编写类String的构造/析构函数，赋值操作符

class String
{
public:
	String(const char * str = NULL);	//普通构造函数
	String(const String &other);		//复制构造函数
	String & operator= (const String &other);	//赋值操作符
	~String();							//析构函数
private:
	char *m_data;
};

String::String(const char *str)
{
	if (str == NULL)
	{
		m_data = new char[1];
		m_data[1] = '\0';
	}
	else
	{
		int lenght = strlen(str);
		m_data = new char[lenght + 1];
		strcpy(m_data, str);
	}
}

String::String(const String &other)
{
	int lenght = strlen(other.m_data);
	m_data = new char[lenght + 1];
	strcpy(m_data, other.m_data);
}


String & String::operator= (const String &other)
{
	if(&other == this)	//证同测试
		return *this;
	delete [] m_data;
	int lenght = strlen(other.m_data);
	m_data = new char[lenght + 1];
	strcpy(m_data, other.m_data);
	return *this;		//第一次写的时候忘记返回*this
}

String & String::operator= (const String &other)
{
	String temp(other);
//	delete [] m_data;
	swap(m_data, other.m_data);
	return *this;	//又忘记返回*this
}

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