string類的實現：

class String
{
public:
	String(const char *str=null);
	String(const String &other);
	~String();
	String &operator =(const String &other);
	String &operator +(const String &other);
	bool operator ==(const String &other);
	int getLength();
private:
	char *m_data;
}
//普通建構函式
String::String(const char *str)
{
	if(str == null)
	{
		m_data = new char[1];
	*m_data = '\0';
	}
	else
	{
	int length = strlen(str);
	m_data = new char[length + 1];
	strcpy(m_data, str);
	}
}
//解構函式
String::~String()
{
	if(m_data)
	{
		delete[] m_data;
		m_data=0;
	}
}
//拷貝建構函式
String::String(const String &other)
{
	if(!other.m_data)
	{
		m_data=0;
	}
	m_data = new char[strlen(other.m_data)+1];
	strcpy(m_data, other.m_data);
}
//賦值函式
String& String::operator =(const String &other)
{
	if(this != &other)
	{
		delete[] m_data;
		if(!other.m_data)
		{
			m_data = 0;
		}
		else
		{
			m_data = new char[strlen(other.m_data)+1];
			strcpy(m_data, other.m_data);
		}
	}
	return *this;
}
//字串連線
String &String::operator+(const String &other)
{
	String newString;
	if(!other.m_data)
	{
		newString = *this;
	}
	else if(!m_data)
	{
		newString = other;
	}
	else
	{
		newString.m_data = new char[strlen(m_data)+strlen(other.m_data)+1];
		strcpy(newString.m_data, m_data);
		strcat(newString.m_data, other.m_data);
	}
	return newString;
}
//判斷相等
bool String::operator ==(const String &other)
{
	if(strlen(m_data) != strlen(other.m_data))
	{
		return false;
	}
	else
	{
		return strcmp(m_data, other.m_data)?false:true;
	}
}
//返回長度
int String::getLength()
{
	return strlen(m_data);
}