json_view: the library's std conversions read views directly

The library's from_json() overloads (containers, maps, std::array, C
arrays, pairs, tuples, optional, valarray, forward_list, enums, binary,
paths, and scalars) accept views: their traits use the view's
basic_json type, and the few accesses that differ (get_ptr) go through
small helpers. from_json() of a view now finds them; only the probe
still looks up by ADL only. get<T>() of a view thus converts these types
without materialize(), with the same values and exceptions; from an
object, a repeated key keeps the last value, as parse() does.

The view keeps its direct paths for vectors of the types it converts
directly (faster than from_json()) and for containers of views; its
other vector and map paths are replaced by from_json(). For basic_json,
std::valarray and std::forward_list are read in one pass (40 % and 3.5 %
fewer instructions); all other conversions are unchanged.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_view/get.md

@@ -19,16 +19,19 @@ For the types below, the conversion works directly on the flat index -- no `Basi
   document's own buffer of decoded strings (see [`get_string()`](get_string.md))
 - `BasicJsonType` -- equivalent to [`materialize()`](materialize.md)
 - `basic_json_view` -- returns `#!cpp *this`
-- `#!cpp std::vector<U, A>` -- from an array, element by element, each converted with `#!cpp get<U>()`; `#!cpp
-  std::vector<basic_json_view>` keeps a view of every element instead of a value
-- `#!cpp std::map<K, V, C, A>` and `#!cpp std::unordered_map<K, V, H, E, A>`, if `K` is constructible from a `#!cpp
-  (const char*, std::size_t)` pair -- from an object, member by member, each value converted with `#!cpp get<V>()`; with a repeated
-  key, the *last* value is kept, as [`BasicJsonType::parse()`](../basic_json/parse.md) (and
-  [`materialize()`](materialize.md)) does; `#!cpp std::map<std::string, basic_json_view>` keeps views of the members
-  instead of values
+- `#!cpp std::vector<U, A>` of the types above -- from an array, element by element, each converted with
+  `#!cpp get<U>()`; `#!cpp std::vector<basic_json_view>` keeps a view of every element instead of a value
+- `#!cpp std::map<K, basic_json_view, C, A>` and `#!cpp std::unordered_map<K, basic_json_view, H, E, A>`, if `K` is
+  constructible from a `#!cpp (const char*, std::size_t)` pair -- views of the members of an object; with a repeated
+  key, the view of the *last* member is kept, as [`BasicJsonType::parse()`](../basic_json/parse.md) keeps its value
 
-A default-constructible `T` whose `from_json()` accepts views reads the view directly as well, member by member, without
-building a `BasicJsonType` value. This covers the types of the
+A default-constructible `T` whose `from_json()` accepts views reads the view directly as well, without building a
+`BasicJsonType` value. This covers the library's own conversions of the standard library types -- all other
+`#!cpp std::vector`s, `#!cpp std::map`/`#!cpp std::unordered_map` (also with other key types), `#!cpp std::list`,
+`#!cpp std::deque`, `#!cpp std::set`, `#!cpp std::forward_list`, `#!cpp std::valarray`, `#!cpp std::array`, C arrays,
+`#!cpp std::pair`, `#!cpp std::tuple`, `#!cpp std::optional`, enumerations, `#!cpp std::filesystem::path`, ... --
+which give the same values as `#!cpp materialize().get<T>()` (from an object, a repeated key keeps the *last* value,
+as [`BasicJsonType::parse()`](../basic_json/parse.md) does), as well as the types of the
 [`NLOHMANN_DEFINE_TYPE_*`](../macros/nlohmann_define_type_intrusive.md) and
 [`NLOHMANN_DEFINE_DERIVED_TYPE_*`](../macros/nlohmann_define_derived_type.md) macros and the enumerations of
 [`NLOHMANN_JSON_SERIALIZE_ENUM`](../macros/nlohmann_json_serialize_enum.md), and any `from_json()` constrained to
@@ -44,8 +47,8 @@ T&)` of [arbitrary type conversions](../../features/arbitrary_types.md), is not
     keeps it: for `#!cpp {"x":1,"x":2}`, `#!cpp get<point>()` reads `#!cpp x == 1`. See
     [Duplicate keys](../../features/json_view.md#duplicate-keys).
 
-Every other `T` -- `#!cpp std::list`, `#!cpp std::pair`, `#!cpp std::array`, user types whose `from_json()` is
-written for `BasicJsonType` only, ... -- is converted by `#!cpp materialize().get<T>()`: the subtree is built into a real `BasicJsonType`
+Every other `T` -- user types whose `from_json()` is written for `BasicJsonType` only, types without a default
+constructor, `#!cpp std::tuple`s holding references, ... -- is converted by `#!cpp materialize().get<T>()`: the subtree is built into a real `BasicJsonType`
 value first (as [`BasicJsonType::parse()`](../basic_json/parse.md) would), and converted from there exactly as
 [`BasicJsonType::get<T>()`](../basic_json/get.md) would convert it.
 
@@ -70,10 +73,10 @@ Strong guarantee: there are no changes to the view or the document it refers to.
   throws for the same JSON type and `T`.
 - For `#!cpp std::vector<U, A>`: throws `type_error.302` if the value is not an array; otherwise, whatever converting
   an element to `U` throws.
-- For `#!cpp std::map`/`#!cpp std::unordered_map`: throws `type_error.302` if the value is not an object; otherwise,
-  whatever converting a member to the mapped type throws.
-- For a `T` whose `from_json()` reads the view: whatever that `from_json()` throws; for the macros, the same
-  exceptions with the same messages as [`BasicJsonType::get<T>()`](../basic_json/get.md) of the same value.
+- For `#!cpp std::map`/`#!cpp std::unordered_map` of views: throws `type_error.302` if the value is not an object.
+- For a `T` whose `from_json()` reads the view: whatever that `from_json()` throws; for the library's conversions and
+  the macros, the same exceptions with the same messages as [`BasicJsonType::get<T>()`](../basic_json/get.md) of the
+  same value.
 - For every other `T`: whatever [`materialize().get<T>()`](../basic_json/get.md) throws -- typically `type_error.302`,
   or whatever a user-provided `from_json()` throws.
 

docs/mkdocs/docs/api/basic_json_view/get_to.md

@@ -9,8 +9,7 @@ Converts the value to `T` and stores it in `v`, in one of three ways:
 
 - For the types that [`get<T>()`](get.md) converts directly -- arithmetic types, `#!cpp bool`,
   `#!cpp std::nullptr_t`, [`string_view_t`](index.md#member-types), `#!cpp std::basic_string`, `BasicJsonType`,
-  `basic_json_view`, `#!cpp std::vector`, and `#!cpp std::map`/`#!cpp std::unordered_map` with string keys -- `v` is
-  assigned: `#!cpp v = get<T>()`.
+  `basic_json_view`, vectors of these, and maps of views -- `v` is assigned: `#!cpp v = get<T>()`.
 - For a type whose `from_json()` accepts views (see [`get<T>()`](get.md)), that `from_json()` is called with the
   view and the existing `v`, without building a `BasicJsonType` value.
 - For every other type, the subtree is built into a `BasicJsonType` value and `#!cpp materialize().get_to(v)` is