Passing Constant Class as argument in function

Question

what is the difference between

void A(const class1 a);

and

void A(const class1 &a);

in C++.

I am not able to differentiate these two.

Sounds like you could use a [good C++ book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — NathanOliver, Mar 22 '17 at 15:34
Well one declaration have an `&` in it, the other does not. That makes a *very* big difference, one that any beginners book would tell you about. — Some programmer dude, Mar 22 '17 at 15:37
Have you done any research on this? What has it told you and where are you stuck with its results? — Angew is no longer proud of SO, Mar 22 '17 at 15:38
Start your research here: http://stackoverflow.com/questions/6877052/use-of-the-operator-in-c-function-signatures — NathanOliver, Mar 22 '17 at 15:38
You shouldn't be writing C++ until you understand the difference between the two, and the way to learn the difference is from [a good C++ book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — Rob K, Mar 22 '17 at 16:40

Frank Puffer · Accepted Answer · 2017-03-22T16:05:55.470

3

The main difference is that

A(const class1 a);

will create a local copy of a inside A which can be expensive (from both memory and computational point of view) while

A(const class1 &a);

will not create a copy.

Therefore passing the reference (2nd version) is usually the preferred way.

edited Mar 22 '17 at 16:05

answered Mar 22 '17 at 15:38

Frank Puffer

1

the first form is not necessarily more expensive all depends of the size of class1 if it is bigger than a reference so what you are saying is true otherwise it is not. – Venom Mar 22 '17 at 15:49
@basslo: That's correct and that's why I wrote "can be expensive". However in my experience it is unlikely that copying a class is less expensive (from memory consumption and computation point of view) than copying a reference. It does not only depend on the size of the class but mainly on what the copy constructor does. – Frank Puffer Mar 22 '17 at 15:55
Generally if `sizeof(type) <= sizeof(void*)` then passing by reference or value doesn't matter. – NathanOliver Mar 22 '17 at 15:57
@NathanOliver: What if the copy constructor does some complicated stuff? (I am not saying that a copy constructor *should* do this.) – Frank Puffer Mar 22 '17 at 16:02
@FrankPuffer There are certainly exceptions. Almost anything that stores a pointer should be passed by reference. – NathanOliver Mar 22 '17 at 16:26

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