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Niels Lohmann da1ca7f9d7 Add dump() and comparisons to json_view

Add basic_json_view::dump() and the comparison operators, and read
floats from the parser's digit layout instead of rescanning the
token.

dump(indent, indent_char, ensure_ascii, number_format) writes a
value the way ordered_json::parse(text).dump() writes it for the
same arguments: members in document order, all of them should a
key occur more than once; strings escaped by the same rules, using
the library's scanning kernels; floats written with the library's
to_chars conversion, so the output equals basic_json's byte for
byte; integers copied from the source, where they are already
canonical, except -0, which parse() reads as 0. There is no
error_handler argument, because the view only holds valid UTF-8.
number_format::source copies numbers exactly as they appear in the
source (e.g. "1.50", "1E2", "-0"), which basic_json cannot provide.
operator<< takes the indentation from the stream width, as for
basic_json. The writer walks iteratively, so nesting depth is
limited by memory only.

operator== and operator!= compare two views, or a view and a
basic_json value in either order, by the rules basic_json's
operator== uses: numbers compare by value across their types,
objects compare by their members with duplicate keys resolved as
parse() resolves them, member order matters only where the object
type keeps one, and discarded views compare as discarded basic_json
values do, including under JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON.
Nothing is materialized except single scalars.

While parsing, the view now records where the integer digits, the
fraction digits, and the exponent of a float token are, so floats
and doubles with at most 19 digits are read from that layout with
the library's decimal_to_float() instead of rescanning the token.
Both round correctly, so the values are those of parse(). get<double>(),
materialize(), dump(), and the comparisons all use it.

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

2026-10-07 16:42:29 +02:00

5.9 KiB

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nlohmann::basic_json_view

Defined in header <nlohmann/json_view.hpp>

template<typename BasicJsonType>
class basic_json_view;

A read-only handle to one value of a basic_json_document: two pointers (a pointer to the document and a pointer into its index), trivially copyable. A view is valid as long as

the document is alive,
the document has not been re-parsed with read() (or parse() into it) or shrunk with shrink_to_fit() since the view was taken, and
if the document borrows its source text, that text is still alive.

Moving the document itself does not invalidate its views: the index is heap-allocated independently of the basic_json_document object.

basic_json_view provides the read-only part of the BasicJsonType interface: the type-inspection functions, element access, lookup, iteration, conversion, and comparison -- get<T>(), get_string(), number_token(), and materialize() to build the BasicJsonType value of a subtree on demand. operator[], at, contains, and value also accept a json_pointer. operator== and operator!= compare two views, or a view and a BasicJsonType value, without ever building a BasicJsonType value for a view; no ordering comparison (#!cpp operator<) is provided.

Template parameters

BasicJsonType: a specialization of basic_json, matching the basic_json_document the view was taken from.

Specializations

json_view - views of a json_document
ordered_json_view - views of an ordered_json_document

Member types

value_t - the JSON type enumeration, see basic_json::value_t
string_t, number_integer_t, number_unsigned_t, number_float_t, json_pointer - the corresponding member types of BasicJsonType
size_type - #!cpp std::size_t
string_view_t - #!cpp std::string_view on C++17 and newer, a minimal internal substitute otherwise
iterator, const_iterator - a forward iterator over the elements of an array or the member values of an object, in document order; both names refer to the same type, since a view is always read-only
item - a (key, value) pair produced by items()
number_format - how dump() writes numbers

Member functions

(constructor)

Object inspection

type - return the type of the value
type_name - return the type as string
is_null - return whether the value is null
is_boolean - return whether the value is a boolean
is_number - return whether the value is a number
is_number_integer - return whether the value is an integer number
is_number_unsigned - return whether the value is an unsigned integer number
is_number_float - return whether the value is a floating-point number
is_string - return whether the value is a string
is_array - return whether the value is an array
is_object - return whether the value is an object
is_binary - return whether the value is a binary array (always #!cpp false)
is_primitive - return whether the type is primitive
is_structured - return whether the type is structured
is_discarded - return whether the view is invalid
operator bool - return whether the view refers to a value

Element access

at - access specified element with bounds checking
operator[] - access specified element
value - access specified element with default value
front - access the first element
back - access the last element

Lookup

find - find an element in an object
count - returns the number of occurrences of a key in an object
contains - check the existence of an element in an object

Iterators

begin - returns an iterator to the first element
cbegin - returns a const iterator to the first element
end - returns an iterator to one past the last element
cend - returns a const iterator to one past the last element
items - wrapper to access iterator member functions in range-based for

Capacity

size - return the number of elements
empty - return whether the value has no elements

Conversion

get - get a value
get_to - get a value and write it to a destination
get_string - get a string value without a copy
number_token - get a number's token text without a copy
materialize - build the BasicJsonType value of this subtree

Comparison

operator== - comparison: equal
operator!= - comparison: not equal

Serialization

dump - serialize to a JSON-formatted string
operator<< - serialize to stream

Source access

source_offset - byte offset of this value in the document's source text

Version history

Added in version 3.13.0.

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