How can I transform an infinite range with a function returning temporary containers in range-v3?

Pipelining Range-v3 Transformations with Temporary Containers

In range-v3, creating a pipeline that transforms elements of a potentially infinite range using a third-party function that returns a vector can be challenging due to the inability to create views of temporary containers.

Issue: Transforming a Range with a Function Returning Temporary Vectors

Consider the following code:

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

where src is an infinite range of type T, and f is a third-party function that returns a std::vector. Executing this code would result in a compilation error because view::transform creates a view of a temporary container returned by f.

Solution: Introducing Cache1 Operator

Recent updates to range-v3 have introduced a solution to this issue: the views::cache1 operator. This operator creates a temporary container that stores the output of a transformation, allowing other views in the pipeline to access its elements.

To implement the desired pipeline correctly, add views::cache1 before views::join:

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

Code Example

The following code sample demonstrates the usage of views::cache1 in practice:

<code class="cpp">auto rng = views::iota(0, 4)
        | views::transform([](int i) { return std::string(i, char('a' + i)); })
        | views::cache1
        | views::join('-');

check_equal(rng, {'-', 'b', '-', 'c', 'c', '-', 'd', 'd', 'd'});</code>

Range Properties

It's important to note that the transformed range rng does not retain all the range properties of its source range. In the example above, it loses the forward range concept due to the creation of temporary containers.

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