C++stringクラスの簡単な実現

#include <string.h>
#include <iostream>
using namespace std;

class String
{
public:
	String(const char* str = NULL);
	String(const String &other);
	String & operator=(const String &other);
	~String();

protected:
private:
	char* m_data;
};
String::String(const char *str )
{
	if (str == NULL)
	{
	m_data = new char[1];
	*m_data = '\0';
	} 
	else
	{
		m_data = new char[strlen(str)+1];
		strcpy(m_data,str);
	}
}
String::~String()
{
	delete m_data;
}
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;
	}
	delete m_data;
	int len = strlen(other.m_data);
	m_data = new char[len+1];
	strcpy(m_data,other.m_data);
	return *this;
}
void test()
{
	String s1 = "hello";  //      
	String s2 ;           //      
	s2 = s1;              //        
	String s3(s1);        //        
}                         //        
int main()
{
	
test();
return 0;

}