Vector initialisation using ranges vs lambda inline initialisation

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I would like to initialise a vector by transforming the other one. I made a test with two ways of inline initialisation a transformed std::vector.

One using lambda inline initialisation (with std::transform):

   std::vector<int> foo(100,42);
   const auto fooTimesTwo = [&]{
            std::vector<int> tmp(foo.size());
            std::transform(foo.begin(), foo.end(), tmp.begin(), convert);
            return tmp;
    }();

And the other one - using std::ranges::views::transform:

   std::vector<int> foo(100,42);
   auto transform_range = (foo | std::ranges::views::transform(convert));
   std::vector<int> fooTimesTwo {
           transform_range.begin(),
           transform_range.end()
   };

I expected that both way of vector initialisation should have similar performance, but from some reason the benchmark of solution with traditional std::transform is much faster that the second one (9.7 times faster -> https://quick-bench.com/q/3PSDRO9UbMNunUpdWGNShF49WlQ ).

My questions are:

• Do I use std::ranges::views::transform incorrectly?
• Why does it work so much slower?

Side note - it can be done using boost::make_transform_iterator, but I couldn't check it on quick-bench as they don't support boost. So I'm not sure about efficiency of such solution.

c++ c++20 std-ranges

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edited Apr 13 at 21:07

Patryk

asked Apr 13 at 19:37

PatrykPatryk

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  Ranges can be inefficient. But it's easy. What's convert doing? – JHBonarius Apr 13 at 20:32
• Multiplies an integer by two. – Patryk Apr 13 at 20:34

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