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json/include/nlohmann/detail/input/parser.hpp
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Niels Lohmann 9c50bdd140 Make the DOM parser cheaper per container and per member (#5882) 
* Reserve the parser's nesting stack once

The stack of open arrays and objects was a std::vector<std::uint8_t> that
allocated at the first push_back and then reallocated at 1, 2, 4, 8 bytes,
so a document nested four levels deep made four allocations before it
parsed a value. A document that starts with a container now reserves 32
levels in one allocation; deeper documents grow as before. The loop that
reads the tokens is the code it was, so large documents are not affected
(a stack object with its own storage, and a reserve at every container,
both cost the token loop instructions or cycles).

Fixed cost of parsing a 124-byte object (macOS instructions): accept
-7.6 %, DOM -2.5 %; no change for a single number.

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

* Build arrays of the DOM parser in one exact-size allocation

Reading an array of unknown length appended its elements to a std::vector
one at a time, so a small array reallocated (and moved its elements) at
sizes 1, 2, 4, ... The DOM SAX parser now collects the elements of arrays
whose length is unknown on one stack shared by all open arrays and moves
them into an array of exactly the right size when the closing bracket
arrives. An array that grows past 16 elements switches to appending to its
own vector, so large arrays pay no second copy. Binary formats, which
announce the length, and builds with JSON_DIAGNOSTICS or
JSON_DIAGNOSTIC_POSITIONS (whose values need their final place from the
start) keep storing into the array directly.

The stacks of open containers are reserved on first use instead of growing
from one element.

The binary readers get a DOM parser of their own, json_sax_dom_binary_parser:
it is the plain parser that stores every value directly in its container
(the code the readers used before). Their containers announce their size, so
the bookkeeping of the collecting parser (frame entries, the destination of a
finished container) was pure overhead for them: with the collecting parser
FromBinaryShape nested was 3-12 % and containers up to 5 % more instructions
(GCC) than before. With the plain parser Linux Ir of FromBinaryShape and
FromBinaryBuffer is within 0.8 % of develop (Clang: identical).

canada.json has 56 000 two-element arrays that took three allocations each
and now take two. DOM parse Ir on Linux against develop (GCC / Clang): canada
-7.0 % / -7.7 %, citm -0.8 % / -1.6 %, 124-byte object -5.4 % / -5.5 %;
accept and SAX are untouched.

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

* Reuse one value for the key events of the callback parser

parse() with a callback built a new string value for every key it passed to
the callback (a heap allocation and release per key), and copied the key a
second time into the object. The callback parser now keeps one string value
for key events, which it assigns, and moves the key into the object.

Callback parse, Linux Ir (GCC / Clang): twitter -13.3 % / -11.7 %, citm
-9.5 % / -7.6 %. (Earlier macOS measurement: twitter -19.5 % instructions.)

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

* Remember where the DOM parser stores the next value

The DOM SAX parser looked up the innermost open container for every value:
the end of its frame stack, then the object or array pointer in the frame.
Compilers have to reload all of that after each value is stored, since a
store of a value may alias the frame. The parser now keeps the innermost
container's kind and, for an array that stores directly, the array itself
in members, updated when a container opens or closes.

Flat arrays of numbers are the case that mattered: parsing floats.json
(1 000 000 doubles) into a DOM took 2 % more cycles than before the
previous commit on macOS and now takes 0.4 % fewer than the code before it
(signed_ints.json -0.8 %). Linux Ir is unchanged (canada -6.1 % GCC /
-6.9 % Clang, citm -0.3 % / -0.8 %, twitter +0.2 % / 0.0 %).

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

* Look up the keys of big ordered_json objects in a hash table

ordered_map keeps its elements in a vector and searches it linearly for
every key it adds, so parsing an object with n keys into ordered_json took
O(n^2) key comparisons: 33 billion instructions for one object with 30 000
keys, where json needs 245 million. From 64 keys on, the DOM parser now
keeps a hash table of the keys of the open object (hashes and positions
only; the elements stay in the vector) and finds duplicate keys there. The
result is the same, including the position of a key that occurs twice.
Smaller objects, std::map based objects and key types without std::hash
are untouched.

ordered_json parse, macOS instructions: object with 1 000 keys 64.8 M ->
14.6 M, 10 000 keys 5 104 M -> 38.7 M, 30 000 keys 33 358 M -> 89 M.
Linux Ir of an object with 2 000 keys: 43.0 M -> 1.9 M (GCC), 37.9 M ->
1.8 M (Clang).

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

* Trim the work the DOM parser does per container and per member

Opening and closing an array or object cost the DOM parser more than before
the exact-size array change, which showed on documents of many small
objects (jeopardy.json: +1.4 % Ir with GCC). This takes it back:

- the stack of open containers keeps its first eight entries in the parser
  object instead of in a vector, so it needs no capacity check beyond the
  push itself and no allocation for typical documents;
- a container is created in the place it ends up in, instead of as a
  temporary that is then moved there;
- a new object member, which is null, is constructed over the null instead
  of being swapped with it (only with the default base class of basic_json);
- the key index that big ordered_json objects use is only a part of the
  stack entries of objects that can have one;
- an array that outgrows the stack reserves twice what it has so far, as a
  vector that grew by itself would have.

DOM parse Ir of the series up to here against develop, Linux GCC / Clang:
canada -6.6 % / -7.3 %, citm +1.2 % / +0.2 %, jeopardy +0.4 % / -0.1 %,
124-byte object -6.5 % / -6.2 %. (twitter measures +6.8 % / +6.6 % at this
point in steady state, almost all of it in glibc's malloc/consolidate for the
new allocation sizes; with the last commit of the series it is -0.1 % / 0.0 %.)

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

* Test parsing containers of the sizes where the DOM parser changes storage

The DOM parser collects the elements of an array on a stack (up to 16) and
the members of an ordered_json object (up to 32) before it moves them into
the container, and uses a hash table for keys from 64 members on. Parse
arrays and objects of the sizes around these limits, nested into each other,
with repeated keys, and as random trees that are built next to their text,
and compare json and ordered_json against the value built through the API.

The tests pass on the code from before these changes as well. The random
trees are what finds the mistakes that are easy to make here: while the
collecting change was being written, a collected array that outgrew the
stack at the moment its parent had just reached the limit left its elements
in the parent, and only these tests (142 failed assertions) showed it.

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

* Do not convert floats that cannot overflow in accept()

accept() only has to find out whether a float token is finite: json_sax_acceptor
ignores the value, and the parser rejects only a float that overflows. A
float whose digits fit into 19 digits and whose decimal exponent is at most
max_exponent10 - 19 is below 10^max_exponent10 and cannot overflow, so the
lexer, told by accept() that the value is not needed, skips the conversion
for those. A token that may overflow, with more digits, or in another float
format is converted as before, so 1e309 and its neighbours are rejected as
they were (the new test compares accept() with parse() on tokens around the
limit).

json::accept(): canada.json Ir -8.6 % (GCC) / -10.9 % (Clang), 124-byte
object -3.2 %; cycles on the M1 (release benchmarks, LTO): canada -7.8 %,
floats.json -9.0 %. The check tests the accept() flag first, so the parser
that needs the value pays one load and branch: SaxParse canada +0.35 % Ir
(GCC and Clang), DOM canada +0.4 %.

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

* Collect the members of ordered_json objects on a stack until they are known

An ordered_json object keeps its members in a vector and appends every new
member to it, so reading an object of unknown size reallocated the vector at
1, 2, 4, 8, ... members, and ordered_map copies all keys on every
reallocation. The DOM SAX parser now collects the members of such objects on
one stack shared by all open objects, as it does the elements of arrays, and
moves them into a vector of exactly the right size when the closing brace
arrives. An object that gets past 32 members switches to appending to its
own vector (and, from 64, to the key index of the earlier commit). The
members are the same, in the same order, and a key that occurs twice keeps
the place of its first occurrence. std::map based objects, objects whose
size the input announces (binary formats), and builds with JSON_DIAGNOSTICS
or JSON_DIAGNOSTIC_POSITIONS are untouched.

What only ordered objects need (the collected members, the key indexes) is
a member of the parser only for them, and the code that stores a value tries
the array that takes its elements directly first. An array that outgrows
the stack now reserves 32 elements, the capacity a vector that grew by
itself has at that point; with the larger reserve before, parsing
floats.json (1 000 000 numbers) was 2 % slower on Apple M1 than before this
series.

ordered_json parse, Linux Ir against develop (GCC / Clang): citm -11.6 % /
-11.7 %, twitter -8.4 % / -8.4 %, jeopardy (12 000 objects of 7 keys)
-10.5 % / -8.0 %, canada -3.9 % / -5.2 %, 124-byte object -20.9 % / -16.5 %.
macOS time (M1, earlier base): citm -19 %, twitter -13 %, jeopardy -13 %,
canada -14 %, 124-byte object -26 %. json parse: steady-state twitter -0.1 %
(see the previous commits), citm -1.2 %.

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

* Rename the key index's match callback so Flawfinder doesn't read it as std::equal

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

* Bind the parser test's strings by reference in the range-for loop

GCC and Clang 14 (-Wrange-loop-construct) reported the copy.

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

* Collect container values only in basic_json's own parse functions

A json_sax_dom_parser that a user constructs (as in the sax_no_exception
example) must hold the values read before a parse error, as on develop:
its value is used after the error. The values of containers of unknown
size are now collected on a stack only with the new template parameter
Collect (default false), which json::parse() and the json_view
materialization set; without it, every value is stored in place as it
arrives. A test checks the values kept after an error for json and
ordered_json.

Also fixes the clang-tidy, cpplint and Infer findings: the frame stack
keeps its first frames in a std::array (left uninitialized) and the
others in a std::unique_ptr, auto for create<>(), NOLINT for member
initializers -Weffc++ needs, and an @infer-ignore for the nullptr that
place_container() does not use.

json::parse() and ordered_json::parse() instructions are unchanged
(Apple clang; twitter, citm, canada, jeopardy: -0.03 % to +0.25 %).

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

* Clarify the DOM parser's collection stacks and rename reenter_enclosing_container()

Address review comments: explain why a closed container erases only the top
of the shared stack, set the payload before the type, rename the helper that
makes the enclosing container current again, and declare the parser's state
stack right before reserving it.

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

* Export json_sax_dom_parser from the C++20 module for MSVC

MSVC needs the detail classes that basic_json befriends to be visible in
the module (#3970, as json_sax_dom_callback_parser); with its new
template parameter, json_sax_dom_parser failed in ci_module_cpp20
(C2039, not a member of nlohmann::detail).

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

* Keep the overflow-limit numbers of the parser test in a std::vector

clang-tidy (cppcoreguidelines-avoid-c-arrays) rejected the C array.

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

* Fix the clang-tidy findings of the ordered_json parser tests

The container sizes are a std::array, the expected text uses raw string
literals, the generator's arithmetic is in parentheses, auto for the
result of make<>(), and single characters are appended as characters.
clang-tidy now reports nothing on the lines the branch adds to
unit-ordered_json.cpp and unit-class_parser.cpp (apart from checks CI's
clang-tidy does not have).

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

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-10-24 07:41:21 +02:00

612 lines
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// __ _____ _____ _____
// __| | __| | | | JSON for Modern C++
// | | |__ | | | | | | version 3.12.0
// |_____|_____|_____|_|___| https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT
#pragma once
#include <cmath> // isfinite
#include <cstdint> // uint8_t
#include <functional> // function
#include <string> // string
#include <utility> // move
#include <vector> // vector
#include <nlohmann/detail/exceptions.hpp>
#include <nlohmann/detail/input/input_adapters.hpp>
#include <nlohmann/detail/input/json_sax.hpp>
#include <nlohmann/detail/input/lexer.hpp>
#include <nlohmann/detail/macro_scope.hpp>
#include <nlohmann/detail/meta/is_sax.hpp>
#include <nlohmann/detail/string_concat.hpp>
#include <nlohmann/detail/value_t.hpp>
NLOHMANN_JSON_NAMESPACE_BEGIN
namespace detail
{
////////////
// parser //
////////////
enum class parse_event_t : std::uint8_t
{
/// the parser read `{` and started to process a JSON object
object_start,
/// the parser read `}` and finished processing a JSON object
object_end,
/// the parser read `[` and started to process a JSON array
array_start,
/// the parser read `]` and finished processing a JSON array
array_end,
/// the parser read a key of a value in an object
key,
/// the parser finished reading a JSON value
value
};
template<typename BasicJsonType>
using parser_callback_t =
std::function<bool(int /*depth*/, parse_event_t /*event*/, BasicJsonType& /*parsed*/)>;
/// whether a SAX handler ignores the text of float tokens (the library's own
/// handlers do), so that the parser need not materialize it
template<typename SAX>
struct sax_ignores_number_text : std::false_type {};
template<typename BasicJsonType, typename InputAdapterType, bool Collect>
struct sax_ignores_number_text<json_sax_dom_parser<BasicJsonType, InputAdapterType, Collect>> : std::true_type {};
template<typename BasicJsonType, typename InputAdapterType>
struct sax_ignores_number_text<json_sax_dom_callback_parser<BasicJsonType, InputAdapterType>> : std::true_type {};
template<typename BasicJsonType>
struct sax_ignores_number_text<json_sax_acceptor<BasicJsonType>> : std::true_type {};
/*!
@brief syntax analysis
This class implements a parser for JSON text. Nested arrays and objects are tracked with an explicit
stack instead of recursion, so deeply nested input does not exhaust the call stack, and what is read
is reported as SAX events.
*/
template<typename BasicJsonType, typename InputAdapterType>
class parser
{
using number_integer_t = typename BasicJsonType::number_integer_t;
using number_unsigned_t = typename BasicJsonType::number_unsigned_t;
using number_float_t = typename BasicJsonType::number_float_t;
using string_t = typename BasicJsonType::string_t;
using lexer_t = lexer<BasicJsonType, InputAdapterType>;
using token_type = typename lexer_t::token_type;
public:
/// a parser reading from an input adapter
explicit parser(InputAdapterType&& adapter,
parser_callback_t<BasicJsonType> cb = nullptr,
const bool allow_exceptions_ = true,
const bool ignore_comments = false,
const bool ignore_trailing_commas_ = false,
const bool discard_number_values_ = false)
: callback(std::move(cb))
, m_lexer(std::move(adapter), ignore_comments, discard_number_values_)
, allow_exceptions(allow_exceptions_)
, ignore_trailing_commas(ignore_trailing_commas_)
{
// read first token
get_token();
}
/*!
@brief public parser interface
@param[in] strict whether to expect the last token to be EOF
@param[in,out] result parsed JSON value
@throw parse_error.101 in case of an unexpected token (including invalid
unicode escapes and surrogate errors, which are reported with a
detailed message)
*/
void parse(const bool strict, BasicJsonType& result)
{
if (callback)
{
json_sax_dom_callback_parser<BasicJsonType, InputAdapterType> sdp(result, callback, allow_exceptions, &m_lexer);
// in case of an error, return a discarded value
if (!parse_dom(sdp, strict))
{
result = value_t::discarded;
return;
}
// set top-level value to null if it was discarded by the callback
// function
if (result.is_discarded())
{
result = nullptr;
}
}
else
{
json_sax_dom_parser<BasicJsonType, InputAdapterType, true> sdp(result, allow_exceptions, &m_lexer);
// in case of an error, return a discarded value
if (!parse_dom(sdp, strict))
{
result = value_t::discarded;
return;
}
}
result.assert_invariant();
}
/*!
@brief public accept interface
@param[in] strict whether to expect the last token to be EOF
@return whether the input is a proper JSON text
*/
bool accept(const bool strict = true)
{
json_sax_acceptor<BasicJsonType> sax_acceptor;
return sax_parse(&sax_acceptor, strict);
}
template<typename SAX>
JSON_HEDLEY_NON_NULL(2)
bool sax_parse(SAX* sax, const bool strict = true)
{
(void)detail::is_sax_static_asserts<SAX, BasicJsonType> {};
const bool result = sax_parse_internal(sax);
if (result)
{
if (strict)
{
// strict mode: next byte must be EOF
if (get_token() != token_type::end_of_input)
{
return syntax_error(*sax, exception_message(token_type::end_of_input, "value"));
}
}
else
{
// the caller keeps using the input: position it right after
// the value by leaving the character that terminated it
m_lexer.release_lookahead();
}
}
return result;
}
private:
/// the text of the current float token for @a sax (see sax_ignores_number_text)
template<typename SAX>
const string_t& number_float_text(SAX* /*sax*/)
{
return number_float_text(std::integral_constant<bool, sax_ignores_number_text<SAX>::value> {});
}
const string_t& number_float_text(std::true_type /*ignored*/) const noexcept
{
return m_lexer.get_ignored_number_text();
}
string_t& number_float_text(std::false_type /*ignored*/)
{
return m_lexer.get_string();
}
/*!
@brief run a DOM SAX parser to completion and position the lexer
Shared by both branches of @ref parse(): builds no SAX parser itself,
but drives an already-constructed @a json_sax_dom_parser or
@ref json_sax_dom_callback_parser through @ref sax_parse_internal(),
then applies the strict-EOF check (reporting parse_error.101 through
@a sdp on failure) or, in non-strict mode, releases the lookahead so
the caller can keep reading the input right after the parsed value.
@param[in,out] sdp the DOM SAX parser to run
@param[in] strict whether to expect the last token to be EOF
@return whether @a sdp did not report an error
*/
template<typename DomSax>
bool parse_dom(DomSax& sdp, const bool strict)
{
sax_parse_internal(&sdp);
if (strict)
{
// in strict mode, input must be completely read
if (get_token() != token_type::end_of_input)
{
syntax_error(sdp, exception_message(token_type::end_of_input, "value"));
}
}
else
{
// the caller keeps using the input: position it right after
// the value by leaving the character that terminated it
m_lexer.release_lookahead();
}
return !sdp.is_errored();
}
template<typename SAX>
JSON_HEDLEY_NON_NULL(2)
bool sax_parse_internal(SAX* sax)
{
// value to avoid a goto (see comment where set to true)
bool skip_to_state_evaluation = false;
// stack to remember the hierarchy of structured values we are parsing
// true = array; false = object (bytes, not std::vector<bool>, whose
// push_back() is a call with GCC)
std::vector<std::uint8_t> states;
// a document that starts with a container pushes onto the stack at
// once: allocate it in one step instead of growing it from 1 byte
if (last_token == token_type::begin_object || last_token == token_type::begin_array)
{
states.reserve(32);
}
while (true)
{
if (!skip_to_state_evaluation)
{
// invariant: get_token() was called before each iteration
switch (last_token)
{
case token_type::begin_object:
{
if (JSON_HEDLEY_UNLIKELY(!sax->start_object(detail::unknown_size())))
{
return false;
}
// closing } -> we are done
if (get_token() == token_type::end_object)
{
if (JSON_HEDLEY_UNLIKELY(!sax->end_object()))
{
return false;
}
break;
}
// parse key
if (JSON_HEDLEY_UNLIKELY(last_token != token_type::value_string))
{
return syntax_error(*sax, exception_message(token_type::value_string, "object key"));
}
if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string_token())))
{
return false;
}
// parse separator (:)
if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
{
return syntax_error(*sax, exception_message(token_type::name_separator, "object separator"));
}
// remember we are now inside an object
states.push_back(0);
// parse values
get_token();
continue;
}
case token_type::begin_array:
{
if (JSON_HEDLEY_UNLIKELY(!sax->start_array(detail::unknown_size())))
{
return false;
}
// closing ] -> we are done
if (get_token() == token_type::end_array)
{
if (JSON_HEDLEY_UNLIKELY(!sax->end_array()))
{
return false;
}
break;
}
// remember we are now inside an array
states.push_back(1);
// parse values (no need to call get_token)
continue;
}
case token_type::value_float:
{
const auto res = m_lexer.get_number_float();
if (JSON_HEDLEY_UNLIKELY(!std::isfinite(res)))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
out_of_range::create(exception_id::number_overflow, concat("number overflow parsing '", m_lexer.get_token_string(), '\''), nullptr));
}
if (JSON_HEDLEY_UNLIKELY(!sax->number_float(res, number_float_text(sax))))
{
return false;
}
break;
}
case token_type::literal_false:
{
if (JSON_HEDLEY_UNLIKELY(!sax->boolean(false)))
{
return false;
}
break;
}
case token_type::literal_null:
{
if (JSON_HEDLEY_UNLIKELY(!sax->null()))
{
return false;
}
break;
}
case token_type::literal_true:
{
if (JSON_HEDLEY_UNLIKELY(!sax->boolean(true)))
{
return false;
}
break;
}
case token_type::value_integer:
{
if (JSON_HEDLEY_UNLIKELY(!sax->number_integer(m_lexer.get_number_integer())))
{
return false;
}
break;
}
case token_type::value_string:
{
if (JSON_HEDLEY_UNLIKELY(!sax->string(m_lexer.get_string_token())))
{
return false;
}
break;
}
case token_type::value_unsigned:
{
if (JSON_HEDLEY_UNLIKELY(!sax->number_unsigned(m_lexer.get_number_unsigned())))
{
return false;
}
break;
}
case token_type::parse_error:
{
// using "uninitialized" to avoid an "expected" message
return syntax_error(*sax, exception_message(token_type::uninitialized, "value"));
}
case token_type::end_of_input:
{
if (JSON_HEDLEY_UNLIKELY(m_lexer.get_chars_read_total() == 1))
{
return syntax_error(*sax, "attempting to parse an empty input; check that your input string or stream contains the expected JSON");
}
return syntax_error(*sax, exception_message(token_type::literal_or_value, "value"));
}
case token_type::uninitialized:
case token_type::end_array:
case token_type::end_object:
case token_type::name_separator:
case token_type::value_separator:
case token_type::literal_or_value:
default: // the last token was unexpected
{
return syntax_error(*sax, exception_message(token_type::literal_or_value, "value"));
}
}
}
else
{
skip_to_state_evaluation = false;
}
// we reached this line after we successfully parsed a value
if (states.empty())
{
// empty stack: we reached the end of the hierarchy: done
return true;
}
if (states.back() != 0) // array
{
// comma -> next value
// or end of array (ignore_trailing_commas = true)
if (get_token_expecting(token_type::value_separator))
{
// parse a new value
get_token();
// if ignore_trailing_commas and last_token is ], we can continue to "closing ]"
if (!(ignore_trailing_commas && last_token == token_type::end_array))
{
continue;
}
}
// closing ]
if (JSON_HEDLEY_LIKELY(last_token == token_type::end_array))
{
if (JSON_HEDLEY_UNLIKELY(!sax->end_array()))
{
return false;
}
// We are done with this array. Before we can parse a
// new value, we need to evaluate the new state first.
// By setting skip_to_state_evaluation to true, the next
// iteration skips parsing a value and evaluates the
// enclosing state directly.
JSON_ASSERT(!states.empty());
states.pop_back();
skip_to_state_evaluation = true;
continue;
}
return syntax_error(*sax, exception_message(token_type::end_array, "array"));
}
// states.back() is false -> object
// comma -> next value
// or end of object (ignore_trailing_commas = true)
if (get_token_expecting(token_type::value_separator))
{
get_token();
// if ignore_trailing_commas and last_token is }, we can continue to "closing }"
if (!(ignore_trailing_commas && last_token == token_type::end_object))
{
// parse key
if (JSON_HEDLEY_UNLIKELY(last_token != token_type::value_string))
{
return syntax_error(*sax, exception_message(token_type::value_string, "object key"));
}
if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string_token())))
{
return false;
}
// parse separator (:)
if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
{
return syntax_error(*sax, exception_message(token_type::name_separator, "object separator"));
}
// parse values
get_token();
continue;
}
}
// closing }
if (JSON_HEDLEY_LIKELY(last_token == token_type::end_object))
{
if (JSON_HEDLEY_UNLIKELY(!sax->end_object()))
{
return false;
}
// We are done with this object. Before we can parse a
// new value, we need to evaluate the new state first.
// By setting skip_to_state_evaluation to true, the next
// iteration skips parsing a value and evaluates the
// enclosing state directly.
JSON_ASSERT(!states.empty());
states.pop_back();
skip_to_state_evaluation = true;
continue;
}
return syntax_error(*sax, exception_message(token_type::end_object, "object"));
}
}
/// get next token from lexer
token_type get_token()
{
return last_token = m_lexer.scan();
}
/// get next token from lexer; true if it is the separator @a expected_type
/// (name_separator or value_separator), which it usually is
bool get_token_expecting(token_type expected_type)
{
return (last_token = m_lexer.scan_expecting(expected_type)) == expected_type;
}
/*!
@brief reports a syntax error at the current token (parse_error.101)
@param[in] sax the SAX parser to report the error to
@param[in] message the error message
@return the result of the SAX parser's parse_error()
*/
template<typename SAX>
bool syntax_error(SAX& sax, const std::string& message)
{
// MSVC 2015 reports C4100 (unreferenced parameter) when SAX::parse_error is static
static_cast<void>(sax);
// computing the position is O(input read so far) for seekable input
const position_t position = m_lexer.get_position();
return sax.parse_error(position, m_lexer.get_token_string(),
parse_error::create(exception_id::syntax_error, position, message, nullptr));
}
std::string exception_message(const token_type expected, const std::string& context)
{
std::string error_msg = "syntax error ";
if (!context.empty())
{
error_msg += concat("while parsing ", context, ' ');
}
error_msg += "- ";
if (last_token == token_type::parse_error)
{
error_msg += concat(m_lexer.get_error_message(), "; last read: '",
m_lexer.get_token_string(), '\'');
}
else
{
error_msg += concat("unexpected ", lexer_t::token_type_name(last_token));
}
if (expected != token_type::uninitialized)
{
error_msg += concat("; expected ", lexer_t::token_type_name(expected));
}
return error_msg;
}
private:
/// callback function
const parser_callback_t<BasicJsonType> callback = nullptr;
/// the type of the last read token
token_type last_token = token_type::uninitialized;
/// the lexer
lexer_t m_lexer;
/// whether to throw exceptions in case of errors
const bool allow_exceptions = true;
/// whether trailing commas in objects and arrays should be ignored (true) or signaled as errors (false)
const bool ignore_trailing_commas = false;
};
} // namespace detail
NLOHMANN_JSON_NAMESPACE_END
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