c++字符串类String的实现

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#include <string.h>

class String
{
private:
    char *str;
    int size;
public:
    String(char *s="")
    {
        size=strlen(s);
        str=new char[size+1];
        strcpy(str,s);
    }
    String(String &strObj)
    {
        size=strObj.getLength();
        str=new char[size+1];
        strcpy(str,strObj.str);
    }
    ~String()
    {
        delete []str;
    }

    const char *toString()
    {
        return str;
    }

    int getLength()
    {
        return strlen(str);
    }

    String &operator =(String &strObj)
    {
        delete []str;
        size=strObj.getLength();
        str=new char[size+1];
        strcpy(str,strObj.str);
        return *this;
    }

    String &operator =(char *s)
    {
        delete []str;
        size=strlen(s);
        str=new char[size+1];
        strcpy(str,s);
        return *this;
    }

    String operator +(String &strObj)
    {
        int tmpsize;
        char *tmpstr;
        tmpsize=getLength()+strObj.getLength();
        tmpstr=new char[tmpsize+1];
        strcpy(tmpstr,str);
        strcat(tmpstr,strObj.str);
        return String(tmpstr);
    }

    String operator +(char *s)
    {
        int tmpsize;
        char *tmpstr;
        tmpsize=getLength()+strlen(s);
        tmpstr=new char[tmpsize+1];
        strcpy(tmpstr,str);
        strcat(tmpstr,s);
        return String(tmpstr);
    }

    friend String operator +(char *s,String &strObj)
    {
        int tmpsize;
        char *tmpstr;
        tmpsize=strlen(s)+strObj.getLength();
        tmpstr=new char[tmpsize+1];
        strcpy(tmpstr,s);
        strcat(tmpstr,strObj.str);
        return String(tmpstr);
    }

    String &operator +=(String &strObj)
    {
        *this=*this+strObj;
        return *this;
    }

    String &operator +=(char *s)
    {
        *this=*this+s;
        return *this;
    }

    String &toUpper()
    {
        for(int i=0;i<=size;i++)
{
     if(*(str+i)>='a' && *(str+i)<='z')
     {
                 *(str+i)-=32;
     }
        }
        return *this;
    }

    String &toLower()
    {
        for(int i=0;i<=size;i++)
        {
            if(*(str+i)>='A' && *(str+i)<='Z')
            {
                *(str+i)+=32;
            }
        }
        return *this;
    }

    char charAt(int i)
    {
        if(i<0 || i>getLength())
        {
            return ('/0');
        }
        else
        {
            return *(str+i);
        }
    }

    char operator [](int i)
    {
        return charAt(i);
    }

    int indexOf(char c)
    {
        for(int i=0;i<=size;i++)
        {
            if(*(str+i)==c){return i;}
        }
        return -1;
    }
};