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学习C++编译环境:Linux
第四十课 前置操作符和后置操作符
1.值得思考的问题
下面的代码有没有区别?为什么?答案是5.真正的区别
i++; //i的值作为返回值,i自增1
++i; // i自增1,i的值作为返回值
40-1 真的有区别吗?
#include <iostream>
#include <string>
using namespace std;
int main()
{
int i = 0;
i++;
++i;
return 0;
}
2.意想不到的事实
现代编译器产品会对代码进行优化
优化是的最终的二进制程序更加高效
优化后的二进制程序丢失了C/C++的原生语义
不可能从编译后的二进制程序还原C/C++程序
3.思考
++操作符可以重载吗?如何区分前置++和后置++?
4.++操作符重载
++操作符可以被重载
全局函数和成员函数均可进行重载
重载前置++操作符不需要额外的参数
重载后置++操作符需要一个int类型的占位参数
40-2 ++操作符的重载
#include <iostream>
#include <string>
using namespace std;
class Test
{
int mValue;
public:
Test(int i)
{
mValue = i;
}
int value()
{
return mValue;
}
//前置++
Test& operator ++ ()
{
++mValue;
return *this;
}
//后置++
Test operator ++ (int)
{
Test ret(mValue);
mValue++;
return ret;
}
};
int main()
{
Test t(0);
//下面两行意义不同,且前置++效率高,不需要构造函数和析构函数
t++;
++t;
return 0;
}
5.真正的区别
对于基础类型的变量
前置++的效率与后置++的效率基本相同
根据项目组编码规范进行选择
对于类类型的对象
前置++的效率高于后置++
尽量使用前置++操作符提供程序效率
class Complex 复数类的进一步改善
test.cpp
#include<iostream>
#include<string>
#include "Complex.h"
using namespace std;
int main()
{
Complex c(0,0);
c++;
cout << c.getA() << endl;
cout << c.getB() << endl;
return 0;
}
Complex.h
#ifndef _COMPLEX_H_
#define _COMPLEX_H_
class Complex
{
double a;
double b;
public:
Complex(double a = 0, double b = 0);
double getA();
double getB();
double getModulus();
Complex operator + (const Complex& c);
Complex operator - (const Complex& c);
Complex operator * (const Complex& c);
Complex operator / (const Complex& c);
bool operator == (const Complex& c);
bool operator != (const Complex& c);
Complex& operator = (const Complex& c);
Complex& operator ++ ();
Complex operator ++ (int);
};
#endif
Complex.cpp
#include "Complex.h"
#include "math.h"
Complex::Complex(double a, double b)
{
this->a = a;
this->b = b;
}
double Complex::getA()
{
return a;
}
double Complex::getB()
{
return b;
}
double Complex::getModulus()
{
return sqrt(a * a + b * b);
}
Complex Complex::operator + (const Complex& c)
{
double na = a + c.a;
double nb = b + c.b;
Complex ret(na, nb);
return ret;
}
Complex Complex::operator - (const Complex& c)
{
double na = a - c.a;
double nb = b - c.b;
Complex ret(na, nb);
return ret;
}
Complex Complex::operator * (const Complex& c)
{
double na = a * c.a - b * c.b;
double nb = a * c.b + b * c.a;
Complex ret(na, nb);
return ret;
}
Complex Complex::operator / (const Complex& c)
{
double cm = c.a * c.a + c.b * c.b;
double na = (a * c.a + b * c.b) / cm;
double nb = (b * c.a - a * c.b) / cm;
Complex ret(na, nb);
return ret;
}
bool Complex::operator == (const Complex& c)
{
return (a == c.a) && (b == c.b);
}
bool Complex::operator != (const Complex& c)
{
return !(*this == c);
}
Complex& Complex::operator = (const Complex& c)
{
if( this != &c )
{
a = c.a;
b = c.b;
}
return *this;
}
Complex& Complex::operator ++ ()
{
a = a + 1;
b = b + 1;
return *this;
}
Complex Complex::operator ++ (int)
{
Complex ret(a, b);
a = a + 1;
b = b + 1;
return ret;
}
运行结果
lkk@lkk-virtual-machine:~/c++/40$ g++ test.cpp Complex.cpp
lkk@lkk-virtual-machine:~/c++/40$ ./a.out
1
1
小结
编译优化使得最终的可执行程序更加高效
前置++操作符和后置++操作符都可以被重载
++操作符的重载必须符合其原生语义
对于基础类型,前置++与后置++的效率几乎相同
对于类类型,前置++的效率高于后置++