C 标头文件，架构X86_64的未定义符号

我正在尝试构建一个将学生成绩存储在C 中的程序。编译时我会收到以下错误：

Undefined symbols for architecture x86_64: "grade1(Student_records const&)", referenced from:
 _main in gradereport-a0007a.o
 "compare(Student_records const&, Student_records const&)", 
referenced from: _main in gradereport-a0007a.o
"read_info(std::__1::basic_istream<char, std::__1::char_traits<char> >&, Student_records&)", 
referenced from: _main in gradereport-a0007a.o
ld: symbol(s) not found for architecture x86_64

我一直（尝试）用于编译的命令：g gradereport.cpp

告诉我，关于我的read_info，比较和一年级函数是错误的（定义不当）。但是，我似乎找不到问题。

从阅读有关"未定义符号"问题的先前帖子，看来（a）我的功能声明不匹配，或者（b）我需要"同时"编译多个文件（因为缺乏更好的更好短语）。我很确定不是（a）。如果（b），我将如何精确地完成这项任务？另外，如果有人受到任何基本批评，我很想听听。预先感谢。

这是我的代码：

gradereport.cpp

#include<iostream>
#include<ios>
#include<iomanip>
#include<algorithm>
#include<vector>
#include<stdexcept>
#include "student_records.h"
#include "Grades.h"
using std::cout;
using std::endl;
using std::vector;
using std::cout;
using std::cin;
using std::string;
using std::max;
using std::domain_error;
int main()
{
    vector<Student_records> students; 
    Student_records record; 
    string::size_type maxlen = 0;
    while(read_info(cin, record))
    {
        maxlen = max(maxlen, record.name.size()); // finds the longest name
        students.push_back(record); // enters each record into the vector
    }
    sort(students.begin(), students.end(), compare); 
    for(vector<Student_records>::size_type i = 0; i != students.size(); ++i)
    {
        cout << students[i].name
            << string(maxlen + 1 - students[i].name.size(), ' ');
        try
        {
            double final_grade = grade1(students[i]);
            /* need to define grade function:
                1) unpack grades;
                2) find homework grades;
                3) find final grade;
            */
            cout << final_grade
        }
        catch(domain_error e)
        {
            cout << e.what();
        }
        cout << endl;
    }
    return 0;
}

student_records.h

#ifndef GUARD_STUDENT_RECORDS
#define GUARD_STUDENT_RECORDS
#include<iostream>
#include<string>
#include<vector>
struct Student_records
{
    std::string name;
    double midterm, final;
    std::vector<double> homeworks;
};
bool compare(const Student_records&, const Student_records&);
std::istream& read_info(std::istream&, Student_records&);
std::istream& read_homework(std::istream&, std::vector<double>&);
#endif

student_records.cpp

#include "student_records.h"
bool compare(const Student_records& x, const Student_records& y)
{
    return x.name < y.name;
}
std::istream& read_info(istream& is, Student_records& s)
{
    std::cout << "Enter name, midterm, and final: ";
    is >> s.name >> s.midterm >> s.final;
    read_homework(std::istream&, std::vector<double>&);
    return is;
}
std::istream& read_homework(std::istream& in, std::vector<double>& homework)
{
    cout << "Enter homework: ";
    if(in)
    {
        hw.clear()
        double x;
        while(in >> x)
            homework.push_back(x);
        in.clear();
    }
    return in;
}

等级

#ifndef GUARD_GRADES
#define GUARD_GRADES
#include<iostream>
#include<vector>
#include<string>
#include "student_records.h"
double grade1(const Student_records&);
double grade2(double, double, std::vector<double>);
double median(std::vector<double>);
#endif

等级

#include<stdexcept>
#include<vector>
#include "grades.h"
#include "Student_records.h"
using std::domain_error;
using std::vector;
double grade2(double midterm, double final, std::vector<double> homework)
{
    double final_homework_grade;
    final_homework_grade = median(homework);
    return .4 * final_homework_grade + .4 * final + .2 * midterm;
}
double median(vector<double> homework)
{
    typedef vector<double>::size_type vec_sz;
    vec_sz size = homework.size();
    if(size == 0)
        throw domain_error("student has done no homework");
    sort(homework.begin(), homework.end());
    return size % 2 == 0 ? ((homework[mid] + homework[mid + 1]) / 2) : homework[mid];
}
double grade1(const Student_records& s) // unpacks grades
{
    return grade2(s.midterm, s.final, s.homework);
}