Document visit_by_index

doc/variant2/changelog.adoc

@@ -12,6 +12,7 @@ https://www.boost.org/LICENSE_1_0.txt
 
 * Added `<boost/variant2.hpp>`.
 * Added `unsafe_get<I>`.
+* Added `visit_by_index`.
 
 ## Changes in 1.76.0
 

doc/variant2/design.adoc

@@ -169,6 +169,10 @@ The main differences between this implementation and `std::variant` are:
   `variant<int, float>` is provided as the member function `subset<U...>`.
   (This operation can throw if the current state of the variant cannot be
   represented.)
+* `unsafe_get`, an unchecked alternative to `get` and `get_if`, is provided
+  as an extension.
+* `visit_by_index`, a visitation function that takes a single variant and a
+  number of function objects, one per alternative, is provided as an extension.
 * The {cpp}20 additions and changes to `std::variant` have not yet been
   implemented.
 

doc/variant2/reference.adoc

@@ -149,6 +149,11 @@ template<class... T>
 template<class R = /*unspecified*/, class F, class... V>
   constexpr /*see below*/ visit(F&& f, V&&... v);
 
+// visit_by_index (extension)
+
+template<class V, class... F>
+  void visit_by_index(V&& v, F&&.. f);
+
 // monostate
 
 struct monostate {};
@@ -891,6 +896,20 @@ Remarks: :: If `R` is given explicitly, as in `visit<int>`, the return
   of `F` to the variant alternatives must have the same return type for
   this deduction to succeed.
 
+### visit_by_index (extension)
+
+```
+template<class V, class... F>
+  void visit_by_index(V&& v, F&&.. f);
+```
+[none]
+* {blank}
++
+Requires: :: `variant_size<V>::value == sizeof...(F)`, or the program is ill-formed.
+Effects: :: `std::forward<Fi>(fi)(get<i>(std::forward<V>(v)))`, where
+  `i` is `v.index()` and `Fi` and `fi` are the `i`-th element of `F...` and `f...`
+  accordingly.
+
 ### swap
 
 ```