Why should I use const and & while declaring class in function?

Question

I don't get the point at all. I just know that; if I don't write like this

void samplefunctionname(const randomclass &classmember) {
  //code goes here...
}

the function will copy every information in classmember to itself. So it's not good for memory. Maybe I just want to write classmember's variables that are chosen by me, but if I do not write const and & it will just write all of the information.

But what actually these guys are doing ? Why class should be const etc.

Answer 1

Why should I use const and & while declaring class in function?

In order to avoid copying the whole class. When you use const&, you pass the object by reference, which doesn't copy any data at all.