format/sepnewline.hpp

The following code example is taken from the book
C++23 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2026
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <utility>
#include <ranges>
#include <format>

// new view type for all elements:
template<std::ranges::range InnerRgT>
struct SepNewlineView : std::ranges::view_interface<SepNewlineView<InnerRgT>>
{
  private:
    InnerRgT innerRange;
  public:
    template<std::ranges::range RgT>
    SepNewlineView(RgT&& rg)
     : innerRange(std::forward<RgT>(rg)) {
    }
    auto begin() { return innerRange.begin(); }
    auto end() { return innerRange.end(); }
};

// deduction guide to deal with lvalues and rvalues:
template<std::ranges::range RgT>
SepNewlineView(RgT&& rg) -> SepNewlineView<std::ranges::views::all_t<RgT>>;

// add special formatter for this view:
template<typename RgT>
struct std::formatter<SepNewlineView<RgT>>
: std::range_formatter<std::ranges::range_value_t<RgT>>
{
  constexpr formatter() {
    this->set_brackets("[", "]");
    this->set_separator("\n ");
  }
};

// make the view usable in a pipe:
struct SepNewlineFO : std::ranges::range_adaptor_closure<SepNewlineFO>
{
  template<std::ranges::range RgT>
  constexpr auto operator()(RgT&& rg) const {
    return SepNewlineView(std::forward<RgT>(rg));
  }
};