P3433R1 Allocator Support for Operation States

Author: AlisdairM

source/exec.tex

@@ -1849,7 +1849,7 @@
 \begin{codeblock}
 []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept -> decltype(auto) {
   auto& [_, data, ...child] = sndr;
-  return std::forward_like<Sndr>(data);
+  return @\exposid{allocator-aware-forward}@(std::forward_like<Sndr>(data), rcvr);
 }
 \end{codeblock}
 
@@ -1895,6 +1895,87 @@
 denoted by \tcode{\exposid{completion-signatures-for}<Sndr, Env>}
 to communicate to users why.
 
+\begin{itemdecl}
+template<@\libconcept{sender}@ Sndr, @\exposconcept{queryable}@ Env>
+  constexpr auto @\exposid{write-env}@(Sndr&& sndr, Env&& env);     // \expos
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\exposid{write-env} is an exposition-only sender adaptor that,
+when connected with a receiver \tcode{rcvr},
+connects the adapted sender with a receiver
+whose execution environment is the result of
+joining the \exposconcept{queryable} argument \tcode{env}
+to the result of \tcode{get_env(rcvr)}.
+
+\pnum
+Let \exposid{write-env-t} be an exposition-only empty class type.
+
+\pnum
+\returns
+\begin{codeblock}
+@\exposid{make-sender}@(@\exposid{write-env-t}@(), std::forward<Env>(env), std::forward<Sndr>(sndr))
+\end{codeblock}
+
+\pnum
+\remarks
+The exposition-only class template \exposid{impls-for}\iref{exec.snd.general}
+is specialized for \exposid{write-env-t} as follows:
+\begin{codeblock}
+template<>
+struct @\exposid{impls-for}@<@\exposid{write-env-t}@> : @\exposid{default-impls}@ {
+  static constexpr auto @\exposid{get-env}@ =
+    [](auto, const auto& state, const auto& rcvr) noexcept {
+      return @\seebelow@;
+    };
+};
+\end{codeblock}
+Invocation of
+\tcode{\exposid{impls-for}<\exposid{write-env-t}>::\exposid{get-env}}
+returns an object \tcode{e} such that
+\begin{itemize}
+\item
+\tcode{decltype(e)} models \exposconcept{queryable} and
+\item
+given a query object \tcode{q},
+the expression \tcode{e.query(q)} is expression-equivalent
+to \tcode{state.query(q)} if that expression is valid,
+otherwise, \tcode{e.query(q)} is expression-equivalent
+to \tcode{get_env(rcvr).que\-ry(q)}.
+\end{itemize}
+\end{itemdescr}
+
+\begin{itemdecl}
+template<class T, class Context>
+  decltype(auto) @\exposid{allocator-aware-forward}@(T&& obj, Context&& context);       // \expos
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\exposid{allocator-aware-forward} is an exposition-only function used to
+either create a new object of type \tcode{T} from \tcode{obj}
+or forward \tcode{obj} depending on whether an allocator is available.
+If the environment associated with context provides an allocator
+(i.e., the expression \tcode{get_allocator(get_env(context))} is valid,
+let \exposid{alloc} be the result of this expression
+and \tcode{P} be \tcode{remove_cvref_t<T>}.
+
+\pnum
+\returns
+\begin{itemize}
+\item
+If \exposid{alloc} is not defined, returns \tcode{std::forward<T>(obj)},
+\item
+otherwise if \tcode{P} is a specialization of \exposid{product-type},
+returns an object of type \tcode{P} whose elements are initialized using
+\tcode{make_obj_using_allocator<decltype(e)> (std::forward_like<T>(e), \exposid{alloc})}
+where \tcode{e} is the corresponding element of \tcode{obj},
+\item
+otherwise, returns \tcode{make_obj_using_allocator<P>(std::forward<T>(obj), \exposid{alloc})}.
+\end{itemize}
+\end{itemdescr}
+
 \rSec2[exec.snd.concepts]{Sender concepts}
 
 \pnum
@@ -3585,7 +3666,8 @@
     args_variant_t @\exposid{args}@;        // \expos
     ops2_variant_t @\exposid{ops2}@;        // \expos
   };
-  return @\exposid{state-type}@{std::forward_like<Sndr>(fn), @\exposid{let-env}@(child), {}, {}};
+  return @\exposid{state-type}@{@\exposid{allocator-aware-forward}@(std::forward_like<Sndr>(fn), rcvr),
+                    @\exposid{let-env}@(child), {}, {}};
 }
 \end{codeblock}