C++基础

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我是靠谱客的博主沉静凉面，最近开发中收集的这篇文章主要介绍C++基础，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

1、简单的C++程序

//
//  main.cpp
//  MyFirstC
//
//  Created by xiaofeifei on 2019/1/27.
//  Copyright © 2019 xiaofeifei. All rights reserved.
//
#include <iostream>
using  namespace std;  //use standard name space
int main(int argc, const char * argv[]) {
    // insert code here...
    //cout is standard out
    //<< 左移操作符在C++中进行了功能改造，语言操作符重载
    cout << "Hello, World!"<<endl;
    return 0;
}

2、C++对C的扩展

2.1、面向对象概念

需求：求圆的周长和面积

数据描述：
半径，周长和面积都用实数表示；
数据处理：
1）输入半径r；
2）计算周长=23.1415r;
3）计算面积=3.1415rr；
4）输出半径r，周长和面积；

方法1：用结构化方法（面向过程）实现

#include <iostream>
using  namespace std;  //use standard name space
//DE01-count the girth and area of circle
int main()
{
    double r, girth, area;
    const double PI = 3.1415;
    cout << "input the radius" << endl;
    cin >> r;
    girth = 2*r*PI;
    area = PI * r * r;
    cout << "radius = " << r << endl;
    cout << "girth = " << girth << endl;
    cout << "area = " << area << endl;
    return 0;
}

输出

input the radius
4
radius = 4
girth = 25.132
area = 50.264
Program ended with exit code: 0

方法2：用面向对象方法实现

#include <iostream>
using  namespace std;  //use standard name space
//DE02-count the girth and are of circle
//面向对象的实现方法
//Circle类的抽象和封装
class Circle
{
public:
    double r;
    double girth;
    double area;
public:
    void setR(double rr)
    {
        r =rr;
    }
    
    double getR()
    {
        return r;
    }
    
    double getGirth()
    {
        return 2*r*3.1415;
    }
    
    double getArea()
    {
        return 3.1415*r*r;
    }
protected:
private:
};
int main()
{
    Circle circle;
    double r;
    cout << "Please input the radius" << endl;
    cin >> r;
    circle.setR(r);
    cout << "radius = " << circle.getR() << endl;
    cout << "girth = " << circle.getGirth() << endl;
    cout << "area = " << circle.getArea() << endl;
    return 0;
}

输出

Please input the radius
5
radius = 5
girth = 31.415
area = 78.5375
Program ended with exit code: 0

思考：C++编译器是如何处理多个对象以及调用类的成员函数的？可查看链接: 编译器对类对象的处理机制

2.2、类中不写成员函数易犯错场景

参考3.1中Circle类的定义中，如果不定义成员函数，直接在成员函数中求得圆的面积并输出，我们来看一下效果怎样，代码如下：


class Circle
{
public:
        double r;
        double area = 3.1415 * r * r;
};

int main()
{
    Circle c1;
    cout << "Please input the radius of circle" << endl;
    cin >> c1.r;
    cout << "Circle area = " << c1.area << endl;
    return 0;
}