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json_view: the library's std conversions read views directly

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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>
This commit is contained in:
Niels Lohmann committed 2026-10-25 13:33:40 +01:00
1 parent 64a5d233d0
commit b3f7f80fb7
10 files changed
+598 -220
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include/nlohmann/detail/view/value.hpp
+7 -5
View File
@@ -130,14 +130,16 @@ struct is_direct_value : std::integral_constant < bool,
template<typename Traits, typename Alloc, typename BasicJsonType, typename View>
struct is_direct_value<std::basic_string<char, Traits, Alloc>, BasicJsonType, View> : std::true_type {};
// (vectors of these, which are faster than through from_json(), and containers
// of views; other containers are read by the library's from_json())
template<typename U, typename A, typename BasicJsonType, typename View>
struct is_direct_value<std::vector<U, A>, BasicJsonType, View> : std::true_type {};
struct is_direct_value<std::vector<U, A>, BasicJsonType, View> : is_direct_value<U, BasicJsonType, View> {};
template<typename K, typename V, typename C, typename A, typename BasicJsonType, typename View>
struct is_direct_value<std::map<K, V, C, A>, BasicJsonType, View> : is_string_key<K> {};
template<typename K, typename C, typename A, typename BasicJsonType, typename View>
struct is_direct_value<std::map<K, View, C, A>, BasicJsonType, View> : is_string_key<K> {};
template<typename K, typename V, typename H, typename E, typename A, typename BasicJsonType, typename View>
struct is_direct_value<std::unordered_map<K, V, H, E, A>, BasicJsonType, View> : is_string_key<K> {};
template<typename K, typename H, typename E, typename A, typename BasicJsonType, typename View>
struct is_direct_value<std::unordered_map<K, View, H, E, A>, BasicJsonType, View> : is_string_key<K> {};
} // namespace view
} // namespace detail