When you do i += integer , you change the value of "i" only not "integer" , the value of integer stays as it is

I cant unerstand how it work,does the value of the integer value preserve the value of 5?
it should increase incorrectly every time, and because of this, values will be lost, isn't it.
5+5=10 = integer
10 + 10 = 20 = integer
and so on

It looks strange

Oh, thank you ;)

I think, that in general, .at() is safer than [] in terms of exceptions throwing.

Is there a difference between using '.at' and '[]'?

If you don't want to copy the parameter (time and memory constraints), but want to use it without altering it, you pass it by const reference.