C++中操作符的前置与后置有什么区别

2022-11-13 09:11:58 操作后置有什么区别

一、值得思考的问题

下面的代码有没有区别？为什么？

二、意想不到的事实

现代编译器产品会对代码进行优化
优化使得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

三、++ 操作符重载

++ 操作符可以重载吗？如何区分前置++ 和后置++？

++ 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

下面来看 ++ 操作符重载的示例：

#include <iOStream>
using namespace std;
class Test
{
    int mValue;
public:
    Test(int i)
    {
        mValue = i;
    }
    int value()
    {
        return mValue;
    }
    Test& operator ++ ()
    {
        ++mValue;
        return *this;
    }
    Test operator ++ (int)
    {
        Test ret(mValue);
        mValue++;
        return ret;
    }
};
int main()
{
    Test t(0);
    Test m(0);
    Test tt = t++;
    cout << "tt = " << tt.value() << endl;
    cout << "t = " << t.value() << endl;
    Test mm = ++m;
    cout << "mm = " << mm.value() << endl;
    cout << "m = " << m.value() << endl;
    return 0;
}

输出结果如下：

前置++的效率高于后置++，因为前置的++没有生成额外的对象，意味着不需要过多的内存，也就是不需要在栈上生成对象。而后置的++需要创建栈空间上的对象，占用栈空间，并且需要调用构造函数，返回后需要调用析构函数。

四、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++操作符提高程序效率

前面写过的复数类可以进一步完善了：

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;
}

test.cpp：

#include <iostream>
#include "Complex.h"
using namespace std;
int main()
{
    Complex a(0, 0);
    Complex b(0, 0);
    Complex aa = a++;
    Complex bb = ++b;
    cout << "aa的实部为: " << aa.getA() << endl;
    cout << "aa的实部为: " << aa.getB() << endl;
    cout << "bb的实部为: " << bb.getA() << endl;
    cout << "bb的实部为: " << bb.getB() << endl;
    return 0;
}