电商专业学习嵌入式软件开发第五十七天

• C++第三天

今天主要讲了class和string以及构造函数和析构函数。最后老师带着我们完成了一个简单的游戏程序。老师留的作业还是不能正确的使用代码表达出来,最多只能将伪代码写出来。因为基础薄弱,今天已经正式向代理班主任曹老师申请到新班重修C基础,等新班正式开办就过去。

class:面向对象的类,可以把class看作struct

#include <iostream>
#include <string>
using namespace std;
//struct Student  //可以把class看作struct
class Student
{
private: //不能通过对象直接访问
    //可以通过共有接口间接访问
    string m_strId;
    string m_strName;
    float m_fScore;
public:  //通过对象直接访问
    string m_strFace;
    //成员函数
    void init(string id, string name, float score = 60.0f, string face = "cool");
    void info();
#if 0   
    //在类中定义的普通成员函数默认为inline函数
    void init(string id, string name, float score = 60.0f, string face = "cool")
    {
        m_strId = id;
        m_strName = name;
        m_fScore = score;
        m_strFace = face;
    }
    void info()
    {
        cout << "id:" << m_strId << " name:" << m_strName << " score:" << m_fScore << " face:" << m_strFace << endl;
    }
#endif
};
void Student::init(string id, string name, float score, string face)
{
    m_strId = id;
    m_strName = name;
    m_fScore = score;
    m_strFace = face;
}
void Student::info()
{
    cout << "id:" << m_strId << " name:" << m_strName << " score:" << m_fScore << " face:" << m_strFace << endl;
}
int main(void)
{
    Student stu;
    stu.m_strFace = "cool";
    cout << stu.m_strFace << endl;  

//  stu.strName = "刘德华";  // X
//  cout << stu.strName << endl;  // X

    stu.init("10101010", "刘德华", 101, "very cool");
    stu.info();

    return 0;
}

构造函数:

#include <iostream>
#include <string>
using namespace std;
class Student
{
private: //不能通过对象直接访问
    //可以通过共有接口间接访问
    string m_strId;
    string m_strName;
    float m_fScore;
public:  //通过对象直接访问
    //如果没有自定义构造函数则会自动生成一个无参的构造函数
    //假如自定义了有参构造函数则不会自动生成一个无参的构造函数
    //要想使用无参构造函数则必须自定义一个无参的构造函数
    //构造函数，在生成对象的时候自动调用，用来构造并初始化对象
#if 1
    Student()
    {
        cout << "Student()\n";
    }
#endif
    Student(string id, string name, float score = 60.0f, string face = "cool")
    {
        m_strId = id;
        m_strName = name;
        m_fScore = score;
        m_strFace = face;
        cout << "Student(...)\n";   
    }
    string m_strFace;
    void info()
    {
        cout << "id:" << m_strId << " name:" << m_strName << " score:" << m_fScore << " face:" << m_strFace << endl;
    }
};
int main(void)
{
    //如果对象后面没有实参则默认调用无参构造函数
    //Student stu;
    //如果对象后面有实参则调用参数匹配的构造函数
    //Student stu("1001", "刘德华", 89, "cool");
    //Student *pstu = new Student[32];
    //Student *p = (Student*)malloc(sizeof(Student));
    //stu.info();

    Student *p1 = new Student;
    Student *p2 = new Student("1001", "刘德华", 89, "cool");
    p1->info();
    p2->info();

    int a(90);
    cout << "a=" << a << endl;

    return 0;
}

string:在C++中使用string不会出现越界现象

#include <iostream>
#include <string>
using namespace std;

struct MyString
{
    char *data;
};

int main(void)
{
#if 0
    MyString str2;
    str2.data = new char[1024];
    sizeof(MyString);sizeof(str2);
    strlen(str2.data);
#endif
    string str = "HelloWorld";
    cout << str << endl;
    cout << sizeof(string) << ' ' << sizeof(str) << endl;
    //length,size:实际元素的个数
    cout << str.length() << endl;
    cout << str.size() << endl;
    //capacity:实际存储空间的大小
    cout << str.capacity() << endl;

    //空间分配策略：
    //若新增数据与原数据大小和小于等于原来的两倍,则新空间大小为原来的两倍    
    //str = str + "123";
    //10+3 <= 10*2 --> 20

    //若新增数据与原数据大小和大于原来的两倍,则新空间大小为新旧数据大小之和
    str = str + "12345678901";
    //10+11 > 10 *2  --> 21 
    cout << str.length() << endl;
    cout << str.capacity() << endl;

    //clear:只是将数据清除并不会释放空间
    str.clear();
    cout << str.length() << endl;
    cout << str.capacity() << endl;

    if (str.empty())
    {
        cout << "str is empty\n";
    }

    return 0;
}

析构函数:释放对象用的,一个文件中有一个就够了

#include <iostream>
#include <string.h>
using namespace std;

class MyString
{
public:
    //析构函数：在对象释放的时候自动被调用
    ~MyString()
    {
        if (NULL != m_data)
        {
            cout << m_data << ':';
            delete []m_data;
        }
        cout << "~MyString()\n";
    }
    MyString(const char *data = NULL)
    {
        if (NULL == data)
        {
            m_data = NULL;
        }
        else
        {
            int len = strlen(data);
            m_data = new char[len+1];
            if (NULL == m_data)
            {
                cout << "new failed\n";
                return;
            }
            strcpy(m_data, data);
            cout << m_data << ':';
        }
        cout << "MyString(...)\n";
    }
    const char *data()
    {
        return m_data;
    }
private:
    char *m_data;
};
void fun()
{
    MyString str("HelloWorld");
    cout << str.data() << endl;
}
int main(void)
{
    //fun();
    //cout << "$$$$$$$$$$$$$$$$\n";
    MyString s1("1111");
    MyString s2("2222");
    MyString s3("3333");
    MyString s4("4444");

    return 0;
}

homework:

写一个类,可以对文件做打开-读写-关闭的操作