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

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* 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>
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docs/mkdocs/docs/features/json_view.md
+101 -11
View File
@@ -23,10 +23,15 @@ sequences are decoded, into one shared buffer owned by the document.
[`basic_json_view`](../api/basic_json_view/index.md) is a small, trivially copyable handle (two pointers) into that
index. It gives you the read-only, type-inspection part of the `basic_json` interface --
[`type()`](../api/basic_json_view/type.md) and the `is_*()` predicates,
[`size()`](../api/basic_json_view/size.md)/[`empty()`](../api/basic_json_view/empty.md) -- without ever allocating a
`basic_json` value. When you do need an actual `basic_json` value for a subtree,
[`materialize()`](../api/basic_json_view/materialize.md) builds exactly the one
[`parse()`](../api/basic_json/parse.md) would have produced for it.
[`size()`](../api/basic_json_view/size.md)/[`empty()`](../api/basic_json_view/empty.md) -- as well as element access
([`operator[]`](../api/basic_json_view/operator%5B%5D.md), [`at`](../api/basic_json_view/at.md),
[`front`](../api/basic_json_view/front.md)/[`back`](../api/basic_json_view/back.md)), lookup
([`find`](../api/basic_json_view/find.md), [`contains`](../api/basic_json_view/contains.md),
[`count`](../api/basic_json_view/count.md)), and iteration
([`begin`](../api/basic_json_view/begin.md)/[`end`](../api/basic_json_view/end.md),
[`items`](../api/basic_json_view/items.md)) -- without ever allocating a `basic_json` value. When you do need an
actual `basic_json` value for a subtree, [`materialize()`](../api/basic_json_view/materialize.md) builds exactly the
one [`parse()`](../api/basic_json/parse.md) would have produced for it.
## How to use it
@@ -62,9 +67,10 @@ table). [`parse_copy()`](../api/basic_json_document/parse_copy.md) always owns a
!!! warning "A borrowed document depends on your buffer"
If a document borrows its text, that text **must outlive the document** (and every view taken from it). Reading
or writing through a view after the underlying buffer is gone is undefined behavior, exactly as it would be for
a dangling `#!cpp std::string_view`.
If a document borrows its text, that text **must outlive the document** (and every view taken from it) and must
stay **unmodified** for as long as the document exists, since floating-point values and unescaped strings are
read from it lazily. Reading or writing through a view after the underlying buffer is gone is undefined behavior,
exactly as it would be for a dangling `#!cpp std::string_view`.
A view is valid only while all of the following hold:
@@ -72,7 +78,7 @@ A view is valid only while all of the following hold:
- the document has not been re-parsed since the view was taken (with [`read()`](../api/basic_json_document/read.md)
or [`parse()`](../api/basic_json_document/parse.md) into it),
- the document has not had [`shrink_to_fit()`](../api/basic_json_document/shrink_to_fit.md) called on it since, and
- if the document borrows its source text, that text is still alive.
- if the document borrows its source text, that text is still alive and unmodified.
Moving the `basic_json_document<>` itself is fine and does **not** invalidate its views: the index is a separate heap
allocation that keeps its address across the move. Take a fresh view from [`root()`](../api/basic_json_document/root.md)
@@ -133,9 +139,93 @@ document: `#!cpp auto v = json_document::parse(text).root();` does not compile.
[`JSON_DIAGNOSTIC_POSITIONS`](../api/macros/json_diagnostic_positions.md) enabled,
[`materialize()`](../api/basic_json_view/materialize.md) does not set them. The index has the start offset of every
value, but not the end of arrays, objects, and escaped strings, which the positions also need.
- **Element access, iteration, `get<T>()`, JSON Pointer, `dump()`, and comparison are not (yet) provided** by
`basic_json_view`. For now, [`materialize()`](../api/basic_json_view/materialize.md) is the way to get a value you
can do those things with.
- **Objects iterate in document order.** [`begin()`](../api/basic_json_view/begin.md)/
[`end()`](../api/basic_json_view/end.md) and [`items()`](../api/basic_json_view/items.md) visit an object's
members in the order they appear in the source text. `basic_json`'s default `object_t` is a `std::map`, which
sorts by key, so iterating a [`materialize()`](../api/basic_json_view/materialize.md)d value can print members in
a different order than iterating the view they came from.
- **Chained access is safe.** [`operator[]`](../api/basic_json_view/operator%5B%5D.md) with a missing key, an index
out of range, or an unresolvable JSON pointer returns a [discarded](../api/basic_json_view/is_discarded.md) view, and
`operator[]` on a discarded view returns a discarded view without throwing: `#!cpp v["a"]["b"][0]` can be tested
once at the end. Type errors on values that exist (a key on an array, an index on an object) still throw, and
[`at`](../api/basic_json_view/at.md) throws for every missing value.
- **Duplicate keys: lookups find the first member, `parse()` keeps the last.** See [Duplicate keys](#duplicate-keys)
below.
- **No [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md) path.** Exceptions thrown by `basic_json_view`'s own
element access and lookup functions never carry the JSON Pointer path `JSON_DIAGNOSTICS` would otherwise add: the
view has no `basic_json` value to point at, so the exception is created without one, regardless of how
`BasicJsonType` was built.
- **`dump()` and comparison are not (yet) provided** by `basic_json_view`. For now,
[`materialize()`](../api/basic_json_view/materialize.md) is the way to get a value you can do those things with.
## Duplicate keys
!!! warning "A lookup in a view and in the parsed value can give different answers"
If an object in the source text repeats a key, [`operator[]`](../api/basic_json_view/operator%5B%5D.md),
[`at`](../api/basic_json_view/at.md), [`find`](../api/basic_json_view/find.md),
[`value`](../api/basic_json_view/value.md), [`contains`](../api/basic_json_view/contains.md), and JSON pointer
resolution all return the **first** member with that key. `basic_json::parse()` instead keeps the **last** value
of a repeated key, so for `#!cpp {"a":1,"a":2}`, `#!cpp view["a"]` is `#!cpp 1` while `#!cpp parse(text)["a"]` is
`#!cpp 2`. [`count`](../api/basic_json_view/count.md) counts every member with the key, so it is `#!cpp 2` here.
[`begin()`](../api/basic_json_view/begin.md)/[`end()`](../api/basic_json_view/end.md) and
[`items()`](../api/basic_json_view/items.md) visit *every* occurrence, in document order, and
[`size()`](../api/basic_json_view/size.md) counts all of them, so `#!cpp view.size()` can be larger than
`#!cpp view.materialize().size()`.
**Why the first?** A lookup can stop as soon as it finds a match. Returning the last member would force every lookup
to scan all members of the object, even when the key is found at the very first one: this made lookups in small
objects 1.6 to 3.4 times slower. Other zero-copy parsers that index the source text, such as yyjson and simdjson, also
return the first member. RFC 8259 only says that names within an object SHOULD be unique and that the behavior of a
receiver that sees duplicates is unpredictable, so neither choice is wrong.
**What stays the same as `parse()`?** [`materialize()`](../api/basic_json_view/materialize.md) and
[`get<std::map<...>>()`](../api/basic_json_view/get.md) replay every member in order, so they keep the *last* value
exactly like `#!cpp basic_json::parse()` (at the position of the first occurrence of the key, for an
[`ordered_json`](../api/ordered_json.md)).
**How do I get the value `parse()` would give?** Either call `#!cpp view.materialize()` and look the key up in the
result, or iterate the members and keep the last match:
```cpp
// the last member with the key "a", as parse() would keep it
json_view last;
for (const auto item : view.items())
{
if (item.key() == "a")
{
last = item.value();
}
}
```
If you cannot trust the source text to have unique keys, check [`size()`](../api/basic_json_view/size.md) against the
number of distinct keys, or reject duplicates when iterating.
## Getting values out without copying
[`get<T>()`](../api/basic_json_view/get.md) converts many `T` directly from the flat index, without ever building a
`basic_json` value for the conversion: `#!cpp bool`, arithmetic types, `#!cpp std::nullptr_t`,
`#!cpp std::string`/other `#!cpp std::basic_string`s (copied once), `basic_json`/`ordered_json` (via
[`materialize()`](../api/basic_json_view/materialize.md)), `basic_json_view` itself, `#!cpp std::vector<U>`, and
`#!cpp std::map`/`#!cpp std::unordered_map` with string-like keys. Every other type -- `#!cpp std::list`,
`#!cpp std::pair`, `#!cpp std::array`, enumerations, user types with a `from_json()` -- goes through
[`materialize()`](../api/basic_json_view/materialize.md)`.get<T>()` instead: the subtree is built into a real
`basic_json` value first, exactly as [`parse()`](../api/basic_json/parse.md) would, and converted from there.
Two conversions never copy at all:
- [`get_string()`](../api/basic_json_view/get_string.md) (equivalently, `#!cpp get<string_view_t>()`) returns a
string as a `string_view_t` pointing into the document's [`source()`](../api/basic_json_document/source.md) text --
or, for a string that contains escape sequences, into the document's own buffer of decoded strings -- instead of
allocating a new `#!cpp std::string`.
- [`number_token()`](../api/basic_json_view/number_token.md) returns a number exactly as it was written in the
source, e.g. `#!cpp "1.50"`, `#!cpp "1E2"`, or an integer with more digits than any number type holds, instead of
rounding it into a `#!cpp double`/`#!cpp int64_t` the way `#!cpp get<T>()` (and
[`basic_json::parse()`](../api/basic_json/parse.md)) would.
Both results are only valid as long as the view -- and, for a string with no escapes, the borrowed source text -- is.
## Choosing between `json`, `ordered_json`, the SAX interface, and `json_view`