Visual C++ :: Program That Get Information From DAT File - Unresolved Ecternals

I am writing a program that grabs information from a .dat file. I have the code all structured up, but I get these 2 errors that does not make any sense to me on how to fix the program.

errors (2):

Code:
Error1error LNK2019: unresolved external symbol "void __cdecl swap(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > *,class std::basic_string<char,struct std::char_traits<char>,
class std::allocator<char> > *)"

[Code] ....

Error2error LNK1120: 1 unresolved externalsC:UsersMomDesktopDriveQuarter 2C++ProjectsDynamicDeptPayrollDebugDynamicDeptPayroll.exeDynamicDeptPayroll
.dat file contents:

Code:
Fogarty Bob 1 40 10.25
Smith John 2 38 8.72
Jones Mary 2 28 6.25
Arrmen William 1 15 8.22
Lavey Betty 1 32 15.00
source contents:

Code:
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
//PROTOTYPES
void sort(int n);
void swap(string *p1, string *p2);

[Code] ....