Last chance
to fix std::nontype
P3740R1

Introduction

• P2472R3 added std::nontype constructors for std::function_ref
• merged into P0792R14, plenary-approved Varna 2023 for C++26
• std::nontype possibly obsoleted by std::constant_wrapper (P2781R9)
  • Wording review completed by LWG
• "non-type template parameter" term was removed by P2841R1 (C++26)

Conclusion

History is messy. std::nontype may be a bad feature now, or at least have a terrible name.

std::function_ref refresher

std::function_ref is a "function wrapper" similar to std::function, std::move_only_function, std::copyable_function, but without ownership.

std::function_ref<R(Args...) cv noex> stores:

• bound-entity — BoundEntityType (basically a cv void*)
• thunk-ptr — R(*)(BoundEntityType, Args...) noex

std::nontype in std::function_ref

With std::constant_wrapper

Alternatives considered

• Low consensus:
  • std::stateless constructor tag
  • remove std::nontype, revisit in C++29
  • wait for P1045R1 constexpr function parameters
  • expose the BoundEntityType
• Plausible:
  • replace std::nontype with std::constant_wrapper
  • rename std::nontype (to std::constant_arg)

std::constant_wrapper concerns

std::cw<&free_function> already behaves much like wrapped function:

• F&& and F* constructors are candidates
  • not a problem, these lose in overload resolution
• danger of inconsistent behavior with std::move_only_function etc.

• Solution: Provide corresponding overloads for all function wrappers.
• Urgency: If done later, meaning of existing code sublty changed.