void* memcpy1(void *dst,const void *src,int count){

	assert(dst != NULL && src!= NULL && count>=0);
	void *temp = dst;
	char *pdst = (char*)dst;
	char *psrc = (char*)src;
	if(pdst>=psrc && pdst <= psrc+count-1)
	{
		pdst = pdst + count -1;
		psrc = psrc + count -1;
		while (count--)
			*pdst-- = *psrc--;
	}
	else
	{
		while(count--)
			*pdst++ = *psrc++;
	}
	return temp;
}

char* strcpy1(char *dst,const char *src){

	assert(dst != NULL && src!= NULL);
	char *temp = dst;
	memcpy1(dst,src,strlen(src)+1);
	return temp;
}

2 string 类的部分实现

class MyString{
public:
	friend ostream& operator<<(ostream&,const MyString&);
	MyString(const char *str = NULL){
		if(NULL == str)
			m_data = 0;
		else
		{
			m_data = new char[strlen(str)+1];
			strcpy(m_data,str);
		}
		
	}
	MyString(const MyString & string)
	{
		if(string.m_data == 0 || string.m_data == NULL)
			m_data = 0;
		else
		{
			m_data = new char[strlen(string.m_data)+1];
			strcpy(m_data,string.m_data);
		}
	}
	MyString& operator=(const MyString &string){
	
		if(this != &string)
		{
			char *temp = m_data;
			if(string.m_data==0 || string.m_data == NULL)
				m_data = 0;
			else
			{
				m_data = new char[strlen(string.m_data)+1];
				strcpy(m_data,string.m_data);
			}
			delete []temp;
		}
	
		return *this;
	}
private:
	char *m_data;
};