构造函数

 下面不太完美，if(m_data)delete [] m_data;if(m_data==NULL)return *this;

class String
{
    public:
    String(const char *str=NULL);
    String(const String& other);
    String& operator=(const String& other);
    ~String();
    private:
    char *m_data;
};
String::String(const char* str)
{
    if(str==NULL)
    {
        m_data=new char[1];
        *m_data='';
    }
    else
    {
        int length=strlen(sizeof(str));
        m_data=new char[length+1];
        strcpy(m_data, str);
    }
}
String::String(const String &other)
{
    int length=strlen(other.m_data);
    m_data=new char[length+1];
    strcpy(m_data, other.m_data);
}
String& operator=(const String& other)
{
    if(this==&other)
        return *this;
    delete []m_data;
    int length = strlen(other.m_data);
    m_data=new char[length+1];
    strcpy(m_data, other.m_data);
    return *this;
}
~String()
{
    delete []m_data;
}