Best way to return a vector in C++ function?

Asked Sep 27 '20 at 05:46

Active Sep 27 '20 at 05:46

Viewed 26 times

Say I have this function:

vector<int> func(vector<int>& a) {
    vector<int> res;
    for (int x : a) {
        res.push_back(a*2);
    }
    return res;
}

As far as I'm aware, when func returns, res is copied onto the stack which basically repeats an O(n) operation. Does C++ have capabilities to return a vector here without copying the entire thing ?

asked Sep 27 '20 at 05:46

doctopus

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Read about [copy elision](https://en.m.wikipedia.org/wiki/Copy_elision) and [return value optimization](https://en.m.wikipedia.org/wiki/Copy_elision#Return_value_optimization) – Some programmer dude Sep 27 '20 at 05:50
**tl;dr** - There likely won't be a redundant copy. At most a (cheap, in this case) move. – StoryTeller - Unslander Monica Sep 27 '20 at 05:51
1

For the specific function you show, I recommend you learn about [`std::transform`](https://en.cppreference.com/w/cpp/algorithm/transformer). – Some programmer dude Sep 27 '20 at 05:52
I see, so ignoring transform aside, the toy function doesn't require any changes to be optimized? – doctopus Sep 27 '20 at 05:57
@doctopus You'd better call `reserve` before pushing items into the vector – for_stack Sep 27 '20 at 06:46

Best way to return a vector in C++ function?

0 Answers0