C++中运算符重载详解及其作用介绍

目录

• 概述
• 函数重载
• 运算符重载
• C++ 的运算符
• 重载运算符的规则
• 成员函数实现 Complex 加法
• 运算符重载的方法
• 多种实现方法
• 实现 operator+=
• 三种运算符重载函数
• 成员函数实现
• 友元函数实现
• 输出结果
• 重载单元运算符
• 例子
• 重载二元运算符
• 例子
• 重载 I/O
• 插入运算符 <<
• 提取运算符 >>
• 总结

概述

运算符重载 (Operator Overloading)

函数重载

重载: 将同一名字重新赋予新的含义.
函数重载: 对一个函数赋予新的含义, 使之实现新功能. 例如:

int max(int x, int y);
double max(double a, double b, double c);

运算符也有重载: 赋予运算符新的含义, 使之一名多用. 例如

int main() {
    int i = 2, j = 3;
    int k = i + j;

    string s1 = "good ", s2 = "morning";
    string s3 = s1 + s2;

    cout << k << endl;
    cout << s3 << endl;

    return 0;
}

输出结果:
5
good morning

运算符重载

通过运算符重载, 扩大了 C++ 已有运算符的作用, 使运算符能用于类对象. 使用运算符重载, 能使程序易于编写, 阅读和维护. 运算符被重载后, 其原有的功能仍然保留, 没有丧失过改变.

运算符重载实质上是函数的重载:

• 定义一个重载运算符的函数
• 需要执行被重载的运算符时, 系统就自动调用该函数, 以实现相应的运算

C++ 的运算符

1. 算数运算符: +(加) -(减) *(乘) %(整除余数) ++(自加) – (自减)
2. 关系运算符: >(大于) <(小于) ==(等于) >=(大于等于) <=(小于等于) !=(不等于)
3. 逻辑运算符: &&(逻辑与) ||(逻辑或) !(逻辑非)
4. 位运算符: <<(按位左移) >>(按位右移) &(按位与) |(按位或) ∧(按位异或) ~(按位取反)
5. 赋值运算符: = 及其扩展赋值运算符
6. 条件运算符: ?:
7. 都好运算符: ,
8. 指针运算符: *
9. 引用运算符合地址运算符: &
10. 求字节数运算符: sizeof
11. 强制类型转换运算符: (类型) 或 类型()
12. 成员运算符: .
13. 指向成员的运算符:->
14. 下标运算符: []
15. 其他: 如函数调用运算符 ()

重载运算符的规则

• 不允许创造新的运算符, 只能对已有 C++ 运算符进行重载.
• C++ 允许重载运算符: 成员运算符(.), 成员指针访问运算符(.*), 域运算符(:😃, 求字节数运算符(sizeof), 条件运算符(?😃
• 重载不能改变运算符运算对象 (即操作数) 的个数
• 重载不能改变运算符的优先级别
• 重载不能改变运算符的结合性
• 重载运算符的函数不能有默认的参数
• 重载的运算符必须和用户定义的自定义类型的对象一起使用. 参数至少有一个是类对象或其 引用

成员函数实现 Complex 加法

Complex 类:

#ifndef PROJECT2_COMPLEX_H
#define PROJECT2_COMPLEX_H

class Complex {
private:
    double real;
    double imag;
public:
    Complex();
    Complex(double r, double i);
    Complex add(Complex &c2);
    void display();
};

#endif //PROJECT2_COMPLEX_H

Complex.cpp:

#include <iostream>
#include "Complex.h"
using namespace std;

Complex::Complex() : real(0), imag(0) {}

Complex::Complex(double r, double i) : real(r), imag(i) {}

Complex Complex::add(Complex &c2) {
    Complex c;
    c.real = real + c2.real;
    c.imag = imag + c2.imag;
    return c;
}

void Complex::display() {
    cout << "(" << real << ", ";
    cout << imag << "i)" << endl;
}

main:

int main() {
    Complex c1(3, 4), c2(5, -10), c3;
    cout << "c1 =";
    c1.display();
    cout << "c2 =";
    c2.display();
    c3 = c1.add(c2);
    cout << "c1 + c2 = ";
    c3.display();

    return 0;
}

输出结果:

c1 =(3, 4i)
c2 =(5, -10i)
c1 + c2 = (8, -6i)

运算符重载的方法

运算符重载格式:

函数类型 operator 运算符名称 (形参流标) {对运算符的重载处理}

Complex 类:

#ifndef PROJECT4_COMPLEX_H
#define PROJECT4_COMPLEX_H

class Complex {
private:
    double real;
    double imag;
public:
    Complex();
    Complex(double, double);
    void display();
    Complex operator+(Complex &c2);
};

#endif //PROJECT4_COMPLEX_H

Complex.cpp:

#include <iostream>
#include "Complex.h"
using namespace std;

Complex::Complex() : real(0), imag(0) {}

Complex::Complex(double r, double i) :real(r), imag(i) {}

void Complex::display() {
    cout << "(" << real << ", ";
    cout << imag << "i)" << endl;
}

Complex Complex::operator+(Complex &c2) {
    Complex c;
    c.real = real + c2.real;
    c.imag = imag + c2.imag;
    return c;
}

main:

#include <iostream>
#include "Complex.h"

using namespace std;

int main() {
    Complex c1(3, 4), c2(5, -10), c3;
    cout << "c1 =";
    c1.display();
    cout << "c2 =";
    c2.display();
    c3 = c1 + c2;
    cout << "c1 + c2 = ";
    c3.display();

    return 0;
}

输出结果:

c1 =(3, 4i)
c2 =(5, -10i)
c3= (8, -6i)

多种实现方法

成员函数实现:

Complex Complex::operator+(Complex &c2) {
    Complex c;
    c.real = real + c2.real;
    c.imag = imag + c2.imag;
    return c;
}

简化:

Complex Complex::operator+(Complex &c2){
    return Complex(real +c2.real, imag + c2.image);
}

友元函数实现:

Complex operator+(Complex &c1, Complex &c2){
    ......
}

实现 operator+=

Complex 类:

#ifndef PROJECT4_COMPLEX_H
#define PROJECT4_COMPLEX_H

class Complex {
private:
    double real;
    double imag;
public:
    Complex();
    Complex(double, double);
    void display();
    Complex operator+=(const Complex &c);
};

#endif //PROJECT4_COMPLEX_H

Complex.cpp:

#include <iostream>
#include "Complex.h"
using namespace std;

Complex::Complex() : real(0), imag(0) {}

Complex::Complex(double r, double i) :real(r), imag(i) {}

void Complex::display() {
    cout << "(" << real << ", ";
    cout << imag << "i)" << endl;
}

Complex Complex::operator+=(const Complex &c) {
    real += c.real;  // this->real += c.real;
    imag += c.imag;  // this->imag += c.imag;
    return *this;
}

main:

#include <iostream>
#include "Complex.h"

using namespace std;

int main() {
    Complex c1(3, 4), c2(5, -10), c3;
    cout << "c1 =";
    c1.display();
    cout << "c2 =";
    c2.display();
    c1 += c2;
    cout << "c1= ";
    c1.display();

    return 0;
}

输出结果:

c1 =(3, 4i)
c2 =(5, -10i)
c1= (8, -6i)

三种运算符重载函数

运算符重载函数可以是类的成员函数:

• 它可以通过 this 指针自由地访问本类的数据成员. 少写一个函数的参数, 但有要求.

运算符重载函数可以是类的友元函数:

• 如果运算符左侧的操作属于 C++ 标准类型 (如 int) 或是一个其他类的对象, 则运算符重载函数不能选用成员函数. 为方便访问类的私有成员, 声明为友元函数为佳.

运算符重载函数还可以是普通函数:

• 只有极少的情况下才使用 (因普通函数一般不能直接访问类的私有成员)

成员函数实现

Complex 类:

#ifndef PROJECT4_COMPLEX_H
#define PROJECT4_COMPLEX_H

class Complex {
private:
    double real;
    double imag;
public:
    Complex();
    Complex(double, double);
    void display();
    Complex operator+(double d);  // 成员函数实现
};

#endif //PROJECT4_COMPLEX_H

Complex.cpp:

#include <iostream>
#include "Complex.h"
using namespace std;

Complex::Complex() : real(0), imag(0) {}

Complex::Complex(double r, double i) :real(r), imag(i) {}

void Complex::display() {
    cout << "(" << real << ", ";
    cout << imag << "i)" << endl;
}

Complex Complex::operator+(double d) {
    return Complex(real + d, imag);
}

友元函数实现

Complex 类:

#ifndef PROJECT4_COMPLEX_H
#define PROJECT4_COMPLEX_H

class Complex {
private:
    double real;
    double imag;
public:
    Complex();
    Complex(double, double);
    void display();
    friend Complex operator+(Complex &c, double d);  // 友元函数
};

#endif //PROJECT4_COMPLEX_H

Complex.cpp:

#include <iostream>
#include "Complex.h"
using namespace std;

Complex::Complex() : real(0), imag(0) {}

Complex::Complex(double r, double i) :real(r), imag(i) {}

void Complex::display() {
    cout << "(" << real << ", ";
    cout << imag << "i)" << endl;
}

Complex operator+(Complex &c, double d) {
    return Complex(c.real + d, c.imag);
}

输出结果

main:

#include <iostream>
#include "Complex.h"

using namespace std;

int main() {
    Complex c1(3, 4), c2(5, -10), c3, c4;
    cout << "c1 =";
    c1.display();
    cout << "c2 =";
    c2.display();

    c3 = c1 + 3.14;
    cout << "c3= ";
    c3.display();

    return 0;
}

输出结果:

c1 =(3, 4i)
c2 =(5, -10i)
c3= (6.14, 4i)

重载单元运算符

单元运算符 (unary operation), 即只有一个运算量. 如: !a, -b, &c, *p, ++i, i-- 等.

例子

重载单元运算符实现分数对象的相反数.

Fraction 类:

#ifndef PROJECT4_FRACTION_H
#define PROJECT4_FRACTION_H

#include <iostream>
using namespace std;

class Fraction {
private:
    int nume;  // 分子
    int deno;  // 分母
public:
    Fraction();
    Fraction(int, int);
    Fraction operator-(const Fraction &c);  // 分数相减
    Fraction operator-();  // 取反一目运算
    friend ostream& operator<<(ostream &output, const Fraction &f);
};

#endif //PROJECT4_FRACTION_H

Fraction.cpp:

#include "Fraction.h"

Fraction::Fraction() : nume(0), deno(0) {}

Fraction::Fraction(int n , int d) : nume(n), deno(d) {}

Fraction Fraction::operator-(const Fraction &c) {
    return Fraction(nume*c.deno - c.nume*deno, deno*c.deno);
}

Fraction Fraction::operator-() {
    return Fraction(-nume, deno);
}

ostream& operator<<(ostream &output, const Fraction &f) {
    double result = (double)f.nume / f.deno;
    output << result << endl;
    return output;
}

main:

#include <iostream>
#include "Fraction.h"
using namespace std;

int main() {
    Fraction f1(1,3), f2(1,5), f3, f4;

    f3 = f1 - f2;  // 分数相减
    f4 = -f1;  // 分数取反

    cout << f3;
    cout << f4;

    return 0;
}

输出结果:

0.133333
-0.333333

重载二元运算符

二元运算符 (binary operation).

• 有两个操作数, 通常在运算符的左右两侧 (例如: 3+2, 5>8, x*=3)
• 重载双目运算符时, 函数中应该有两个参数

例子

要求:

• 定义字符串类 String, 用来存放不定长的字符串
• 重载关系运算符, 用于两个字符串的比较运算

步骤:

• 定义类的 “框架”
• 完善运算符重载

String 类:

#ifndef PROJECT4_STRING_H
#define PROJECT4_STRING_H

#include <cstdlib>

class String {
private:
    char *p;
public:
    String(){p=nullptr;}
    String(char *str);
    void display();
};

#endif //PROJECT4_STRING_H

String.cpp:

#include <iostream>
#include <cstring>
#include "String.h"
using namespace std;

String::String(char *str) {
    p = new char[sizeof(str)];
    strcpy(p, str);
}

void String::display() {
    cout << p;
}

main:

#include <iostream>
#include "String.h"

using namespace std;

int main() {
    String s1("Hello");
    String s2("China");
    s1.display( );
    cout<<" ";
    s2.display( );
    cout<<endl;

    return 0;
}

输出结果:

Hello China

重载 I/O

通过重载输入流 (input stream) 和输出流 (output stream), 我们可以用来输出用户自己定义的数据.

格式:

ostream &operator<<(ostream&, const 自定义类&);
istream &operator>>(istream&,自定义类&);

插入运算符 <<

Complex 类:

#ifndef PROJECT4_COMPLEX_H
#define PROJECT4_COMPLEX_H

#include <iostream>
using namespace std;

class Complex {
private:
    double real;
    double imag;
public:
    Complex();
    Complex(double, double);
    void display();
    Complex operator+(Complex &c);
    friend ostream& operator<<(ostream &output, const Complex &c);
};

#endif //PROJECT4_COMPLEX_H

Complex.cpp:

#include <iostream>
#include "Complex.h"
using namespace std;

Complex::Complex() : real(0), imag(0) {}

Complex::Complex(double r, double i) :real(r), imag(i) {}

void Complex::display() {
    cout << "(" << real << ", ";
    cout << imag << "i)" << endl;
}

Complex Complex::operator+(Complex &c) {
    return Complex(real + c.real, imag + c.imag);
}

ostream &operator<<(ostream &output, const Complex &c) {
    output<<"("<<c.real<<" + "<<c.imag<<"i)";
    return output;
}

main:

#include <iostream>
#include "Complex.h"
using namespace std;

int main() {
    Complex c1(2, 4),c2(6, 10),c3;
    c3 = c1 + c2;
    cout << c1 << " + " << c2 << " = " << c3 << endl;

    return 0;
}

输出结果:

(2 + 4i) + (6 + 10i) = (8 + 14i)

提取运算符 >>

Complex 类:

#ifndef PROJECT4_COMPLEX_H
#define PROJECT4_COMPLEX_H

#include <iostream>
using namespace std;

class Complex {
private:
    double real;
    double imag;
public:
    Complex();
    Complex(double, double);
    void display();
    Complex operator+(Complex &c);
    friend ostream& operator<<(ostream &output, const Complex &c);
    friend istream& operator>>(istream &input, Complex &c);
};

#endif //PROJECT4_COMPLEX_H

Complex.cpp:

#include <iostream>
#include "Complex.h"
using namespace std;

Complex::Complex() : real(0), imag(0) {}

Complex::Complex(double r, double i) :real(r), imag(i) {}

void Complex::display() {
    cout << "(" << real << ", ";
    cout << imag << "i)" << endl;
}

Complex Complex::operator+(Complex &c) {
    return Complex(real + c.real, imag + c.imag);
}

ostream &operator<<(ostream &output, const Complex &c) {
    output<<"("<<c.real<<" + "<<c.imag<<"i)";
    return output;
}

istream &operator>>(istream &input, Complex &c) {
    cout << "input real part and imaginary part:\n";
    input >> c.real >> c.imag;
    return input;
}

main:

#include <iostream>
#include "Complex.h"
using namespace std;

int main() {
    Complex c1, c2;
    cin >> c1 >> c2;
    cout << "c1=" << c1 << endl;
    cout << "c2=" << c2 << endl;

    return 0;
}

输出结果:

input real part and imaginary part:
2 4
input real part and imaginary part:
6 10
c1=(2 + 4i)
c2=(6 + 10i)

总结

运算符重载使类的设计更加丰富多彩, 扩大了类的功能和使用范围. 运算符重载使得程序易于理解, 易于对对象进行操作. 有了运算符重载, 在声明了类之后, 我们就可以像使用标准类型一样来使用自己声明的类.

类的声明往往是一劳永逸的. 有了好的类, 用户在程序中就不必定义许多成员函数去完成运算和 I/O 的功能, 使主函数更加简单易读. 好的运算符重载能细心啊面向对象程序设计思想.

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