编写类String的构造函数、析构函数和赋值函数

class String
{
public:
	String(const char *str = NULL);	//默认构造函数
	String(const String &other);	//赋值构造函数
	~String(void);					
	String & operator =(const String & other);	//赋值函数
private:
	char *m_data;
};

String::String(const char *str )
{
	if ( str != NULL )
	{
		int len = strlen(str);
		m_data = new char[len+1];
		strcpy(m_data,str);
	}
	else	//这部分不要拉掉
	{
		m_data = new char[1];
		*m_data = '\0';
	}
}

String::String(const String &other)	//赋值
{
	int len = sizeof(other.m_data);
	m_data = new char[len+1];
	strcpy(m_data, other.m_data);
}

String & String::operator =(const String &other)
{
	//1：检测是否是自赋值
	if ( this == &other )
		return *this;

	//2：释放原有的内存资源
	delete [] m_data;

	//3：分配新的内存资源，并复制内容
	int len = strlen(other.m_data);
	m_data = new char[len+1];
	strcpy(m_data, other.m_data);

	//4：返回本对象的引用
	return *this;
}

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