不正确地将成员变量设置为 null

我想我正确地声明了所有内容，但我在 main.cpp 的第 22-26 行中得到了未声明的标识符。我也得到这个 = 在我的员工第 12-16 行中模棱两可.cpp。我还有一个奇怪的表达式，必须在员工.cpp的第 55 行中具有指向工资对象类型的指针。我真的很感激任何帮助，因为我的学校目前没有导师。

//Employee.h
using namespace std;
class Employee {
private:
public:
    string FirstName;
    string LastName;
    string DisplayFirstName;
    string DisplayLastName;
    string DisplaySalary;
    string SearchName;
    float Salary;
    Employee( string FirstName, string LastName, float Salary )
    {
            setFirstName(FirstName);
            setLastName(LastName);
            setSalary(Salary);
    }
    string setFirstName(string FirstName);
    string setLastName(string LastName);
    float setSalary(float Salary);
    void ReadFile(ifstream& MyinFile, string FirstName, string LastName, float Salary);
    string EmployeeSearch(string LastName[], string SearchName);
    void DisplayEmployee (string DisplayFirstName, string DisplayLastName, string DisplaySalary);
    Employee();
};
//Employee.cpp
#include <iostream>
#include <fstream>
#include <string>
#include "Employee.h"
using namespace std;
string setFirstName(string FirstName)
{
**FirstName = NULL;** //ambiguous error
}
string setLastName(string LastName)
{
**LastName = NULL;** //ambiguous error
}
float setSalary(float Salary)
{
Salary = 0.0;
}
void ReadFile(ifstream& MyinFile, string FirstName, string LastName, float Salary)
{
char exit_char;
int MaxSize;
int count = 0;
MyinFile.open("employee.dat"); 
    if (!MyinFile)
    {    //no
        cout << "Can't open input file." << endl; //Tests the right file.
        char exit_char;                         //End Program
        cout << "Press any key to exit" << endl;
        cin >> exit_char;
    }
    for(count = 0; count < MaxSize; count++)
    {
        MyinFile >> LastName[count];
        MyinFile >> FirstName[count];
        MyinFile >> **Salary[count];** //error
    }
MyinFile.close();
}
string EmployeeSearch(string LastName[], string FirstName[], float Salary, string SearchName, string DisplayFirstName, string DisplayLastName, string DisplaySalary)
{
    cout << "Please enter the name of the employee you would like to search." <<  endl;
    cin >> SearchName;
    for (int i = 0; i < 10; i++ )
    {
        if (LastName[i] == SearchName)
    {
        DisplayFirstName = FirstName[i];
        DisplayLastName = LastName[i];
        DisplaySalary = **Salary[i];**  //error
    }
    else 
        cout << "Could not find employee." << endl;
    }
};
void DisplayEmployee (string DisplayFirstName, string DisplayLastName, string DisplaySalary)
{
cout << DisplayFirstName << "   ";
cout << DisplayLastName << "    ";
cout << DisplaySalary << endl;
};
//Main.cpp
#include <iostream>
#include <fstream>
#include <string>
#include "Employee.h"
using namespace std;
const int MaxSize = 100;
int main()
{
char Redo;          //Input a character to redo the program
ifstream MyinFile;
cout << "Your Salary Machinenn";
Employee  Employee;
Employee.ReadFile(**MyinFile, FirstName, LastName, Salary**); //undeclared identifier error
do
{
    Employee.EmployeeSearch(**LastName[], SearchName**); //undeclared identifier error
    Employee.DisplayEmployee(**DisplayFirstName,DisplayLastName,DisplaySalary**); //undeclared identifier error
    //Asks user if they want redo the program
    cout << "Would you like to redo the program?n";
    cout << "Please enter Y or N: n";
    cin >> Redo;
}while(Redo == 'Y' || Redo == 'y');
return 0;
}

编写该程序是为了读取具有名字和姓氏然后薪水的文件然后能够输入文件中任何人的姓氏，它将显示姓名和薪水，然后重复。我假设使用构造函数将名字和姓氏初始化为 NULL，然后将薪水初始化为 0.0。我还应该使用 get 和设置成员函数。

以下是错误：

------ Build started: Project: Lab3Project, Configuration: Debug Win32 ------
Main.cpp
\psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectmain.cpp(22): error C2065: 'FirstName' : undeclared identifier
 \psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectmain.cpp(22): error C2065: 'LastName' : undeclared identifier
\psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectmain.cpp(22): error C2065: 'Salary' : undeclared identifier
\psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectmain.cpp(25): error C2065: 'SearchName' : undeclared identifier
\psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectmain.cpp(25): error C3861: 'LastName': identifier not found
\psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectmain.cpp(26): error C2065: 'DisplayFirstName' : undeclared identifier
\psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectmain.cpp(26): error C2065: 'DisplayLastName' : undeclared identifier
\psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectmain.cpp(26): error C2065: 'DisplaySalary' : undeclared identifier
Employee.cpp
\psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectemployee.cpp(12): error C2593: 'operator =' is ambiguous
      c:program filesmicrosoft visual studio 10.0vcincludexstring(772): could be  'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(_Elem)'
      with
      [
          _Elem=char,
          _Traits=std::char_traits<char>,
          _Ax=std::allocator<char>
      ]
      c:program filesmicrosoft visual studio 10.0vcincludexstring(767): or       'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(const _Elem *)'
      with
      [
          _Elem=char,
          _Traits=std::char_traits<char>,
          _Ax=std::allocator<char>
      ]
      while trying to match the argument list '(std::string, int)'
 \psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectemployee.cpp(16): error C2593: 'operator =' is ambiguous
      c:program filesmicrosoft visual studio 10.0vcincludexstring(772): could be 'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(_Elem)'
      with
      [
          _Elem=char,
          _Traits=std::char_traits<char>,
          _Ax=std::allocator<char>
      ]
      c:program filesmicrosoft visual studio 10.0vcincludexstring(767): or       'std::basic_string<_Elem,_Traits,_Ax> &std::basic_string<_Elem,_Traits,_Ax>::operator =(const _Elem *)'
      with
      [
          _Elem=char,
          _Traits=std::char_traits<char>,
          _Ax=std::allocator<char>
      ]
      while trying to match the argument list '(std::string, int)'
\psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectemployee.cpp(40): error C2109: subscript requires array or pointer type
\psfhomedocumentsvisual studio 2010projectscsci112lab3projectlab3projectemployee.cpp(55): error C2109: subscript requires array or pointer type