c++简单实现string类

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

class mystring
{
public:
    mystring(char *s);
    mystring();
    ~mystring();//虚构函数用来释放动态内存
    void addstring(char *s);//连接两个字符串
    void stringup(char *s);//字符串大写换小写
    void change(char *s);//改变字符串
    void show();//输出字符串
private:
    int len;
    char *p;
};

mystring::mystring(char *s)
{
    len=strlen(s);
    p=new char [len+1];
    strcpy(p,s);
}

mystring::mystring()
{
    len=0;
    p=new char [len+1];
}

mystring::~mystring()
{
    delete []p;
    len=0;
}

void mystring::addstring(char *s)
{
    int tlen=strlen(s);
    char *temp=new char [len+1];

    strcpy(temp,p);
    
    delete []p;
    p=new char [len+tlen+1];//p的动态内存重新申请
    strcpy(p,temp);
    strcat(p,s);

    delete temp;//释放临时动态内存
}

void mystring::stringup(char *s)
{
    int i;
    if(p!=NULL)
        delete []p;
    len=strlen(s);
    p=new char [len+1];
    strcpy(p,s);

    for(i=0;i<len;i++)
    {
        if(p[i]>='a'&&p[i]<='z')
        {
            //s[i]=s[i]-32;b不行
            p[i]=s[i]-32;

        }
    }
}

void mystring::change(char *s)
{
    delete []p;
    len=strlen(s);
    p=new char [len+1];
    strcpy(p,s);
}

void mystring::show()
{
    cout<<p<<endl;
}

void main()
{
    mystring ms("awdseswsa");
    ms.show();

    ms.addstring("123");
    ms.show();

    ms.change("dedede");
    ms.show();

    ms.stringup("abc");
    ms.show();
}