Hi,

For continuous containers, like vector, there is easy workaround - lambda
can accept its argument by reference instead of by value, so index could be
calculated internally in lambda expression as in next example:

[&primitives](Primitive& a) {

        return &a - primitives.data();

});

--
Tihomir
On Tue, Dec 29, 2020, 00:39 Allen Liu via Std-Proposals <
std-proposals_at_[hidden]> wrote:
> Hi,
>
>
>
> I recently encountered a feature for C++ that I think might improve the
> performance of the code in certain cases. Currently, I haven’t found a way
> to obtain the index of the array element along with the value for the C++
> lambda expression, but having the index could be useful for performance in
> cases where operations on the original elements might be too expensive
> (e.g. moving and swapping structures around during sorting). For instance,
> consider the need to repeatedly sort the vector of primitives during BVH
> build. Sorting the primitives structure would be expensive because data
> members of the primitives need to be swapped within the memory during
> sorting. However, if the sorting is implemented on primitive ids, then it
> could be done efficiently.
>
>
>
> A concrete example is:
>
>
>
> struct Primitive {
>
>                 int data1;
>
>                 int data2;
>
>                 // ... more data
>
> }
>
>
>
> std::vector<Primitive> primitives;
>
> std::vector<int> ids;
>
> for (int i = 0; i < primitives.size(); ++i) {
>
>                 ids.push_back(i);
>
> }
>
>
>
> while (condition) {
>
>                 sort(ids.begin(), ids.end(), [&primitives](int a, int b) {
>
>                                 return primitives[a].data1 <
> primitives[b].data1;
>
> });
>
> // operations on sorted data
>
> }
>
>
>
> sort(primitives.begin(), primitives.end(), [&ids](const Primitive & a, int
> index_a, const Primitive & b, int index_b) {
>
>                 return ids[index_a] < ids[index_b];
>
> });
>
>
>
> With the array indices in lambda, sorting the vector of primitive at the
> last step can be done efficiently without creating extra space while having
> a constant time look up for each comparison.
>
>
>
> Thank you,
>
> Zihao
> --
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