赋值符重载有误

[duguxianzhi]

class String
{
public:
	String(const char *str=NULL);
	String(const String &another);
	~String();
	String & operate =(const String &rhs);
private:
	char *m_data;
}

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

String::String(const String &another)
{
	//m_data is a private member
	m_data = new char[strlen(another.m_data)+1];
	strcpy(m_data,another.m_data);
}

String &String::operate =(const String &rhs)
{
	if(this==&rhs)
	{
		return *this;
	}
	delete []m_data;//delete original object's m_data 

	m_data= new char[strlen(rhs.m_data)+1];
	strcpy(m_data,rhs.m_data);
	return *this;
}

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