C++类默认的四个函数，及手工实现String类

#include 
#include 
using namespace std;

class String
{
public:
    //构造函数
    String(const char *str = NULL);

    //析构函数
    ~String(void);

    //复制构造函数
    String(const String &other);

    //赋值函数
    String & operator=(const String &other);


private:
    // used to save the string
    char *m_data;
};


String::~String(void)
{
    delete [] m_data;
}

String::String(const char * str)
{
    if(str == NULL)
    {
        m_data = new char(1);
        *m_data = '\0';
    }
    else
    {
        size_t length_of_string = strlen(str);
        m_data = new char(length_of_string + 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 & String::operator=(const String &other)
{
    if (this == &other)
    {
        return *this;
    }
    //释放原有的内存资源
    delete [] m_data;
    // 分配新的内存资源
    size_t length = strlen(other.m_data);
    m_data = new char(length + 1);
    strcpy(m_data, other.m_data);
}


int main(int argc, char *argv[])
{
    String str1("hello");
    String str2(str1);
    String str3;
    str3 = str1;
    return 0;
}