Consider this:

#include <vector>

std::vector<int> f() {
    std::vector<int> v{1, 2};
    return v;
}

void test() {
    auto v = f();
    v = f();           <---- HERE
}

Will the compiler “reuse” the space assigned to v in the call marked with HERE or create a complete new vector?

Alternatively, consider:

#include <vector>

std::vector<int> f(std::vector<int>&& v = {}) {
    // v might not be empty, so be careful!
    v = {1, 2};
    return v;
}

void test() {
    auto v = f();
    v = f(std::move(v));
}

In that approach, a) I don’t like that v might not be empty, so one has to be careful in the function and b) v is initialised with a default with is also not optimal.

What’s a good pattern to design f in a way that it would potentially get a reusable vector?