Last chance
to fix std::nontype
P3740R0

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 (P2781R8)
  • in LWG 10:30 AM today
• "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

Proposed alternative: std::stateless

Alternatives considered

• remove std::nontype, revisit in C++29
  • ... but this feature is too important
• wait for P1045R1 constexpr function parameters
  • ... but paper looks abandoned, no C++29 guarantee
• expose the BoundEntityType
  • ... but leaking non-portable implementation details
• rename std::nontype
  • ... to what? confusion with std::constant_wrapper

Why not just use std::constant_wrapper

std::constant_wrapper<int(*)()> already behaves much like the function pointer it wraps. Therefore:

• F&& and F* constructors are candidates, but lose in overload resolution
• std::constant_wrapper also works in std::move_only_function
  • ... but with totally different meaning?!
• std::nontype reference impl. also supports std::move_only_function
  • ... which would make a lot more sense
• design still ages poorly given constexpr function parameters

Most viable solutions

• use std::stateless constructor tag instead
• use std::constant_wrapper instead of std::nontype
• rename std::nontype
  • Author's proposal: std::constant_arg / std::constant_arg_t