Why does the C + + copy constructor parameter have to be a reference? Must the assignment constructor argument also be a reference?

Prior to writing the copy constructor, the parameter is referenced and not passed for the value, just to reduce the memory copy. Today, however, I see an article that finds that it is wrong to understand the parameters of the copy construction. The argument is a reference and is not passed as a value to prevent infinite recursion of the copy constructor, resulting in a stack overflow.

Let's look at an example:

2. class test
4. {
6. public:
8. test()
10. {
12. cout << "constructor with argument\n";
14. }
16. ~test()
18. {
22. }
24. test(test& t)
26. {
28. cout << "copy constructor\n";
30. }
32. test&operator=(const test&e)
34. {
36. cout << "assignment operator\n";
38. return *this;
40. }
42. };
44. int _tmain(int argc, _TCHAR* argv[])
46. {
48. test ort;
50. test a(ort);
52. test b = ort ;
54. a = b;
56. return 0;
58. }

Output: If you can understand this knowledge. Let's explain why value passing is infinitely recursive! If the copy constructor is this:

1. test(test t);

We call

2. test ort;
4. test a(ort）; --> test.a(test t=ort)==test.a(test t(ort))
6. -->test.a(test t(test t = ort))
8. ==test.a(test t(test t(ort)))
10. -->test.a(test t(test t(test t=ort)))
12. ...
14.     就这样会一直无限递归下去。

Here we also understand why the parameters of a copy constructor must be a reference and cannot be passed for a value. Next, we test the parameters of the assignment constructor, and if we change its parameters to a value pass, do a test.

2. class test
4. {
6. public:
8. test()
10. {
12. cout << "constructor with argument\n";
14. }
16. ~test()
18. {
22. }
24. test(test& t)
26. {
28. cout << "copy constructor\n";
30. }
32. test&operator=(test e)
34. {
36. cout << "assignment operator\n";
38. return *this;
40. }
42. };
44. int _tmain(int argc, _TCHAR* argv[])
46. {
48. test ort;
50. test a(ort);
52. test b = ort ;
54. a = b;
56. return 0;
58. }

Output: Assignment constructor If it is passed as a value, it is only one copy more, and does not have infinite recursion.Summary: The parameters of the copy constructor must be references. An assignment constructor parameter can be either a reference or a value pass, and a value pass will be copied more than once. Therefore, it is recommended that the assignment constructor also be written as a reference type. (Ckk see just now I understand still have deviation: Left and right value is not the key, reduce copy number increase assignment efficiency is the focus)

Why does the C + + copy constructor parameter have to be a reference? Must the assignment constructor argument also be a reference?