运算符重载
1.概念
所谓重载,就是重新赋予新的含义。函数重载就是对一个已有的函数赋予新的含义,使之实现新功能,因此,一个函数名就可以用来代表不同功能的函数,也就是”一名多用”。
运算符也可以重载。实际上,我们已经在不知不觉之中使用了运算符重载。例如,大 家都已习惯于用加法运算符”+”对整数、单精度数和双精度数进行加法运算,如5+8, 5.8 +3.67等,其实计算机对整数、单精度数和双精度数的加法操作过程是很不相同的, 但由于C++已经对运算符”+”进行了重载,所以就能适用于int, float, doUble类型的运算。
又如”<<“是C++的位运算中的位移运算符(左移),但在输出操作中又是与流对 象cout 配合使用的流插入运算符,”>>“也是位移运算符(右移),但在输入操作中又是与流对象 cin 配合使用的流提取运算符。这就是运算符重载(operator overloading)。C++系统对”<<“和”>>“进行了重载,用户在不同的场合下使用它们时,作用是不同 的。对”<<“和”>>“的重载处理是放在头文件stream中的。因此,如果要在程序中用”<< “和”>>”作流插入运算符和流提取运算符,必须在本文件模块中包含头文件stream(当然还应当包括”using namespace std“)。
现在要讨论的问题是:用户能否根据自己的需要对C++已提供的运算符进行重载,赋予它们新的含义,使之一名多用。?
- 运算符重载入门技术推演
- 为什么会用运算符重载机制
用复数类举例
Complex c3 = c1 + c2;
原因 Complex是用户自定义类型,编译器根本不知道如何进行加减.因此编译器给提供了一种机制,让用户自己去完成,自定义类型的加减操作。这个机制就是运算符重载机制 - 运算符重载的本质是一个函数
#include <iostream>using namespace std;class Complex{public: int a; int b;public: Complex(int a=0, int b=0) { this->a = a; this->b = b; } void printCom() { cout<<a<<" + " << b << "i" <<endl; }};Complex myAdd(Complex &c1, Complex &c2){ Complex tmp(c1.a + c2.a, c1.b+ c2.b); return tmp; }Complex operator+(Complex &c1, Complex &c2){ cout<<"12345上山 打老虎"<<endl; Complex tmp(c1.a + c2.a, c1.b+ c2.b); return tmp; }int main(){ int a = 0, b = 0; int c; c = a + b; Complex c1(1, 2), c2(3, 4); Complex c3; Complex c4 = c1 + c2; c4.printCom(); cout<<"hello..."<<endl; return 1;}
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2.限制
3.基础
例如:
//全局函数 完成 +操作符 重载Complex operator+(Complex &c1, Complex &c2)//类成员函数 完成 -操作符 重载Complex operator-(Complex &c2)
例如1:通过类成员函数完成-操作符重载
Complex c4 = c1 - c2; c4.printCom();
例如2:通过全局函数方法完成+操作符重载
int main(){ Complex c1(1, 2), c2(3, 4); Complex c3 = c1 + c2; c3.printCom();}
例如3
++ ++ . ++ . ++
//调用方法
++c1 ; //=需要写出操作符重载函数原形
c1.printCom();
//运算符重载函数名定义
//首先承认操作符重载是一个函数 定义函数名operator++
//分析函数参数 根据左右操作数的个数,operator++(Complex &c1)
//分析函数返回值Complex& operator++(Complex &c1)
返回它自身
例如4
Complex& operator--(){ this->a--; this->b--; return *this;}
//4.2调用方法
--c1;
c1.printCom();
//4.3前置—运算符重载函数名定义
//c1.operator–()
例如5
Complex operator++(Complex &c1, int) { Complex tmp = c1; c1.a++; c1.b++; return tmp;}
//5.2 调用方法
c1 ++ ; //先使用 后++
//5.3 后置++运算符重载函数名定义
Complex operator++(Complex &c1, int)
函数占位参数 和 前置++ 相区别
例如6
Complex operator--(int) { Complex tmp = *this; this->a--; this->b--; return tmp; }
//6.2 调用方法
c1 ++ ; //先使用 后++
//6.3 后置–运算符重载函数名定义
Complex operator--(int)
//函数占位参数 和 前置– 相区别
前置和后置运算符总结
C++中通过一个占位参数来区分前置运算和后置运算
#include <iostream>using namespace std;class Complex{private: int a; int b; friend Complex operator+(Complex &c1, Complex &c2); friend Complex& operator++(Complex &c1); friend Complex operator++(Complex &c1, int);public: Complex(int a=0, int b=0) { this->a = a; this->b = b; } void printCom() { cout<<a<<" + " << b << "i" <<endl; }public: Complex operator-(Complex &c2) { Complex tmp(this->a - c2.a, this->b - c2.b); return tmp; } Complex& operator--() { this->a --; this->b --; return *this; } Complex operator--(int) { Complex tmp = *this; this->a--; this->b--; return tmp; }};Complex operator+(Complex &c1, Complex &c2){ Complex tmp(c1.a + c2.a, c1.b + c2.b); return tmp;}Complex& operator++(Complex &c1){ c1.a++; c1.b++; return c1;}Complex operator++(Complex &c1, int){ Complex tmp = c1; c1.a ++; c1.b ++; return tmp;}void main(){ Complex c1(1, 2), c2(3, 4); Complex c3 = c1 + c2; c3.printCom(); Complex c4 = c1 - c2; c4.printCom(); ++c1; c1.printCom(); --c1; c1.printCom(); c1++; c1.printCom(); c1--; c1.printCom(); cout<<"hello..."<<endl; system("pause"); return ;}
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定义运算符重载函数名的步骤
全局函数、类成员函数方法实现运算符重载步骤
1)要承认操作符重载是一个函数,写出函数名称operator+ ()
2)根据操作数,写出函数参数
3)根据业务,完善函数返回值(看函数是返回引用 还是指针 元素),及实现函数业务
运算符重载的正规写法
#include <iostream>using namespace std;class Complex{private: int a; int b; friend ostream& operator<<(ostream &out, Complex &c1);public: Complex(int a=0, int b=0) { this->a = a; this->b = b; } void printCom() { cout<<a<<" + " << b << "i" <<endl; }public: Complex operator+(Complex &c2) { Complex tmp(a + c2.a, b + c2.b); return tmp; } Complex& operator++() { a++; b++; return *this; } Complex operator++(int) { Complex tmp = *this; this->a ++; this->b ++; return tmp; } Complex operator-(Complex &c2) { Complex tmp(this->a - c2.a, this->b - c2.b); return tmp; } Complex& operator--() { this->a --; this->b --; return *this; } Complex operator--(int) { Complex tmp = *this; this->a--; this->b--; return tmp; }};void main31(){ Complex c1(1, 2), c2(3, 4); Complex c3 = c1 + c2; c3.printCom(); Complex c4 = c1 - c2; c4.printCom(); ++c1; c1.printCom(); --c1; c1.printCom(); c1++; c1.printCom(); c1--; c1.printCom(); cout<<"hello..."<<endl; system("pause"); return ;}ostream& operator<<(ostream &out, Complex &c1){ out<<"12345 生活真是苦"<<endl; out<<c1.a << " + " << c1.b << "i" << endl; return out;}void main(){ int a = 10; Complex c1(1, 2), c2(3, 4); cout<<a<<endl; cout << c1 ; cout << c1 << "aaddddd"; system("pause");}
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友元函数实现操作符重载的应用场景
1)友元函数和成员函数选择方法
- 当无法修改左操作数的类时,使用全局函数进行重载
- =, [], ()和->操作符只能通过成员函数进行重载
2)用友元函数 重载 << >>操作符
- istream 和 ostream 是 C++ 的预定义流类
- cin 是 istream 的对象,cout 是 ostream 的对象
- 运算符 << 由ostream 重载为插入操作,用于输出基本类型数据
- 运算符 >> 由 istream 重载为提取操作,用于输入基本类型数据
- 用友员函数重载 << 和 >> ,输出和输入用户自定义的数据类型
a)用全局函数方法实现 << 操作符
ostream& operator<<(ostream &out, Complex &c1){ //out<<"12345,生活真是苦"<<endl out<<c1.a<<" + "<<c1.b<<"i "<<endl return out}
//调用方法
cout<<c1;
//链式编程支持
cout<<c1<<"abcc";
//cout.operator<<(c1).operator<<("abcd");
/函数返回值充当左值 需要返回一个引用
b)类成员函数方法无法实现 << 操作符重载
- 因无法拿到cout这个类的源码
- cout.operator<<(c1);
友元函数重载操作符使用注意点
a) 友员函数重载运算符常用于运算符的左右操作数类型不同的情况
b)其他
- 在第一个参数需要隐式转换的情形下,使用友员函数重载运算符是正确的选择
- 友员函数没有 this 指针,所需操作数都必须在参数表显式声明,很容易实现类型的隐式转换
- C++中不能用友员函数重载的运算符有
= () [] ->
友元函数案例vector类
#include <iostream>using namespace std;class vector{ public : vector( int size =1 ) ; ~vector() ; int & operator[]( int i ) ; friend ostream & operator << ( ostream & output , vector & ) ; friend istream & operator >> ( istream & input, vector & ) ;private : int * v ; int len ;};vector::vector( int size ) { if (size <= 0 || size > 100 ) { cout << "The size of " << size << " is null !\n" ; abort() ; } v = new int[ size ] ; len = size ;}vector :: ~vector() { delete[] v ; len = 0 ; }int &vector::operator[]( int i ) { if( i >=0 && i < len ) return v[ i ] ; cout << "The subscript " << i << " is outside !\n" ; abort() ;}ostream & operator << ( ostream & output, vector & ary ){ for(int i = 0 ; i < ary.len ; i ++ ) output << ary[ i ] << " " ; output << endl ; return output ;}istream & operator >> ( istream & input, vector & ary ) { for( int i = 0 ; i < ary.len ; i ++ ) input >> ary[ i ] ; return input ;}void main(){ int k ; cout << "Input the length of vector A :\n" ; cin >> k ; vector A( k ) ; cout << "Input the elements of vector A :\n" ; cin >> A ; cout << "Output the elements of vector A :\n" ; cout << A ; system("pause");}
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4.提高
1.运算符重载机制
编译器实现运算符重载实际上就是通过函数重载实现的,可分为全局函数方式,也可分为成员函数方式进行重载,并没有改变原操作符的属性和语义。只是针对某个特定类定义一种新的数据类型操作。
2.重载赋值运算符
- 赋值运算符重载用于对象数据的复制
- operator= 必须重载为成员函数
- 重载函数原型为:
类型 & 类名 :: operator= ( const 类名 & ) ;
结论:
1 先释放旧的内存
2 返回一个引用
3 =操作符 从右向左
#define _CRT_SECURE_NO_WARNINGS #include <iostream>using namespace std;class Name{public: Name(const char *myp) { m_len = strlen(myp); m_p =(char *) malloc(m_len + 1); strcpy(m_p, myp); } Name(const Name& obj1) { m_len = obj1.m_len; m_p = (char *)malloc(m_len + 1); strcpy(m_p, obj1.m_p); } Name& operator=(Name &obj1) { if (this->m_p != NULL) { delete[] m_p; m_len = 0; } this->m_len = obj1.m_len; this->m_p = new char [m_len+1]; strcpy(m_p, obj1.m_p); return *this; } ~Name() { if (m_p != NULL) { free(m_p); m_p = NULL; m_len = 0; } }protected:private: char *m_p ; int m_len; };void objplaymain(){ Name obj1("abcdefg"); Name obj2 = obj1; Name obj3("obj3"); obj3 = obj1; obj1 = obj2 = obj3; }void main(){ objplaymain(); cout<<"hello..."<<endl; system("pause"); return ;}
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3.重载下标运算符
- [ ]运算符用于访问数据对象的元素
- 重载格式
类型 类 :: operator[] ( 类型 ) ;
- 只能用成员函数重载,不能用友元函数重载
示例:
设 x 是类 X 的一个对象,则表达式
x [ y ]
可被解释为
x . operator [ ] ( y )
函数返回值当左值需要返回一个引用!
4.带下标和相等操作符的数组类
#ifndef NEWARRAY_H#define NEWARRAY_H#include <iostream>#include <stdlib.h>class NewArray{public: NewArray(); NewArray(int _len); NewArray(const NewArray & obj); ~NewArray(); void setData(int index,int var); int getData(int index); int length(); int& operator[](int i); NewArray& operator=(NewArray& obj); bool operator==(NewArray& obj); bool operator!=(NewArray& obj);private: int m_len; int *m_buf;};#endif // NEWARRAY_H
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#include "newarray.h"NewArray::NewArray(){ m_buf = NULL; m_len = -1;}NewArray::NewArray(int _len){ if(_len < 0) _len = 0; m_len = _len; m_buf = new int[m_len];}NewArray::NewArray(const NewArray & obj){ m_len = obj.m_len; m_buf = new int[m_len]; for(int i = 0;i < m_len;i++) { m_buf[i] = obj.m_buf[i]; }}NewArray::~NewArray(){ if(m_buf != NULL) { delete []m_buf; m_buf = NULL; m_len = -1; }}void NewArray::setData(int index,int var){ m_buf[index] = var;}int NewArray::getData(int index){ return m_buf[index];}int NewArray::length(){ return m_len;}int& NewArray::operator[](int i){ return m_buf[i];}NewArray& NewArray::operator=(NewArray& obj){ if(m_buf != NULL) { delete []m_buf; m_len = -1; m_buf = NULL; } m_len = obj.m_len; m_buf = new int[m_len]; for(int i = 0;i < m_len;i++) { m_buf[i] = obj.m_buf[i]; } return *this;}bool NewArray::operator==(NewArray& obj){ if(m_len != obj.m_len) { return false; } for(int i = 0;i < m_len;i++) { if(m_buf[i] != obj.m_buf[i]) { return false; } } return true;}bool NewArray::operator!=(NewArray& obj){ return !((*this) == obj);}
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#include "newarray.h"using namespace std;int main(){ NewArray a1(10); for (int i=0; i<a1.length(); i++) { a1.setData(i, i); a1[i] = i; } cout<<"\na1: "; for (int i=0; i<a1.length(); i++) { cout<<a1[i]<<"\t"; } cout<<endl; NewArray a2 = a1; cout<<"\na2: "; for (int i=0; i<a2.length(); i++) { cout<<a2.getData(i)<<" "; } cout<<endl; NewArray a3(5); { a3 = a1; a3 = a2 = a1; cout<<"\na3: "; for (int i=0; i<a3.length(); i++) { cout<<a3[i]<<" "; } } if (a3 == a1) { printf("\nequal\n"); } else { printf("\nnot equal\n"); } if (a3 != a1) { printf("\nnot equal\n"); } else { printf("\nequal\n"); } cout<<"hello..."<<endl; return 1;}
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5.重载函数调用运算符
- () 运算符用于函数调用
- 重载格式
类型 类 :: operator() ( 表达式表 ) ;
- 只能用成员函数重载,不能用友元函数重载
例1
设 x 是类 X 的一个对象,则表达式
x ( arg1, arg2, … )
可被解释为
x . operator () (arg1, arg2, … )
案例:
#include <iostream>class F { public : double operator ( ) ( double x , double y ) ; } ;double F :: operator ( ) ( double x , double y ) { return x * x + y * y ; }void main ( ) { F f ;f.getA(); cout << f ( 5.2 , 2.5 ) << endl ; }
#include <iostream.h>class F { public : double memFun ( double x , double y ) ; } ;double F :: memFun ( double x , double y ) { return x * x + y * y ; }void main ( ) { F f ; cout << f.memFun ( 5.2 , 2.5 ) << endl ;}
6.不建议重载的运算符
理论知识:
1)&&和||是C++中非常特殊的操作符
2)&&和||内置实现了短路规则
3)操作符重载是靠函数重载来完成的
4)操作数作为函数参数传递
5)C++的函数参数都会被求值,无法实现短路规则
#include <cstdlib>#include <iostream>using namespace std;class Test{ int i;public: Test(int i) { this->i = i; } Test operator+ (const Test& obj) { Test ret(0); cout<<"执行+号重载函数"<<endl; ret.i = i + obj.i; return ret; } bool operator&& (const Test& obj) { cout<<"执行&&重载函数"<<endl; return i && obj.i; }};void main(){ int a1 = 0; int a2 = 1; cout<<"注意:&&操作符的结合顺序是从左向右"<<endl; if( a1 && (a1 + a2) ) { cout<<"有一个是假,则不在执行下一个表达式的计算"<<endl; } Test t1 = 0; Test t2 = 1; if( (t1 + t2) && t1) { cout<<"两个函数都被执行了,而且是先执行了+"<<endl; } { cout<<"两个函数都被执行了,而且是先执行了+"<<endl; } system("pause"); return ;}
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5.字符串类的实现
#ifndef MYSTRING_H#define MYSTRING_H#include <iostream>using namespace std;#include <stdlib.h>#include <string.h>class MyString{public: MyString(); MyString(int _len); MyString(const char *_str); MyString(const MyString & obj); ~MyString(); MyString& operator =(const MyString & obj); MyString& operator =(const char * _str); bool operator ==(const MyString & obj); bool operator ==(const char * _str); bool operator !=(const MyString & obj); bool operator !=(const char * _str); bool operator >(const MyString & obj); bool operator >(const char * _str); bool operator <(const MyString & obj); bool operator <(const char * _str); char& operator [](int index); friend ostream& operator<<(ostream & out,MyString & obj); friend istream& operator>>(istream & in,MyString & obj);private: int m_len; char *m_str;};#endif
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#include "mystring.h"MyString::MyString(){ m_len = 0; m_str = NULL;}MyString::MyString(int _len){ if(_len < 0) _len = 0; m_len = _len; m_str = new char[m_len+1]; memset(m_str,0,m_len);}MyString::MyString(const char *_str){ if(_str == NULL) { m_len = 0; m_str = new char[m_len+1]; strcpy(m_str,""); }else { m_len = strlen(_str); m_str = new char[m_len+1]; strcpy(m_str,_str); }}MyString::MyString(const MyString & obj){ m_len = obj.m_len; m_str = new char[m_len+1]; strcpy(m_str,obj.m_str);}MyString::~MyString(){ if(m_str != NULL) { delete []m_str; m_str = NULL; m_len = 0; }}MyString& MyString::operator =(const MyString & obj){ if(m_str != NULL) { delete []m_str; m_str = NULL; m_len = 0; } m_len = obj.m_len; m_str = new char[m_len+1]; strcpy(m_str,obj.m_str); return *this;}MyString& MyString::operator =(const char * _str){ if(m_str != NULL) { delete []m_str; m_str = NULL; m_len = 0; } if(_str == NULL) { m_len = 0; m_str = new char[m_len+1]; strcpy(m_str,""); }else { m_len = strlen(_str); m_str = new char[m_len+1]; strcpy(m_str,_str); } return *this;}bool MyString::operator ==(const MyString & obj){ if(m_len != obj.m_len) { return false; } return !strcmp(m_str,obj.m_str);}bool MyString::operator ==(const char * _str){ if(_str == NULL) { if(m_len == 0) { return true; }else{ return false; } }else{ if(m_len == strlen(_str)){ return !strcmp(m_str,_str); }else{ return false; } }}bool MyString::operator !=(const MyString & obj){ return !((*this) == obj);}bool MyString::operator !=(const char * _str){ return !((*this) == _str);}bool MyString::operator >(const MyString & obj){ if(strcmp(m_str,obj.m_str) > 0) { return true; }else{ return false; }}bool MyString::operator >(const char * _str){ if(strcmp(m_str,_str) > 0) { return true; }else{ return false; }}bool MyString::operator <(const MyString & obj){ if(strcmp(m_str,obj.m_str) < 0) { return true; }else{ return false; }}bool MyString::operator <(const char * _str){ if(strcmp(m_str,_str) < 0) { return true; }else{ return false; }}char& MyString::operator [](int index){ return m_str[index];}ostream& operator<<(ostream & out,MyString & obj){ out<<obj.m_str; return out;}istream& operator>>(istream & in,MyString & obj){ in>>obj.m_str; return in;}
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#define _CRT_SECURE_NO_WARNINGS#include "mystring.h"void main01(){ MyString s1; MyString s2("s2"); MyString s2_2 = NULL; MyString s3 = s2; MyString s4 = "s4444444444"; s4 = s2; s4 = "s2222"; s4[1] = '4'; printf("%c", s4[1]); cout<<s4 <<endl; cout<<"hello..."<<endl; system("pause"); return ;}void main02(){ MyString s1; MyString s2("s2"); MyString s3 = s2; if (s2 == "aa") { printf("相等"); } else { printf("不相等"); } if (s3 == s2) { printf("相等"); } else { printf("不相等"); }}void main03(){ MyString s1; MyString s2("s2"); MyString s3 = s2; s3 = "aaa"; if (s3 < "bbbb" ) { printf("s3 小于 bbbb"); } else { printf("s3 大于 bbbb"); } MyString s4 = "aaaaffff"; cout<<s4<<endl;}void main011(){ MyString s1(128); cout<<"\n请输入字符串(回车结束)"; cin>>s1; cout<<s1; system("pause");}int main(){ MyString s1(128); cout<<"\n请输入字符串(回车结束)"; cin>>s1; cout<<s1<<endl; system("pause"); return 0;}
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总结
- 操作符重载是C++的强大特性之一
- 操作符重载的本质是通过函数扩展操作符的语义
- operator关键字是操作符重载的关键
- friend关键字可以对函数或类开发访问权限
- 操作符重载遵循函数重载的规则
- 操作符重载可以直接使用类的成员函数实现
- =, [], ()和->操作符只能通过成员函数进行重载
- ++操作符通过一个int参数进行前置与后置的重载
- C++中不要重载&&和||操作符
附录:运算符和结合性
友元函数