Passing vectors by reference

Question

If I have a vector of objects in one class which I want to change in another, I would try and pass all the information by reference.

What exactly do I need to pass by reference though? The vector? The objects? Both?

Essentially what I'm asking is: What is the difference between these?

vector&<object> blah; // A reference to a vector of objects?

vector<object&> blah; // A vector of references to objects?

vector&<object&> blah; // A reference to a vector of references to objects???

I'm not actually sure how referencing of array like containers work. Are these legal?

What do you need to change exactly? If you need to change one object, you pass it by reference. If you need to change the container (e.g., add or delete an object), you pass the container. — Vlad, Nov 01 '11 at 12:16
I'll recommend [a good introductory C++ book](http://stackoverflow.com/q/388242/46642). — R. Martinho Fernandes, Nov 01 '11 at 12:19
@R.MartinhoFernandes thanks, I've bought some. I think the one I had was really crap actually. — Dollarslice, Nov 02 '11 at 12:31

score 55 · Accepted Answer · edited Sep 05 '15 at 12:50

55

vector&<object> is a syntax error. vector<object&> is invalid, because a vector's value type must be assignable. vector&<object&> blah is a syntax error.

A reference to a vector is vector<T>&.

edited Sep 05 '15 at 12:50

Peter Mortensen

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answered Nov 01 '11 at 12:16

Cat Plus Plus

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ok so you just store a reference to a vector... and when accessing an element of said vector by index you're actually accessing the original object? – Dollarslice Nov 01 '11 at 16:08

score 4 · Answer 2 · answered Nov 01 '11 at 12:17

You cannot have a vector of references. Vector elements must be copyable and assignable, which references are not. So only the first option is actually an option, but it's spelled std::vector<Object> &.

Note that v[1] already returns a reference to the second element, so you can happily pass individual elements by reference.

_{It is possible to have a vector of reference-wrappers a la std::ref, but if you don't know what that is, you probably shouldn't be using it at this point.}

score 2 · Answer 3 · edited May 13 '19 at 12:14

Another option is to pass around iterators instead of containers. This is the approach the standard library takes in <algorithm>. They are slightly more verbose at the calling site, but they have the advantage that they work for parts of a collection as well as complete collections and decouples the algorithm from the container.

Lastly, of course, it's worth while checking you know your algorithms as there may already be one that does what you want.

Passing vectors by reference

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