C++ Noob question

cocos2d-x C++
1

I have some code

for (auto &touch : touches)
{
    auto start = touch->getStartLocation();
}

where touches is

const std::vector<Touch*> &touches

All is well with the world.

But I accidentally missed off the &, so had
for (auto touch : touches)
{
auto start = touch->getStartLocation();
}

and everything still works.

Can someone explain to a noob like me , why is the & ‘optional’ in this case?

(I have tried a few variations, and it seems to me it is always optional?)

2

The & mean in c++ address of

As you see the Touch is pointer *

I think the auto keyword, taking the care in that case… Maybe

3

It isn’t really optional.
for (auto touch : touches) creates copies of the pointers.
for (auto & touch : touches) doesn’t.

Actually it is recommended to always write

for (auto && element : range)

(with 2 &) when doing range-based loops.

It has something to do with “reference collapsing” and “universal references”… Never mind, just do it…
More information here: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3853.htm
and here: https://isocpp.org/blog/2012/11/universal-references-in-c11-scott-meyers