infile)
{
    string FileName;

    cout << "Filenames must not contain blanks ." << endl;
    cout << "Please enter the file name to decode ->" << endl;
    cin >> FileName;


        infile.open(FileName.c_str());

        while(!infile)
        {

                FileName.clear();

                cout << "Sorry \"" << FileName << "\" is not a valid file name. Please try again." << endl;

                cin >> FileName;
                infile.open(FileName.c_str());
        }

        return;
}


bool checkData(bool passorfail, AccountInfo Accounts[Max_Persons], int attempts)
{

    string givenID;
    string givenPass;
    int givenPin;

    do
    {
        cout << "Login" << endl;
        cout << "UserID: ";
        cin >> givenID;
        cout << endl << "Password: ";
        cin >> givenPass;
        cout << endl << "Pin: ";
        cin >> givenPin;

        for(int i = 0; i < Max_Persons; i++)
        {
            if(givenID == Accounts[i].userID)
            {
                if (givenPass == Accounts[i].password)
                {
                    if (givenPin == Accounts[i].PIN)
                    {
                        passorfail = true;
                    }
                }
            }
        }

        if(!passorfail)
            {
                Login_Failed(attempts);
                attempts++;
            }

    } while (!passorfail || attempts < 3);

    return passorfail;
}

void PrintHeading()
{
    cout << "-----------------------------------------------------------\n";
    cout << "------         Welcome to MyGreatWebsite.com         ------\n";
    cout << "-----------------------------------------------------------\n";
    cout << "**Unauthorized access to this stie is strictly prohibited**";

    return;
}

void Login_Failed(int attempts)
{
    cout << "Sorry, that username and/or password was incorrect" << endl;
    cout << "You have used " << attempts << " of your attempts to login. " << endl;
    cout << "You only have 3 attempts total before you are automatically " << endl;
    cout << "kicked from the system" << endl; 

    return;
}

void Kicked()
{
    cout << "Sorry, you have reached the maximum number attempts " << endl;
    cout << "to access this site, please have a good day" << endl;

    return;
}

void Login_Success()
{
    cout << "Welcome to MyGeatWebsite.com" << endl;
    cout << "Enjoy your stay" << endl;

    return;
}

提前感谢您的帮助。

Answer 1

LNK2019不是“语法错误”。它是链接器发出的诊断。这意味着某些已声明和使用的函数（在本例中为ReadData）尚未定义，或者如果已定义，尚未编译，或者已编译，尚未传递给链接器。

ReadData的前瞻声明与实施不符。

避免这种情况的一种简单方法是不使用前向声明。前向声明在语言中是有原因的（支持递归，并支持单独的编译），但作为一般约定，它们没有优势并且确实存在一些问题，包括额外的维护工作和您现在遇到的问题。而是将每个函数定义放在首次使用之前。

Answer 2

只是为了澄清，在Visual Studio上，以LNK开头的错误是Linker错误。现在出于此错误的原因，您似乎已将ReadData原型声明为

UserRecord ReadData(ifstream &, AccountInfo);

在函数定义中它是：

AccountInfo ReadData(ifstream & infile, AccountInfo Accounts[Max_Persons]) { }

链接器可能假定有两个不同的函数，一个接收单个AccountInfo，另一个接收AccountInfo[]数组。更改ReadData的原型以匹配函数定义，它应该解决问题。

继续得到错误lnk 2019

2 个答案: