operator+()
addScore()函式
以下程式碼，在類別外定義addScore()函式，並在Student類別中把addScore()設為friend(友情/朋友)，代表addScore()類別外的函式可以使用Student的private成員變數。
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#include <iostream>
using namespace std;
class Student {
//私有成員變數
int m_math;
int m_english;
int m_chinese;
public:
//將類別外的函式設為friend，才能讀取物件私有成員變數
friend void addScore(Student& s, int score);
string m_name;
Student(){}
//建構子，姓名，數學成績，英文成績，國文成績
Student(string name, int math, int english, int chinese) {
m_name = name;
m_math = math;
m_english = english;
m_chinese = chinese;
}
//印出學生姓名與分數
void print() {
cout << "name = " << m_name << endl;
cout << "math = " << m_math << endl;
cout << "english = " << m_english << endl;
cout << "chinese = " << m_chinese << endl;
}
};
//為每個科目增加分數
void addScore(Student& s, int score) {
s.m_math += score;
s.m_chinese += score;
s.m_english += score;
}
int main() {
//建立物件
Student student("Bill", 50, 60, 70);
//為每個分數增加15分
addScore(student, 15);
//印出學生姓名與各科分數
student.print();
return 0;
}
name = Bill
math = 65
english = 75
chinese = 85
operator+()函式
把addScore函式名全改成operator+
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class Student {
//私有成員變數
int m_math;
int m_english;
int m_chinese;
public:
//將類別外的函式設為friend，才能讀取物件私有成員變數
friend void operator+(Student& s, int score);
string m_name;
Student(){}
//建構子，姓名，數學成績，英文成績，國文成績
Student(string name, int math, int english, int chinese) {
m_name = name;
m_math = math;
m_english = english;
m_chinese = chinese;
}
//印出學生姓名與分數
void print() {
cout << "name = " << m_name << endl;
cout << "math = " << m_math << endl;
cout << "english = " << m_english << endl;
cout << "chinese = " << m_chinese << endl;
}
};
//為每個科目增加分數
void operator+(Student& s, int score) {
s.m_math += score;
s.m_chinese += score;
s.m_english += score;
}
int main() {
//建立物件
Student student("Bill", 50, 60, 70);
//為每個分數增加15分
operator+(student, 15);
//印出學生姓名與各科分數
student.print();
return 0;
}
student + 15
如果函式名是operator+(student,30)，寫法就可以換成student + 30，student是operator+()第一個參數，30是operator+()第二個參數。
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class Student {
//私有成員變數
int m_math;
int m_english;
int m_chinese;
public:
//將類別外的函式設為friend，才能讀取物件私有成員變數
friend void operator+(Student& s, int score);
string m_name;
Student(){}
//建構子，姓名，數學成績，英文成績，國文成績
Student(string name, int math, int english, int chinese) {
m_name = name;
m_math = math;
m_english = english;
m_chinese = chinese;
}
//印出學生姓名與分數
void print() {
cout << "name = " << m_name << endl;
cout << "math = " << m_math << endl;
cout << "english = " << m_english << endl;
cout << "chinese = " << m_chinese << endl;
}
};
//為每個科目增加分數
void operator+(Student& s, int score) {
s.m_math += score;
s.m_chinese += score;
s.m_english += score;
}
int main() {
//建立物件
Student student("Bill", 50, 60, 70);
//為每個分數增加15分
student + 15;
//印出學生姓名與各科分數
student.print();
return 0;
}
name = Bill
math = 65
english = 75
chinese = 85
傳回值型態是物件
student = student + 15;
以上的程式碼，是要把運算的結果傳回給型態是Student的物件，所以必須把傳回值型態改成物件的參考。
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Student& operator+(Student& s, int score)
運算的過程也可以是很多數字的相加，不變的是，要把運算的結果傳回給型態是Student的物件。
student = student + 15 + 5 + 5;
以上的運算過程，函式的呼叫如下
student = (((student + 15) + 5) + 5);
每一個括號就有一個傳回值，+加號左邊的參數是operator+()第一個參數，+加號右邊是operator+()第二個參數。
再次分解如下
student = operator+(operator+(operator+(student, 15), 5), 5);
程式的傳回值型態改寫如下
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#include <iostream>
using namespace std;
class Student {
//私有成員變數
int m_math;
int m_english;
int m_chinese;
public:
//將類別外的函式設為friend，才能讀取物件私有成員變數
friend Student& operator+(Student& s, int score);
string m_name;
Student(){}
//建構子，姓名，數學成績，英文成績，國文成績
Student(string name, int math, int english, int chinese) {
m_name = name;
m_math = math;
m_english = english;
m_chinese = chinese;
}
//印出學生姓名與分數
void print() {
cout << "name = " << m_name << endl;
cout << "math = " << m_math << endl;
cout << "english = " << m_english << endl;
cout << "chinese = " << m_chinese << endl;
}
};
//為每個科目增加分數
Student& operator+(Student& s, int score) {
s.m_math += score;
s.m_chinese += score;
s.m_english += score;
return s;
}
int main() {
//建立物件
Student student("Bill", 50, 60, 70);
//為每個分數增加15分
student = student + 15;
//印出學生姓名與各科分數
student.print();
return 0;
}
operator+()成員函式
之前的方式是把類別之外的函式透過friend實現，接下來將operator+()在類別內實作。
把operator+()變為成員函式，成員函式本身就有隱藏的this指標，指向呼叫函式的物件，所以原本的第一個參數可以刪掉。
Student& operator+(int score){}
而原本
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s.m_math += score;
s.m_chinese += score;
s.m_english += score;
可以改為
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m_math += score;
m_chinese += score;
m_english += score;
原本的friend函式也可以刪掉
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friend Student& operator+(Student& s, int score);
傳回值也改為隱藏的this指標，this指標為物件存放的記憶體位址，使用取值運算子，傳回物件。
return *this;
完整程式碼
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#include <iostream>
using namespace std;
class Student {
//私有成員變數
int m_math;
int m_english;
int m_chinese;
public:
string m_name;
Student(){}
//建構子，姓名，數學成績，英文成績，國文成績
Student(string name, int math, int english, int chinese) {
m_name = name;
m_math = math;
m_english = english;
m_chinese = chinese;
}
Student& operator+(int score) {
m_math += score;
m_chinese += score;
m_english += score;
return *this;
}
//印出學生姓名與分數
void print() {
cout << "name = " << m_name << endl;
cout << "math = " << m_math << endl;
cout << "english = " << m_english << endl;
cout << "chinese = " << m_chinese << endl;
}
};
int main() {
//建立物件
Student student("Bill", 50, 60, 70);
//為每個分數增加15分
student = student + 15 + 5;
//印出學生姓名與各科分數
student.print();
return 0;
}
operator+()參數替換
若想使用以下的方式，數值在加號+左邊，物件在加號+右邊。
student = 15 + student;
只能使用類別外的friend函式實現，無法使用成員函式實現。
增加函式如下
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Student& operator+(int score, Student& s){...}
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#include <iostream>
using namespace std;
class Student {
//私有成員變數
int m_math;
int m_english;
int m_chinese;
public:
//將類別外的函式設為friend，才能讀取物件私有成員變數
friend Student& operator+(Student& s, int score);
friend Student& operator+(int score, Student& s);
string m_name;
Student(){}
//建構子，姓名，數學成績，英文成績，國文成績
Student(string name, int math, int english, int chinese) {
m_name = name;
m_math = math;
m_english = english;
m_chinese = chinese;
}
//印出學生姓名與分數
void print() {
cout << "name = " << m_name << endl;
cout << "math = " << m_math << endl;
cout << "english = " << m_english << endl;
cout << "chinese = " << m_chinese << endl;
}
};
//為每個科目增加分數
Student& operator+(Student& s, int score) {
s.m_math += score;
s.m_chinese += score;
s.m_english += score;
return s;
}
Student& operator+(int score, Student& s) {
s.m_math += score;
s.m_chinese += score;
s.m_english += score;
return s;
}
int main() {
//建立物件
Student student("Bill", 50, 60, 70);
//為每個分數增加15分
student = 15 + student;
//印出學生姓名與各科分數
student.print();
return 0;
}
operator+()參數都為物件
若想使用以下的方式。
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Student s1("Bill", 50, 60, 70);
Student s2("Tom", 50, 60, 70);
s1 = s1 + s2;
寫一個operator+傳入的參數為2個物件
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Student& operator+(Student& s1, Student& s2){}
程式碼如下
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#include <iostream>
using namespace std;
class Student {
//私有成員變數
int m_math;
int m_english;
int m_chinese;
public:
//將類別外的函式設為friend，才能讀取物件私有成員變數
friend Student& operator+(Student& s, int score);
friend Student& operator+(int score, Student& s);
friend Student& operator+(Student& s1, Student& s2);
string m_name;
Student(){}
//建構子，姓名，數學成績，英文成績，國文成績
Student(string name, int math, int english, int chinese) {
m_name = name;
m_math = math;
m_english = english;
m_chinese = chinese;
}
//印出學生姓名與分數
void print() {
cout << "name = " << m_name << endl;
cout << "math = " << m_math << endl;
cout << "english = " << m_english << endl;
cout << "chinese = " << m_chinese << endl;
}
};
//為每個科目增加分數
Student& operator+(Student& s, int score) {
s.m_math += score;
s.m_chinese += score;
s.m_english += score;
return s;
}
Student& operator+(int score, Student& s) {
s.m_math += score;
s.m_chinese += score;
s.m_english += score;
return s;
}
Student& operator+(Student& s1, Student& s2) {
s1.m_math += s2.m_math;
s1.m_chinese += s2.m_chinese;
s1.m_english += s2.m_english;
return s1;
}
int main() {
//建立物件
Student s1("Bill", 50, 60, 70);
Student s2("Tom", 50, 60, 70);
s1 = s1 + s2;
//印出學生姓名與各科分數
s1.print();
return 0;
}
混合使用
以上程式碼支援以下寫法
s1 = s1 + 10 + 5 + s2;
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int main() {
//建立物件
Student s1("Bill", 50, 60, 70);
Student s2("Tom", 50, 60, 70);
s1 = s1 + 10 + 5 + s2;
//印出學生姓名與各科分數
s1.print();
return 0;
}