'Chuncho' said:

So, is it correct to pass a value by const-reference in this situation? For what I read, it isn't, so I won't do it. But is it always incorrect, or there are some situations where is correct?

What if I pass the value just as const?

Like : …::operator+(const int)…

Would that be correct? Because I'm not supossed to change the value of that argument.

Thanks again.

You can do that, but using const in conjunction with pass-by-value only serves to make sure that the function doesn't internally modify the copy of that value. E.g. It means that you can look at the function signature and know that the value will be the same at the end of the function as it was at the top.
It enforces things upon itself rather than enforcing things on the caller.

So, is it correct to pass a value by const-reference in this situation? For what I read, it isn't, so I won't do it. But is it always incorrect, or there are some situations where is correct?