string类和标准模版库

来学习了，来学习了，写有问题的话，希望大佬指出下。

1.string类的七种构造函数使用

#include <iostream>
#include <string>
int main(int argc, const char * argv[]) {using namespace std;string one{"Lotter Winner:"};cout << one << endl;string two(20, '$');    // 20个'$'字符cout << two << endl;string three(one);      // 把one给threecout << three << endl;one += "Oops!";         // string类可以直接加cout << one << endl;two = "Sorry! That was ";three[0] = 'p';string four;four = two + three;     // string类可以直接加cout << four << endl;char alls[] = "All's well that ends well";string five(alls, 20);  // 前20个字符，没有wellcout << five << "!\n";string six(alls + 6, alls + 10);    // 像两个指针一样 指向起始和结束cout << six << ", ";string seven(&five[6], &five[10]);  // 那么也可以这样等价写
//  string seven(&alls[6], &alls[10]);  // 这样可以是可以的cout << seven << "...\n";string eight(four, 7, 16);          // 从four的第七个到第十六个cout << eight << "in motion!" << endl;return 0;
}

还是先学第十一章，感觉类还是需要连贯的学习，索性刷完十一章。

2.time类的运算符重载对P390

main.cpp

#include <iostream>
#include "mytime.hpp"
int main()
{using std::cout;using std::endl;Time planning;Time coding(2, 40);Time fixing(5, 55);Time total;cout << "planning time = ";planning.show();cout << "planning time = ";coding.show();cout << "planning time = ";fixing.show();total = coding + fixing;cout << "coding + fixing = ";total.show();Time diff = fixing - coding;cout << "fixing - coding = ";diff.show();Time ajusted = total * 1.5;cout << "total * 1.5 = ";ajusted.show();return 0;
}

mytime.hpp

#ifndef mytime_hpp
#define mytime_hpp#include <stdio.h>
class Time
{int hours;int minutes;
public:Time();Time(int h, int m = 0);void AddMin(int m);void AddHr(int h);void Reset(int h = 0, int m = 0);Time operator+(const Time & t)const;Time operator-(const Time & t)const;Time operator*(double n)const;void show() const;
};
#endif /* mytime_hpp */

mytime.cpp

#include <iostream>
#include "mytime.hpp"
Time::Time()
{hours = minutes = 0;
}
Time::Time(int h, int m )
{hours = h;minutes = m;
}
void Time::AddMin(int m)
{minutes += m;hours += minutes/60;minutes %= 60;
}void Time::AddHr(int h)
{hours += h;
}
void Time::Reset(int h, int m)
{hours = h;minutes = m;
}Time Time::operator+(const Time &t) const
{Time sum;sum.minutes = this->minutes + t.minutes;sum.hours = this->hours + t.hours + sum.minutes/60;sum.minutes %= 60;return sum;
}Time Time::operator-(const Time & t) const
{// 默认 大-小Time diff;int tot1, tot2;tot1 = t.minutes + t.hours * 60;tot2 = this->minutes + this->hours * 60;diff.minutes = (tot2 - tot1) % 60;diff.hours = (tot2 - tot1) / 60;return diff;
}
Time Time::operator*(double n)const
{Time result;long totalminutes = hours * n * 60 + minutes * n;result.hours = (int)totalminutes / 60;result.minutes = totalminutes % 60;return result;
}void Time::show() const
{std::cout << hours << ":" << minutes << std::endl;
}

3.友元函数重载<<运算符

.hpp文件
friend std::ostream & operator<<(std::ostream & os,const Time& t);
.cpp文件
std::ostream & operator<<(std::ostream & os,const Time& t)
{os << t.hours << ":" << t.minutes << std::endl;return os;
}

为什么要返回一个 ostream &？
其实前辈在写这个operator<<()函数时的时候
例子：cout << x << y
其实是这样的 (cout << x) << y
第一步 cout << x 其实这里就是先显示x, 然后返回值被cout代替，返回一个ostream &，
第二步 cout << y 显示y，并结束
所以去重载它的时候可以这样模仿前辈去写。

	Time diff = fixing - coding;cout << "fixing - coding = " << diff;// fixing - coding = 3:15 结果如上所示，完成了重载Time ajusted = total * 1.5;cout << "total * 1.5 = " << ajusted;// total * 1.5 = 12:52Time ajusted1 = 2 * coding;cout << "2 * coding = " << ajusted1;// 2 * coding = 5:20return 0;

4.矢量随机漫步的实现

main.cpp

#include <iostream>
#include <cstdlib>  // rand(), srand()
#include <ctime>    // time()
#include "vector.hpp"int main()
{using namespace std;using VECTOR::Vector;   // 导入类名后不需要用VECTOR::srand(time(0)); // 随机数生成器double direction;   // 角度Vector step;Vector result(0.0, 0.0);unsigned long steps = 0;    // 走了几步 计数器double target;double dstep;   // 步子的大小cout << "Enter target distance (q to quit): ";while (cin >> target){cout << "Enter step length: ";if (!(cin >> dstep))break;while (result.magval() < target){direction = rand() % 360;step.reset(dstep, direction, Vector::POL);result = result + step;steps ++;}cout << "After " << steps << " steps, the subject has"" the following location:\n";cout << result << endl;result.polar_mode();cout << "or\n" << result << endl;cout << "Average outward distance per step = "<< result.magval()/steps << endl;steps = 0;result.reset(0.0, 0.0);cout << "Enter target distance (q to quit): ";}cout << "bye!\n";cin.clear();while (cin.get()!='\n') {continue;}return 0;
}

vector.hpp

#ifndef vector_hpp
#define vector_hpp#include <stdio.h>
#include <iostream>
namespace VECTOR
{class Vector{public:enum Mode{RECT, POL};   // RECT表示直角坐标模式 POL表示极坐标模式private:double x;double y;double mag;  // 长度double ang;  // 角度Mode mode;void set_x();   // 如果输入的极坐标，那么相应地计算出x, y坐标void set_y();void set_mag(); // 如果输入的是x, y，那么相应的计算出距离和角度void set_ang();public:Vector();Vector(double n1, double n2, Mode form = RECT);void reset(double n1, double n2, Mode form = RECT);     // 重置~Vector();double xval() const{return x;}double yval() const{return y;}double magval() const{return mag;}double angval() const{return ang;}void polar_mode();  // 设置成极坐标void rect_mode();   // 设置成rect模式Vector operator+(const Vector &b) const;    // x,y分量相加Vector operator-(const Vector &b) const;    // 相减Vector operator-() const;                   // 取反Vector operator*(double n) const;           // 乘nfriend Vector operator*(double n, const Vector& a);friend std::ostream& operator<<(std::ostream &os, const Vector& v); // 重载 显示};
}
#endif /* vector_hpp */

vector.cpp

#include "vector.hpp"
#include <cmath>
using std::sqrt;
using std::sin;
using std::cos;
using std::atan;
using std::atan2;
using std::cout;
namespace VECTOR
{const double Rad_to_deg = 45.0 / atan(1.0);void Vector::set_mag(){ mag = sqrt(x * x + y * y);}void Vector::set_ang(){if(x == 0.0 && y == 0.0)ang = 0;elseang = atan2(y, x);}//从极坐标来设x, yvoid Vector::set_x(){ x = mag *cos(ang);}void Vector::set_y(){ y = mag *cos(ang);}Vector::Vector(){ x = y = mag = ang = 0.0;mode = RECT;}Vector::Vector(double n1, double n2, Mode form){mode = form;if (form == RECT){x = n1;y = n2;set_ang();set_mag();}else if(form == POL){mag = n1;ang = n2 / Rad_to_deg;set_x();set_y();}else{cout << "Enter the Mode!!!";cout << " vecter set to 0.";x = y = mag = ang = 0.0;mode = RECT;}}// 重置void Vector::reset(double n1, double n2, Mode form){mode = form;if (form == RECT){x = n1;y = n2;set_ang();set_mag();}else if(form == POL){mag = n1;ang = n2 / Rad_to_deg;set_x();set_y();}else{cout << "Enter the Mode!!!";cout << " vecter set to 0.";x = y = mag = ang = 0.0;mode = RECT;}}Vector::~Vector(){}void Vector::polar_mode(){mode = POL;}void Vector::rect_mode(){mode = RECT;}Vector Vector::operator+(const Vector &b) const{return Vector(x + b.x, y + b.y);}Vector Vector::operator-(const Vector &b) const{return Vector(x - b.x, y - b.y);}Vector Vector::operator-() const{return Vector(-x, -y);}Vector Vector::operator*(double n) const{return Vector(x * n, n * y);}Vector operator*(double n, const Vector& a){return a * n;}std::ostream& operator<<(std::ostream &os, const Vector& v){// 这个函数 是一个友元函数，不在类作用域内，所以用的是Vector::RECTif(v.mode == Vector::RECT)os << "(x, y) = (" << v.x << ", " << v.y << ")";else if (v.mode == Vector::POL)os << "(m, a) = (" << v.mag << ", " << v.ang << ")";elseos << "No mode!";return os;}
}

分析：

srand(time(0));

这句中的time(0)返回当前时间，通俗的说这个就是返回一个从不知道何时开始的到现在的秒数，你可以输出看看，它一直再按秒数不断增加.
而strand()函数呢是覆盖默认种子值，重新启动一个随机序列.为什么要覆盖？
因为后面的rand()函数，生成的随机数是伪随机数，所以要通过不断的覆盖种子来达到。

回顾下这个代码：

1.首先在VECTOR的命名空间下定义了Vector类，定义了枚举性的意义在于，可以使得每个对象都拥有一个状态，确定他是什么模式RECT表示直角坐标模式 POL表示极坐标模式。
2.有x，y，meg，ang四个属性，当处于RECT坐标模式的时候，你需要输入x,y值，私有方法set_mag()，set_ang()，会帮你生成meg，ang。POL同理。
3.xval()，yval()，magval()，angval()用于返回各个属性。
4.void polar_mode(); void rect_mode(); 这两个用于状态的转换
5.Vector operator+(const Vector &b) const;等运算符重载，我们就分析这一个，其实他返回的是调用了Vector()构造函数，返回的时候，构造默认是直角坐标系，所以直接用x,y坐标的相减、相加、乘n，然后返回一个新的Vector对象。
这里提一嘴和对比下两个*运算
声明

Vector operator*(double n) const;           // 乘n
friend Vector operator*(double n, const Vector& a);

实现

    Vector Vector::operator*(double n) const{return Vector(x * n, n * y);}Vector operator*(double n, const Vector& a){return a * n;}

首先后面运算符重载这个是基于前者的，return a * n;这一句的实现其实是调用上一个a.operator(n)实现的，没有前一个就无法用后一个。对比的是前者他可以访问成员变量，而后者可以把Vector对象当作一个整体去乘。
6.std::ostream& operator<<(std::ostream &os, const Vector& v)
cout << 呢他是左结合一步步去做的，然后返回值是ostream&。
【注】最后这几行就是清理下输入行中多的字符。