C++ oop 多个链接器错误

Question

我在头文件中定义了一个 Student class 和一个 Name class，函数在 cpp 文件中实现。使用 Visual Studio 2015 编译时出现以下错误：

Severity    Code    Description Project File    Line
Error   LNK2019 unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Student const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABVStudent@@@Z) referenced in function _main    3512-lab8   c:\Users\Tess\documents\visual studio 2015\Projects12-lab812-lab8\main.obj    1
Error   LNK2019 unresolved external symbol "class std::basic_istream<char,struct std::char_traits<char> > & __cdecl operator>>(class std::basic_istream<char,struct std::char_traits<char> > &,class Student &)" (??5@YAAAV?$basic_istream@DU?$char_traits@D@std@@@std@@AAV01@AAVStudent@@@Z) referenced in function _main  3512-lab8   c:\Users\Tess\documents\visual studio 2015\Projects12-lab812-lab8\main.obj    1
Error   LNK1120 2 unresolved externals  3512-lab8   c:\users\tess\documents\visual studio 2015\Projects12-lab8\Debug12-lab8.exe   1

这是我的文件

Name.h

#include <string>
#ifndef NAME_H
#define NAME_H
class Name {
public:
    Name() {}
    Name(const std::string& first, const std::string& last)   :first_(toLowerCase(first)), last_(toLowerCase(last)) {}
    std::string toLowerCase(const std::string& s);
    friend std::istream& operator>>(std::istream& is, Name& n);
    friend std::ostream& operator<<(std::ostream& os, const Name& n);
    friend bool isValidName(const Name& n);
private:
    std::string first_;
    std::string last_;
    static bool isValidName(const Name& n);
};
#endif

Student.h

#include "Name.h"
#include <string>
#ifndef STUDENT_H
#define STUDENT_H
class Student {
public:
    Student(){}
    explicit Student(const Name& name, const std::string& id = "A11111111") :id_(id), name_(name) {
    if (!isValidId(id_)) {
        throw "invalid id";
    }
    else if (!isValidName(name_)) {
        throw "invalid name";
    }
}
    virtual ~Student(){}
    friend std::ostream& operator<<(std::ostream& os, const Student& s);
    friend std::istream& operator>>(std::istream& is, Student& s);
    friend bool operator<(const Student& lhs, const Student& rhs);
private:
    std::string id_;
    Name name_;
    static bool isValidId(const std::string& id);
};
#endif

Name.cpp

#include "Name.h"
std::istream& operator>>(std::istream& is, Name& n) {
    return is >> n.first_ >> n.last_;
}
std::ostream& operator<<(std::ostream& os, const Name& n) {
    return os << n.first_ << " " << n.last_;
}
std::string Name::toLowerCase(const std::string& s) {
    std::string str = s;
    for (std::string::size_type i = 0; i < s.size(); i++) {
        tolower(str[i]);
    }
    return str;
}
bool Name::isValidName(const Name & n){
    std::string::size_type i;
    std::string::size_type first_size = n.first_.size();
    std::string::size_type last_size = n.last_.size();

    if (first_size == 0 || last_size == 0) {
        return false;
    }
    for (i = 0; i < first_size; ++i) {
        if (!isalpha(n.first_[i])) {
            return false;
        }
    }
    for (i = 0; i < last_size; ++i) {
        if (!isalpha(n.last_[i])) {
            return false;
        }
    }
    return true;
}

Student.cpp

#include "Student.h"
#define SID_LENGTH 9
bool operator<(const Student& lhs, const Student& rhs) {
    return lhs.id_ < rhs.id_;
}
bool Student::isValidId(const std::string & id){
    std::string::size_type i = 0;

    if (id.length() == SID_LENGTH) {
        if (id[i] == 'A' || id[i] == 'a') {
            for (i = 1; i < id.size(); i++) {
                if (!isdigit(id[i])) {
                    return false;
                } 
            }
            return true;
        }
    }
    return false;
}

Main.cpp

#include "Student.h"
#include <iostream>
#include <map>
int main() {
    std::map<Student, int> m;
    Student s;
    int score;
    while (std::cin >> s >> score) {
        m[s] += score;
    }
    for (const auto& it : m) {
        std::cout << it.first << "" << it.second << std::endl;
    }
}

Answer 1

#include .h 文件在您的 main() 文件中而不是 .cpp 文件

Answer 2

您已为 Student class 声明了重载的 operator<< 和 operator>>，但您从未在 .cpp 文件中定义它们。我认为这就是您看到的错误。

我不确定它在你编辑之前是什么样子，但你确实需要在你的 Name.h 文件（以及你的 Main.cpp 文件中包含 <iostream> )

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