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C++之内存模型与名称空间

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C++之内存模型与名称空间……

多文件编译

  • 头文件中常包含的内容：

    • 使用#define或const定义的符号常量
    • 函数原型
    • 结构声明
    • 类声明
    • 模板声明
    • 内联函数
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    #ifndef __COORDIN_H__
    #define __COORDIN_H__

    #include<iostream>
    #include<cmath>

    struct polar
    {
        double distance;
        double angle;
    };

    struct rect
    {
        double x;
        double y;
    };

    polar rect_to_polar(rect xypos);
    void show_polar(polar pplace);

    #endif

  • VScode中如何对多个CPP文件进行编译

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    g++ 文件名1.cpp 文件名2.cpp -o 重命名
    .\重命名

    CPP文件：

    • coordin.cpp
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    #include"coordin.h"

    using namespace std;

    polar rect_to_polar(rect xypos)
    {
        polar pplace;
        pplace.distance=sqrt(xypos.x*xypos.x+xypos.y*xypos.y);
        pplace.angle=atan2(xypos.y,xypos.x);//返回的是弧度制
        return pplace;
    }

    void show_polar(polar pplace)
    {
        const double Rad_to_deg=57.29577951;
        cout<<"Distance="<<pplace.distance<<endl;
        //将弧度制转化为角度制
        cout<<"Angle="<<pplace.angle*Rad_to_deg<<" degree"<<endl;
    }
    • main1.cpp
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    #include<iostream>//编译器自带的用<>
    #include"coordin.h"//用户自己写的用""

    using namespace std;

    int main(void)
    {
        rect rplace;
        polar pplace;
        cout<<"Please enter x and y(q for quit):";
        while(cin>>rplace.x>>rplace.y)
        {
            pplace=rect_to_polar(rplace);
            show_polar(pplace);
            cout<<"Please enter x and y(q for quit):";
        }
        return 0;
    }

    image-20230105135539339

Extern与::的使用

  • extern访问别的源文件中的全局变量，通常在头文件中声明

  • ::强制访问全局变量，而不访问离得最近的局部变量

  • 头文件：support.h

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    #ifndef __SUPPORT_H__
    #define __SUPPORT_H__

    #include<iostream>

    extern double warming;

    void update(double dt);
    void local(void);

    #endif

  • 源文件：

    • support.cpp
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    #include"support.h"

    using namespace std;

    void update(double dt)
    {
        warming+=dt;
        cout<<"Updating global warming to "<<warming<<" degrees"<<endl;
    }

    void local(void)
    {
        double warming=0.8;
        cout<<"Local warming="<<warming<<" degrees"<<endl;
        cout<<"Global warming in Local ="<<::warming<<" degrees"<<endl;
    }
    • main.cpp
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    #include<iostream>
    #include"support.h"

    using namespace std;

    double warming=0.3;

    int main(void)
    {
        cout<<"Global warming is "<<warming<<" degrees"<<endl;
        update(0.1);
        cout<<"Global warming is "<<warming<<" degrees"<<endl;
        local();
        cout<<"Global warming is "<<warming<<" degrees"<<endl;
        
        return 0;
    }

  • 结果如下：

    image-20230105142522719

Static的使用

  • 当对全局变量使用Static时，表示此全局变量只用于当前文件，不能被其他文件访问

  • 在函数中使用Static时：只初始化一次，且在函数调用完后变量不会消失

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    #include<iostream>

    using namespace std;

    const int ArSize=10;

    void strcount(const char *str);

    int main(void)
    {
        char input[ArSize];
        char next;

        cout<<"Enter a line:";
        cin.get(input,ArSize);//实际只会读取ArSize-1个char
        while(cin)
        {
            cin.get(next);//读取缓冲区的换行符
            while(next!='\n')//如果读取的不是换行符，说明读取的字符超出了Arsize-1的范围
            {
                cin.get(next);//则把超出范围的字符全部读掉，直到读到换行符
            }
            strcount(input);
            cout<<"Enter next line(empty line to quit):";
            cin.get(input,ArSize);        
        }
        cout<<"Test Over!"<<endl;
        return 0;
    }

    void strcount(const char *str)
    {
        static int total=0;//static在函数中定义的变量，只初始化一次，且在函数调用完后变量不会消失
        int count=0;

        while(*str++)//++的优先级更高,且先使用后自加，即先*str，再str+1
        {
            count++;
        }
        total+=count;
        cout<<count<<" characters"<<endl;
        cout<<total<<" characters total"<<endl;
    }

    结果如下：

    image-20230105153222956

定位new运算符

  • 首先需要包含头文件new，它提供了这种版本的new运算符的原型#include<new>

  • 与常规的new使用相比，在new后面用（起始地址）指定new出空间的起始地址

  • 定位new不能使用delete删除，只有常规new需要使用delete删除

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#include <iostream>
#include <new>

using namespace std;

const int BUF = 512;
const int N = 5;
char buffer[BUF];

int main(void)
{
	double *pd1, *pd2;
	int i;

	cout << "Calling new and placement new: " << endl;
	pd1 = new double[N];
	pd2 = new(buffer) double[N];
	for(i = 0; i < N; i++)
		pd2[i] = pd1[i] = 1000 + 20.0 * i;
	cout << "pd1 = " << pd1 << ", buffer = " << (void *)buffer << endl;
	for(i = 0; i < N; i++)
	{
		cout << pd1[i] << " at " << &pd1[i] << "; ";
		cout << pd2[i] << " at " << &pd2[i] << endl;
	}

	cout << "\nCalling new and placement new a second time: " << endl;
	double *pd3, *pd4;
	pd3 = new double[N];
	pd4 = new(buffer) double[N];
	for(i = 0; i < N; i++)
		pd4[i] = pd3[i] = 1000 + 40.0 * i;
	for(i = 0; i < N; i++)
	{
		cout << pd3[i] << " at " << &pd3[i] << "; ";
		cout << pd4[i] << " at " << &pd4[i] << endl;
	}

	cout << "\nCalling new and placement new a third time: " << endl;
	delete[] pd1;
	pd1 = new double[N];
	pd2 = new(buffer + N*sizeof(double)) double[N];
	for(i = 0; i < N; i++)
		pd2[i] = pd1[i] = 1000 + 60.0 * i;
	for(i = 0; i < N; i++)
	{
		cout << pd1[i] << " at " << &pd1[i] << "; ";
		cout << pd2[i] << " at " << &pd2[i] << endl;
	}

	delete[] pd1;
	delete[] pd3;

	return 0;
}

结果如下：

image-20230106231745122

名称空间

  • 使用新的关键字namespace创建名称空间
  • using 名称空间的名字::使用的名称空间中的某个内容//会将同名变量覆盖
  • using namespace 名称空间的名字//此时引用区域可以使用该名称空间的所有内容，隐藏有变量 名称空间名字::变量
  • namesp.h
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#include<iostream>
#include<string>

namespace pers
{
    using std::cout;//使用名称空间std中某个内容
    using std::cin;
    using std::endl;

    struct Person
    {
        std::string fname;
        std::string lname;
    };
    
    void getPerson(Person &rp);
    void showPerson(const Person &rp);
}

namespace debts
{
    using namespace pers;//可以使用pers名称空间的所有内容

    struct Debt
    {
        Person name;
        double amount;
    };

    void getDebt(Debt &rd);
    void showDebt(const Debt &rd);
    double sumDebts(const Debt ar[],int n);
}
  • namesp.cpp
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#include"namesp.h"

namespace pers
{
    void getPerson(Person &rp)
    {
        cout<<"Enter first name:";
        cin>>rp.fname;
        cout<<"Enter last name:";
        cin>>rp.lname;
    }

    void showPerson(const Person &rp)
    {
        cout<<"The last and first name is:"<<rp.lname<<" "<<rp.fname<<endl;
    }
}

namespace debts
{
    void getDebt(Debt &rd)
    {
        getPerson(rd.name);
        cout<<"Enter debt:";
        cin>>rd.amount;
    }

    void showDebt(const Debt &rd)
    {
        showPerson(rd.name);
        cout<<"The debt money is:$"<<rd.amount<<endl;
    }

    double sumDebts(const Debt ar[],int n)
    {
        double total=0;
        for(int i=0;i<n;i++)
        {
            total+=ar[i].amount;
        }
        return total;
    }
}
  • main.cpp
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#include<iostream>
#include"namesp.h"

int main(void)
{
    using debts::Debt;
    using debts::showDebt;

    Debt golf={{"Micheal","Jordan"},120.0};

    showDebt(golf);
    
    return 0;
}

结果如下：

image-20230109215711229

练习

1.多文件编译

  • 存储高尔夫选手的姓名及成绩并显示，且能修改

  • golf.h

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    #include<iostream>
    #include<cstring>

    using namespace std;

    const int Len = 40;

    struct golf
    {
    	char fullname[Len];
    	int handicap;
    };

    void setgolf(golf &g, const char *name, int hc);
    int setgolf(golf &g);
    void handicap(golf &g, int hc);
    void showgolf(const golf &g);

  • golf.cpp

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    #include "golf.h"

    void setgolf(golf &g, const char *name, int hc)
    {
    	strcpy(g.fullname, name);
    	g.handicap = hc;
    }

    int setgolf(golf &g)
    {
    	int temp = 1;

    	cout << "The fullname is: ";
    	cin.getline(g.fullname, Len);

    	if(strcmp(g.fullname, "") == 0)
    	{
    		temp = 0;
    		return temp;
    	}
    	else
    	{
    		cout << "The handicap is: ";
    		cin >> g.handicap;
    		cin.get();
    		return temp;
    	}
    }

    void handicap(golf &g, int hc)
    {
    	g.handicap = hc;
    }

    void showgolf(const golf &g)
    {
    	cout << "The fullname is: " << g.fullname << endl;
    	cout << "The handicap is: " << g.handicap << endl;
    }

  • main.c

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    #include <iostream>
    #include "golf.h"

    using namespace std;

    const int SIZE = 2;

    int main(void)
    {
    	golf g[SIZE];
    	int count = 0;

    	cout << "Please enter the information of golf players: " << endl;
    	while((count < SIZE) && (setgolf(g[count])))
    	{
    		cout << "Please enter next player: " << endl;
    		count++;
    	}

    	cout << "\nShow all golf players informations: " << endl;
    	for(int i = 0; i < count; i++)
    		showgolf(g[i]);

    	cout << "\nReset all the players information: " << endl;
    	for(int i = 0; i < count; i++)
    	{
    		handicap(g[i], 90);
    		showgolf(g[i]);
    	}

    	return 0;
    }

  • 结果如下：

    image-20230201230346528

2.定位new运算符

  • 体会普通new与定位new的区别
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#include <iostream>
#include <cstring>
#include <new>

using namespace std;

struct chaff
{
	char dross[20];
	int slag;
};

const int BUF = 512;
char buffer[BUF];

void show(const chaff &p);

int main(void)
{
	char dross[20];
	int slag;

	chaff *pd1 = new chaff[2];
	chaff *pd2 = new(buffer) chaff[2];

	for(int i = 0; i < 2; i++)
	{
		cout << "#" << i+1 << ": " << endl;
		cout << "Enter the dross: ";
		cin.getline(dross, 20);
		cout << "Enter the slag: ";
		cin >> slag;
		cin.get();

		strcpy(pd1[i].dross, dross);
		strcpy(pd2[i].dross, dross);

		pd1[i].slag = pd2[i].slag = slag;
	}

	for(int i = 0 ; i < 2; i++)
	{
		show(pd1[i]);
		show(pd2[i]);
	}

    cout<<"-----------------------"<<endl;
    cout<<"The address of buffer is:"<<&buffer<<endl;
    cout<<"The address of pd2 point is:"<<pd2<<endl;

	delete[] pd1;

	return 0;
}

void show(const chaff &p)
{
    cout<<"----------------------------"<<endl;
    cout<<"The address is: "<<&p<<endl;
	cout << "The dross is: " << p.dross << endl;
	cout << "The slag is: " << p.slag << endl;
}

结果如下：

image-20230201234040224

3.名称空间的使用

  • 利用名称空间SALES求商品的平均值、最大值与最小值

  • p3.h

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    #ifndef __P3_H__
    #define __P3_H__

    #include <iostream>

    namespace SALES
    {
    	const int QUARTERS = 4;
    	struct Sales
    	{
    		double sales[QUARTERS];
    		double average;
    		double max;
    		double min;
    	};

    	void setSales(Sales &s, const double ar[], int n);
    	void setSales(Sales &s);
    	void showSales(const Sales &s);
    }

    #endif

  • p3.cpp

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    #include "p3.h"

    using namespace std;

    namespace SALES
    {
    	void setSales(Sales &s, const double ar[], int n)
    	{
    		double total = 0.0;
    		int i;

    		for(i = 0; i < n && i < 4; i++)
    		{
    			s.sales[i] = ar[i];
    			total += s.sales[i];
    		}

    		s.average = total / i;
    		s.max = s.sales[0];
    		s.min = s.sales[0];

    		for(int k = 1; k < i; k++)
    		{
    			s.max = (s.max > s.sales[k]) ? s.max : s.sales[k];
    			s.min = (s.min < s.sales[k]) ? s.min : s.sales[k];
    		}

    		if(n < 4)
    		{
    			for(int k = n; k < 4; k++)
    				s.sales[k] = 0;
    		}
    	}

    	void setSales(Sales &s)
    	{
    		double total = 0.0;

    		cout << "Enter 4 sales quarters: " << endl;
    		for(int i = 0; i < QUARTERS; i++)
    		{
    			cout << "#" << i+1 << ": ";
    			cin >> s.sales[i];
    			total += s.sales[i];

    			if(i == 0)
    			{
    				s.max = s.sales[i];
    				s.min = s.sales[i];
    			}
    			else
    			{
    				s.max = (s.max > s.sales[i]) ? s.max : s.sales[i];
    				s.min = (s.min < s.sales[i]) ? s.min : s.sales[i];
    			}
    		}
    		s.average = total / QUARTERS;
    	}

    	void showSales(const Sales &s)
    	{
    		cout << "Sales of 4 quarters: " << endl;

    		for(int i = 0; i < QUARTERS;i ++)
    			cout << s.sales[i] << " ";
    		cout << endl;

    		cout << "Average = " << s.average << endl;
    		cout << "Max = " << s.max << endl;
    		cout << "Min = " << s.min << endl; 
    	}
    }

  • main.cpp

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    #include <iostream>
    #include "p3.h"

    using namespace std;

    int main(void)
    {
    	using namespace SALES;

    	double ar[4] = {11.1, 22.2, 33.3, 44.4};
    	Sales sl;

    	setSales(sl, ar, 3);
    	showSales(sl);

    	setSales(sl);
    	showSales(sl);

    	return 0;
    }

  • 结果如下：

    image-20230202152638213

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