面試碰到的題，答案來自網路搜尋。

class myString{
private:
	char* m_data;

public:
	myString(const char *str=NULL);
	myString(const myString &other);
	~myString();
	myString operator=(const myString &other);

	showString();
};

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

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

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

myString myString::operator=(const myString &other)
{
	if (this == &other)
	{
		return *this;
	}
	else
	{
		delete []m_data;
		int len = strlen(other.m_data);
		m_data = new char[len+1];
		strcpy(m_data, other.m_data);
		return *this;
	}
}

myString::showString(){
	cout<<m_data<<endl;
}