C++ 为什么拷贝构造函数参数必须为引用？赋值构造函数参数也必须为引用吗？

class test
{
public:
	test()
	{
		cout << "constructor with argument\n"; 
	}
	~test()
	{
	}
	test(test& t)
	{
	 cout << "copy constructor\n"; 
	}
	test&operator=(const test&e)
	{
		cout << "assignment operator\n";  
		return *this;
	}
};
int _tmain(int argc, _TCHAR* argv[])
{
	test ort;
	test a(ort);
	test b = ort ;
	 a = b;
	return 0;
}

test ort;
test a(ort）; --> test.a(test t=ort)==test.a(test t(ort))
              -->test.a(test t(test t = ort))
                 ==test.a(test t(test t(ort)))
              -->test.a(test t(test t(test t=ort)))
               ...
    就这样会一直无限递归下去。

class test
{
public:
	test()
	{
		cout << "constructor with argument\n"; 
	}
	~test()
	{
	}
	test(test& t)
	{
	 cout << "copy constructor\n"; 
	}
	test&operator=(test e)
	{
		cout << "assignment operator\n";  
		return *this;
	}
};
int _tmain(int argc, _TCHAR* argv[])
{
	test ort;
	test a(ort);
	test b = ort ;
	 a = b;
	return 0;
}