I write so (using VS2013) :

Student. H

 # pragma once 
#include 
using namespace std; 

The class Student 
{
Public: 
Student (string num, string name) 
{
This - & gt; Num=num; 
This - & gt; Name=name; 
} 
To the Student (); 
Void ShowData (Score & amp;); 
Private: 
String num. 
String name; 
}; 

Student. CPP

 # include "stdafx. H" 
H # include "Student." 
# include "Score. H" 
# include & lt; Iostream> 
using namespace std; 

Student: : ~ Student () 
{
} 

Void Student: : ShowData (Score & amp; Stu) 
{
Cout & lt; <"==========the student information \ n"; 
Cout & lt; Cout & lt; <"\ n name: \ t" & lt; Cout & lt; <"\ n high number: \ t" & lt; Cout & lt; Cout & lt; Cout & lt; <"\ nC++ : \ t" & lt; Cout & lt; <"\ n===================\ n"; 
} 

Score. H

 # pragma once 
The class Student. 

The class Score 
{
Public: 
Score (float Math, float English, float Politics says, float Cplusplus) 
{
This - & gt; Math=Math; 
This - & gt; English=English; 
This - & gt; Politics says=Politics says; 
This - & gt; Cplusplus=Cplusplus; 
} 
To Score (); 
Friend void Student: : ShowData (Score & amp;); 
Private: 
Float Math; 
Float English; 
Float Politics says; 
Float Cplusplus; 
}; 

Score. CPP

 # include "stdafx. H" 
# include "Score. H" 

Score: : ~ Score () 
{
} 

lab 3. CPP

 # include "stdafx. H" 
# include "Score. H" 
H # include "Student." 
# include & lt; String> 
# include & lt; Iostream> 
using namespace std; 

Int _tmain (int arg c, _TCHAR * argv []) 
{
String num, name; 
Float math, English, politics says, cpluslus; 
Cout<" Please enter the student id, name, and mathematics, English, politics and c + + : \ n "; 
Cin & gt;> Num & gt;> The name & gt;> Math & gt;> English & gt;> Politics says & gt;> Cpluslus; 
Student stu (num, name); 
Score stu1 (math, English, politics says, cpluslus); 
Stu. ShowData (stu1); 
return 0; 
} 

But in Student. CPP file ShowData function referenced in the Score display an error when data members


Then when debugging the compiler hint has the following error:
Error 1 error C2027: use the undefined type "Student" c: \ users \ XXXX \ documents \ visual studio 2013 \ projects \ experiment three \ \ score three h 17 1 lab 3
Error 2 error C2061: syntax errors: identifier "Score" c: \ documents \ \ users \ XXXX visual studio 2013 \ projects \ three experiments for three \ \ student h 16 1 lab 3
Error 3 error C2245: will not exist a member function "Student: : ShowData" designated as friend yuan (with all overloaded member function signature does not match) c: \ documents \ \ users \ XXXX visual studio 2013 three \ test \ \ projects \ experiment score. 17 1 lab 3 h
Mistake 4 error C2511: "void Student: : ShowData (Score & amp;) ":" Student "found no overloaded member function in c: \ users \ XXXX \ documents \ visual studio 2013 \ projects \ experiment three \ \ Student three CPP 12 1 lab 3
Error 5 error C2597: the non-static member "Student:" num "illegal reference to c: \ documents \ \ users \ XXXX visual studio 2013 \ projects \ experiment three \ \ Student three CPP 14 1 lab 3
Error 6 error C2597: the non-static member "Student:" name "of illegal reference to c: \ users \ XXXX \ documents \ visual studio 2013 \ projects \ experiment three \ \ Student three CPP 15 1 lab 3
Error and the error C2248: "Score: : Math" : unable to access private members (statements) in the "Score" class c: \ documents \ \ users \ XXXX visual studio 2013 \ projects \ experiment three \ \ student three CPP 16 1 lab 3
8 error error C2248: "Score: : English:" can't access private members (statements) in the "Score" class c: \ documents \ \ users \ XXXX visual studio 2013 \ projects \ experiment three \ \ student three CPP 17 1 lab 3
Error error of 9 C2248: "Score: : Politics says:" don't have access to private member (statements) in the "Score" class c: \ documents \ \ users \ XXXX visual studio 2013 \ projects \ experiment three \ \ student three CPP 18 1 lab 3
Error 10 error C2248: "Score: : Cplusplus" : unable to access private members (statements) in the "Score" class c: \ documents \ \ users \ XXXX visual studio 2013 \ projects \ experiment three \ \ student three CPP 19 1 lab 3
Error and the error C2027: use the undefined type "Student" c: \ documents \ \ users \ XXXX visual studio 2013 \ projects \ experiment three \ \ score three h 17 1 lab 3
12 IntelliSense: do not allow the use of incomplete type c: \ Documents \ \ Users \ XXXX Visual Studio 2013 \ Projects \ experiment three \ \ Score three h 17 14 experiment three

Excuse me, where is wrong? The two class how to declare?