libc++ Ranges and unique_ptr

Ranges: Pipe Operator

For detailed implementation, see the range-v3 pipe operator chapter.

libc++'s Ranges implementation evolved directly from range-v3, with the same core mechanism:

• CRTP base class __range_adaptor_closure<T> for tag dispatch
• __pipeable<Fn> wraps function objects
• Two operator| overloads: range|closure and closure|closure

std::unique_ptr: compressed_pair

sizeof(unique_ptr<T, default_delete<T>>) = 8 bytes (64-bit)

  _LIBCPP_COMPRESSED_PAIR expansion:
  ┌──────────────────────────────────┐
  │ [[no_unique_address]] T* __ptr_  │  ← 8 bytes
  │ [[no_unique_address]] default_   │  ← 0 bytes (empty type)
  │            delete<T> __deleter_  │
  └──────────────────────────────────┘
  sizeof = 8

trivial_abi: libc++ marks unique_ptr with [[clang::trivial_abi]], allowing it to be passed in registers (instead of via an implicit pointer on the stack), reducing function call overhead. GCC and MSVC do not support this attribute.