Accelerated C++ 교재로 공부하신 분들께 질문입니다.

아.. 가뭄에 단비같은 답변이네요 ㅠㅠ 도움을 주신다니 감사합니다.
전부 공부를 하셨다니 다 아시겠다고 생각하고 올리겠습니다^^
먼저 main. cpp 입니다.

#include<algorithm>
#include<iomanip>
#include<ios>
#include<iostream>
#include<stdexcept>
#include<string>
#include<vector>
#include "grade.h"
#include "Student_info.h"
#include "median.h"

using namespace std;

int main()
{
	vector<Student_info> students;
	Student_info record;
	string::size_type maxlen = 0;

	while(read(cin, record)) {
		maxlen = max(maxlen, record.name.size() );
		students.push_back(record);
	}

	sort(students.begin(), students.end(), compare);

	for(vector<Student_info>::size_type i =0;
	i != students.size(); ++i) {
		cout << students[i].name
			<< string(maxlen + 1 - students[i].name.size(), ' ');

		try {
			double final_grade = grade(students[i]);
			streamsize prec = cout.precision();
			cout << setprecision(3) << final_grade
				<< setprecision(prec);
		} catch (domain_error e) {
			cout << e.what();
		}
		cout << endl;
	}
	return 0;
}

그다음 Student_info.h 입니다.

#ifndef GUARD_Student_info_h
#define GUARD_Student_info_h

#include<iostream>
#include<string>
#include<vector>

struct Student_info {
	std::string name;
	double midterm, final;
	std::vector<double> homework;
};

bool compare(const Student_info&, const Student_info&);
std::istream& read(std::istream&, Student_info&);
std::istream& read_hw(std::istream&, std::vector<double>&);
#endif

Student_info.cpp입니다.

#include "Student_info.h"
using namespace std;

bool compare(const Student_info& x, const Student_info& y)
{
	return x.name < y.name;
}

istream& read(istream& is, Student_info& s)
{
	cout << "Enter the name : ";
	is >> s.name ;
	cout << "Enter the midterm :;
	is >> s.midterm;
	cout << "Enter the final :";
	is >> s.final;
	read_hw(is, s.homework);
	return is;
}

istream& read_hw(istream& in, vector<double>& hw)
{
	if(in) {
		hw.clear();
		double x;
		while (in >> x)
			hw.push_back(x);
		in.clear();
	}
	return in;
}

median.h입니다.

#ifdef GUARD_median_h
#define GUARD_median_h
#include<vector>
double median(std::vector<double>);
#endif

median.cpp입니다.

#include<algorithm>
#include<stdexcept>
#include<vector>

using namespace std;

double median(vector<double> vec)
{
	typedef vector<double>::size_type vec_sz;

	vec_sz size = vec.size();
	if (size == 0)
		throw domain_error("median of an empty vector");

	sort(vec.begin(), vec.end());

	vec_sz mid = size/2;

	return size % 2 == 0 ? (vec[mid] + vec[mid+1]) /2 : vec[mid];
}

grade.h입니다.

#ifdef GUARD_grade_h
#define GUARD_grade_h
#include<vector>
#include "Student_info.h"
double grade(double, double, double);
double grade(double, double, const std::vector<double>&);
double grade(const Student_info&);
#endif

grade.cpp입니다.

#include<iostream>
#include<vector>
#include "grade.h"
#include "median.h"
#include "Student_info.h"

using std::domain_error; using std::vector;

double grade(double midterm, double final, double homework)
{
	return 0.2 * midterm  0.4 * final + 0.4 * homework;
}


double grade(double midterm, double final, const vector<double>& hw)
{
	if (hw.size() == 0)
		throw domain_error("student has done no homework");
	return grade(midterm, final, median(hw));
}


double grade(const Student_info& s)
{
	return grade(s.miterm, s.final, s.homework);
}

이상 헤더파일 3개와 cpp파일 3개입니다.
내용이 좀 깁니다만은... 챕터 4의 각 예제들이 각 파일설명인지라;;;
에러메시지 부분은요..

--------------------Configuration: main - Win32 Debug--------------------
Compiling...
main.cpp
D:\Visual_C++6.0\Chap4\main.cpp(33) : error C2065: 'grade' : undeclared identifier
Error executing cl.exe.

main.exe - 1 error(s), 0 warning(s)

너무 조잡해서 도움주시기 곤란하시겠지만... 부탁드립니다~