Add element access, iteration, values, and JSON pointers to json_view (#5622)

* Add element access, iteration, values, and JSON pointers to json_view Give basic_json_view the read-only access functions of basic_json: operator[] and at() with keys and indices, front()/back(), find(), contains(), count(), begin()/end() and cbegin()/cend(), items() with structured bindings from C++17 on, and type_name(). Exceptions have the ids and messages of the const functions of basic_json. Where basic_json has undefined behavior the view answers safely: operator[] with a missing key or an out-of-range index returns a discarded view, and front()/back() of an empty container throw invalid_iterator.214. Objects are iterated in document order, and all members are visited; duplicate-key lookups find the first member (as yyjson and simdjson do), while parse(), materialize(), and the map conversions keep the last value, as parse() does. Keys of up to 16 bytes are compared with two overlapping loads. Add value conversions: get<T>()/get_to() for arithmetic types, bool, nullptr_t, strings (std::basic_string copied, string_view_t without a copy), BasicJsonType, views, std::vector, and maps with string keys; get_string() for the string without a copy; number_token() for the number exactly as written in the source; value() with keys and JSON pointers; and operator[]/at()/ contains() with JSON pointers. Everything else, including types with from_json(), goes through materialize() of that subtree. get<T>() of arithmetic types is inlined down to the conversion, so reading an integer needs no call. detail::json_pointer_access exposes a pointer's reference tokens to code outside basic_json. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Make view lookups last-wins and chained access safe * Lookups (operator[], at, find, contains, count, value, JSON pointers) return the last member of a duplicate key, as materialize() and parse() keep it. * operator[] on a discarded view returns a discarded view instead of throwing, so v["a"]["b"] is safe for a missing "a". * operator[] and at() take any integer type (not only int and size_t), fixing ambiguous calls with unsigned, long, std::int64_t, ... * Fix the operator[] documentation, which claimed a discarded view for a type mismatch where type_error.305 is thrown. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Replace the removed operator bool of basic_json_view in the header and its tests A view is tested with is_discarded(); the lookups in at(), value() and contains(json_pointer) and the unit tests no longer rely on the explicit conversion to bool. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Use named documents in the json_view tests that called root() on a temporary root() of an rvalue document is deleted, because the views would dangle. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Test discarded views with is_discarded() in the operator[] docs and examples The explicit conversion to bool of basic_json_view is gone. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Return the first member of duplicate keys from lookups again Looking up the last member of a duplicate key cannot stop at a match, so every lookup scanned the whole object (1.6 to 3.4 times slower for small objects). operator[](key), at, find, value, contains, count, and JSON pointer resolution return the first member again, as yyjson and simdjson do; materialize() and get<map>() keep the last value, as parse(). The documentation says so in the feature page and on each lookup page, and explains how to get the value parse() would give. The integer index templates and the discarded chaining of operator[] stay. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Do not force-inline the view's functions on MSVC __forceinline makes MSVC report C4714 for function templates it cannot inline (get<std::string>), an error under /WX. The forced inlining is only a performance hint, so use plain inline there. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Make json_view.hpp pass include-what-you-use Mark the includes that are needed by design (json.hpp, the macro unscope, <tuple> for the tuple_size specialization), take size_t from <cstring>, and drop the forward declaration of basic_json_document that the friend declaration makes redundant. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Avoid GCC useless-cast warnings in the integer index tests Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Make json_view.hpp pass include-what-you-use ci_single_binaries runs IWYU with --error on every header. For json_view.hpp it suggested adding <array> (std::array is used), the headers that json.hpp already provides (abi_config, abi_macros, input_adapters, json_pointer, cpp_future, string_concat, value_t, json_fwd), and <version> for std::nullptr_t, and removing <cstddef>. - include <array> - keep <cstddef> (nullptr_t, size_t; IWYU attributes them to <version> and <cstring>) - tell IWYU not to suggest the headers that json.hpp provides: the amalgamated json_view.hpp only includes json.hpp, so including them here would duplicate their definitions Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Fix the clang-tidy 22 findings in json_view and its tests The same changes as on the dump branch, where they were first made, so that this branch passes clang-tidy on its own. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Regenerate the amalgamated headers Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Report one-character non-numeric array indices like basic_json in the view Develop's #5800 made basic_json report a JSON pointer array index such as "/a" as parse_error.109 instead of out_of_range.404, as longer ones were; the view's array_index() follows, with "-" and the empty token still unresolved. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Address the review of json_view element access From gregmarr's review of #5622: - count() counts every member with the key, so count(key) > 1 detects duplicate keys; it was contains() as 0/1 before. - get_to() passes types that get<T>() does not convert directly to from_json with the existing value, as basic_json::get_to() does, so that a from_json which sets only the fields present keeps the others. - array_index() checks the empty token and "-" first; the results are unchanged. - Docs: no "if an exception is thrown" where a view never changes anything; the borrowed source must also stay unmodified; items() on a temporary view is safe; back()/front() notes shortened; the basic_json see-also entries say how the view's results can differ. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Regenerate the amalgamated headers Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Count matching keys as unsigned in the json_view access test The conditional added 1 or 0 as int to a std::size_t, which GCC rejects with -Werror=sign-conversion (ci_test_standards_gcc). Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Address the second review round of json_view element access From gregmarr's review of #5622: - begin()/end(): "a null value or a discarded view iterates as an empty range", singular like the primitive case before it. - The class page's list of function groups has no trailing commas. - document_data::key_equals() compares a key node with a string, for the key comparisons that were spelled out in each lookup. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Regenerate the amalgamated headers Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Write the negated enable_if conditions of json_view with negation<> With a ! or && in the condition, astyle spaces the whole template header (template < ... >); detail::negation and a named trait keep it compact, as the library writes such conditions elsewhere. From gregmarr's review of #5630. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Regenerate the amalgamated headers Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Keep detail::negation out of json_view.hpp's enable_ifs include-what-you-use asks json_view.hpp to include detail/meta/type_traits.hpp for it, which the single header must not do (the amalgamation would duplicate json's internals). The two conditions of value() go through a trait in value.hpp, like the others. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Regenerate the amalgamated headers Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-10-21 11:29:53 +00:00 · 2026-10-21 10:21:27 +02:00
1 parent 122d718330
commit 43e97b629b
@@ -0,0 +1,133 @@
+# <small>nlohmann::basic_json_view::</small>get
+
+```cpp
+template<typename T>
+T get() const;
+```
+
+Converts the value to `T`.
+
+For the types below, the conversion works directly on the flat index -- no `BasicJsonType` value is built for it:
+
+- `#!cpp bool`
+- arithmetic types other than `#!cpp bool` (for a boolean, `#!cpp 0`/`#!cpp 1`, exactly as for
+  [`BasicJsonType::get<T>()`](../basic_json/get.md))
+- `#!cpp std::nullptr_t`
+- `#!cpp std::basic_string<char, Traits, Alloc>` (including `string_t`) -- a copy of the string
+- [`string_view_t`](index.md#member-types) -- **no copy**: the returned view points into the document's
+  [`source()`](../basic_json_document/source.md) text, or, for a string that contains escape sequences, into the
+  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
+
+Every other `T` -- `#!cpp std::list`, `#!cpp std::pair`, `#!cpp std::array`, enumerations, user types with a
+`from_json()`, ... -- 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.
+
+## Template parameters
+
+`T`
+:   the type to convert the value to
+
+## Return value
+
+the value, converted to `T`
+
+## Exception safety
+
+Strong guarantee: there are no changes to the view or the document it refers to.
+
+## Exceptions
+
+- For the directly-converted types listed above (other than `BasicJsonType` and `basic_json_view`, which never
+  throw): throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if the value's type does not
+  match `T` -- the same exception, with the same message, that [`BasicJsonType::get<T>()`](../basic_json/get.md)
+  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 every other `T`: whatever [`materialize().get<T>()`](../basic_json/get.md) throws -- typically `type_error.302`,
+  or whatever a user-provided `from_json()` throws.
+
+None of the exceptions thrown directly by this function (the first three bullets above) carry a
+[`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) path: the view has no `BasicJsonType` value to point at. An
+exception thrown while converting through `materialize()` (the last bullet) is different: it is thrown by a real
+`BasicJsonType` value, so it **does** carry a `JSON_DIAGNOSTICS` path if `BasicJsonType` was built with it enabled.
+That path is relative to the root of the view's subtree, because `materialize()` builds only the subtree (with parent
+pointers): it is empty if the failing value is the subtree's root itself, and it never is the path within the whole
+document.
+
+## Complexity
+
+- `#!cpp bool`, arithmetic types, `#!cpp std::nullptr_t`, [`string_view_t`](index.md#member-types), `basic_json_view`:
+  constant.
+- `#!cpp std::basic_string<char, Traits, Alloc>`: constant, plus one allocation and a copy of the string's bytes.
+- `BasicJsonType`: linear in the size of the subtree, see [`materialize()`](materialize.md).
+- `#!cpp std::vector<U, A>`: linear in the number of elements, times the complexity of converting one element to `U`.
+- `#!cpp std::map`/`#!cpp std::unordered_map`: linear in the number of members for walking them, plus the container's
+  own insertion cost per member (logarithmic for `#!cpp std::map`, amortized constant for `#!cpp
+  std::unordered_map`), times the complexity of converting one member to the mapped type.
+- every other `T`: the cost of [`materialize()`](materialize.md) and of destroying the `BasicJsonType` value
+  afterwards, plus that of [`BasicJsonType::get<T>()`](../basic_json/get.md).
+
+## Notes
+
+!!! info "Floating-point values"
+
+    A floating-point `T` is converted from the same digits the lexer would see during `#!cpp BasicJsonType::parse()`,
+    using the same conversion, so the result is bit-for-bit identical to `#!cpp BasicJsonType::parse(text).get<T>()`
+    for the same source text.
+
+!!! info "Duplicate keys"
+
+    `#!cpp std::map`/`#!cpp std::unordered_map` conversions keep the *last* value of a repeated key, like
+    [`materialize()`](materialize.md) and [`BasicJsonType::parse()`](../basic_json/parse.md) do. This is the opposite
+    of [`operator[]`](operator[].md)/[`at`](at.md)/[`find`](find.md)/[`contains`](contains.md), which resolve to the
+    *first* occurrence (see the [Notes on duplicate keys](operator[].md#notes)).
+
+!!! info "No pointers, references, or implicit conversion"
+
+    Unlike `BasicJsonType`, `basic_json_view` has no stored value anywhere to hand out a pointer or a reference to, so
+    it provides neither `#!cpp get_ptr()`, `#!cpp get_ref()`, nor `#!cpp operator ValueType()`.
+    [`get_string()`](get_string.md) (equivalently, `#!cpp get<string_view_t>()`) is the zero-copy alternative for
+    strings.
+
+## Examples
+
+??? example
+
+    The example below reads typed fields straight into C++ variables, collects a view of every array element with
+    `#!cpp get<std::vector<basic_json_view>>()` instead of a value, and converts a nested object into a user type
+    through its `from_json()` -- which runs on a `BasicJsonType` value `materialize()` builds for just that one
+    member.
+
+    ```cpp
+    --8<-- "examples/basic_json_view__get.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_view__get.output"
+    ```
+
+## See also
+
+- [get_to](get_to.md) - convert and write into a passed value
+- [get_string](get_string.md) - the string, without a copy
+- [number_token](number_token.md) - a number's token text, without a copy
+- [materialize](materialize.md) - build the `BasicJsonType` value of this subtree
+- [`BasicJsonType::get`](../basic_json/get.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0.