如果两段内存重叠，用memcpy函数可能会导致行为未定义

如果两段内存重叠，用memcpy函数可能会导致行为未定义，改进：

void* memmove(void* str1,const void* str2,size_t n)
{
	char* pStr1= (char*) str1;
	const char* pStr2=(const char*)str2;
	if  (pStr1 < pStr2 )
	{
		for(size_t i=0;i!=n;++i)
		{
			*(pStr1++)=*(pStr2++);
		}
	}
	else
	{
		pStr1+=n-1;
		pStr2+=n-1;
		for(size_t i=0;i!=n;++i)
		{
			*(pStr1--)=*(pStr2--);
		}
	}
	return (str1);
}
int main()
{
	// 内存重叠
	char c1[] = "hello world";
	memmove(c1+3, c1, 8);
	cout<<"memmove result:   "<<c1<<endl; //输出结果：helhello wo

	memcpy(c1+3,c1,8);
	//cout<<"memcpy:   "<<c1<<endl;  // <span style="line-height: 28.7999992370605px; font-family: arial, STHeiti, 'Microsoft YaHei', 宋体;">输出</span><span style="line-height: 28.7999992370605px; font-family: arial, STHeiti, 'Microsoft YaHei', 宋体;">结果：</span><span style="line-height: 28.7999992370605px; font-family: arial, STHeiti, 'Microsoft YaHei', 宋体;">helhelhell0</span>

	// 内存不重叠
	char c2[] = "hello world";
	char c3[] = "love you";
	memmove(c2,c3,8);
	//cout<<"memmove result:"<<c2<<endl;//<span style="line-height: 28.7999992370605px; font-family: arial, STHeiti, 'Microsoft YaHei', 宋体;">输出结果：</span><span style="line-height: 28.7999992370605px; font-family: arial, STHeiti, 'Microsoft YaHei', 宋体;">love yourld</span>

	memcpy(c2,c3,8);
	cout<<"memcpy:   "<<c2<<endl; //输出结果：love yourld

	memcpy(c1+3,c1,8);

}