How Can I Flatten Vectors Returned from a Function Using Range-V3 Pipelines?

Range Pipelining with Temporary Containers Using Range-V3

Problem:

You have a range of elements (src) and a third-party function (f) that takes an element of the range as input and returns a vector. You want to transform each element in the range using f and flatten the resulting vectors into a single range. However, using view::transform and view::join directly won't work because views cannot be created for temporary containers.

Solution:

To support such pipelines, range-v3 introduces the views::cache1 operator. This operator allows you to store the results of a view in a temporary container and then create a view of that container. By including views::cache1 in the pipeline, you can create a view of the flattened vectors:

auto rng = src | views::transform(f) | views::cache1 | views::join;

Code Sample:

The following code sample demonstrates how to use views::cache1 to create a range pipeline that transforms and flattens elements from a range:

<code class="cpp">#include <range/v3/view.hpp>
#include <range/v3/algorithm.hpp>
#include <vector>

std::vector<std::string> f(int i) {
  return std::vector<std::string>(i, char('a' + i));
}

int main() {
  auto rng = views::iota(0, 4)
        | views::transform([](int i) { return f(i); })
        | views::cache1
        | views::join('-');
  
  check_equal(rng, {'-', 'b', '-', 'c', 'c', '-', 'd', 'd', 'd'});
  
  CPP_assert(input_range<decltype(rng)>);
  CPP_assert(!range<const decltype(rng)>);
  CPP_assert(!forward_range<decltype(rng)>);
  CPP_assert(!common_range<decltype(rng)>);

  return 0;
}</code>

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