C/C++ :: VS Errors - Unresolved External Symbol

This is a file called namespaces.h

#pragma once
namespace mycode {
void foo();
}

This is a file called source.cpp

#include <iostream>
#include "namespaces.h"
namespace mycode {
void foo() {
std::cout << "foo() called in the mycode namespace" << std::endl;
}
}

When I try to run this code i keep getting these errors;

Error1error LNK2019: unresolved external symbol "void __cdecl mycode::foo(void)" (?foo@mycode@@YAXXZ) referenced in function _mainC:UsersHomeDesktopgameDevWin32Project1Win32Project1Source1.objWin32Project1

Error2error LNK1120: 1 unresolved externalsC:UsersHomeDesktopgameDevWin32Project1DebugWin32Project1.exeWin32Project1

this is a win32 console application, like the books says, the book i am working from that is.