Ranged-based for loop over array within function

Question

im pretty new to C++, learning by doing right now. Within the class material, I have the following function:

std::vector<std::complex<double>> material::get_idx(const std::vector<double>& numbers) {

  std::vector<std::complex<double>> res;

  for (auto& num : numbers) {
    // calculate with num (we don't modify it) and push_back to res 
    ...
  }

  return res;
}

A couple of minutes ago I used the normal for loop like this: for(size_t i=0; i<numbers.size(); i++). Since I've read that using ranged-based for loop with auto can be more efficient than plain for loops, I changed it. Now I'm asking myself if it's correct/senseful/good practice to write the ranged-based for loop with a reference like this: for(auto& num : numbers) and what's the difference to using for(auto num : numbers). I'm kind of confused because I'm already handing over the numbers vector per reference in the signature of the get_idx function. Of course I want to hand over the vector numbers to the function get_idx per reference, but how about looping over those elements via reference?

My advice, const auto& if you don't want to modify, auto&& if you do. — NathanOliver
– NathanOliver, Commented Mar 11, 2021 at 13:20
Hey @NathanOliver, thanks for your answer, what is the difference of using const auto& instead of just const auto? — Brosch
– Brosch, Commented Mar 11, 2021 at 13:21
The first is a reference, so no copy. The second is a value so a copy is made. Copying can be expensive. — NathanOliver
– NathanOliver, Commented Mar 11, 2021 at 13:22
you never run out of new things to learn in C++, though i am not always sure if thats a good thing ;) — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Mar 11, 2021 at 13:49
Well at least it's a good thing for sure to have people here being able to explain things voluntarily. Thanks again, have a great day folks! — Brosch
– Brosch, Commented Mar 11, 2021 at 13:51

463035818_is_not_an_ai · Accepted Answer · 2021-03-15 10:22:23Z

1

auto makes a copy, auto& makes an lvalue reference of any kind but will not bind to rvalues, auto const& makes an lvalue reference to const that can bind to rvalues and induce reference lifetime extension, auto&& generates a forwarding reference that can bind to anything and can induce reference lifetime extension. auto const makes a copy and makes it immutable.

Which is more correct depends on which you want.

You could just type double const&, double const or double instead of anything to do with auto.

I start with auto&&, which is shorthand for "I do not care, just make it work". Next I'd use auto, as values make reasoning easier. And if I had a worry about making it clear I am not editing, auto const or auto const&.

If I wanted to edit the elements inplace, auto&.

edited Mar 15, 2021 at 10:22

463035818_is_not_an_ai

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answered Mar 11, 2021 at 15:10

Yakk - Adam Nevraumont

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1 Comment

Brosch Over a year ago

Its for(double const& num : numbers) rather than for(std::complex<double> const& num : numbers)

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