The view_closure class template in range-v3 that is an implementation strategy for what in C++20 became the range adaptor closure object concept:
A range adaptor closure object is a unary function object that accepts a viewable_range argument and returns a view. For a range adaptor closure object C and an expression R such that decltype((R)) models viewable_range, the following expressions are equivalent and yield a view:
C(R)
R | C
Given an additional range adaptor closure object D, the expression C | D is well-formed and produces another range adaptor closure object such that the following two expressions are equivalent:
R | C | D
R | (C | D)
The result of transform(f) is a range adaptor closure object, which you can apply to a viewable_range either via pipe as r | transform(f) or via call as transform(f)(r), either of which will give you some kind of transform_view adaptor.
More broadly, transform itself is a range adaptor object which is defined in a way such that transform(f) gives you a range adaptor closure object such that transform(r, f), r | transform(f), and transform(f)(r) are all equivalent.
view_closure, the class template, is necessary to ensure that stuff like this works:
auto adaptor = transform(f) | filter(g) | chunks(n);
That is, you can build up a pipeline without having a range, and the result of that is a range adaptor closure object that you can then apply to a range - r | adaptor would be equivalent to r | transform(f) | filter(g) | chunks(n) which is equivalent to chunks(filter(transform(r, f), g), n).
That class template basically affects what operator| does, among other things. You only need to use it if you're creating your own range adaptor, it's not something you need to care about as a user of ranges otherwise.
