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json/include/nlohmann/detail/input/parser.hpp
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Niels Lohmann da7b9bdb3d fix: restore the character that terminates a number (#5340) 
operator>> is documented to leave the stream positioned right after the
parsed value, so that concatenated JSON values can be read back to back.
That did not hold for numbers: a number is only terminated by the
character following it, and lexer::scan_number() reads that character
and calls unget() -- which is simulated and rewinds only the lexer's own
bookkeeping. input_stream_adapter consumes via sbumpc() with no matching
sungetc(), so the terminating character stayed consumed and the next
extraction started one byte too late ('1true' left the stream at 'rue').

Propagating unget() to the adapter directly does not work: next_unget
makes the following get() replay the cached character, so the terminator
would be delivered twice. Instead, restore the still-pending character
once at the end of a non-strict parse, where the input is handed back to
the caller:

- input_stream_adapter gains unget_character() (sungetc()) and advertises
  it via supports_unget, detected the same way as supports_seek.
- lexer::restore_pending_unget() turns a pending simulated unget of a
  real (non-EOF) character into a real one and clears next_unget so the
  character is not also replayed. It is a no-op for adapters that cannot
  unget, and reports failure when sungetc() fails, in which case the
  input is left as it was before.
- parser calls it on the three non-strict paths, i.e. for operator>> and
  sax_parse(strict = false).

Strict parse()/accept() are unaffected: they require the input to end
after the value, so the character is consumed by the end-of-input check
anyway. Parse error messages and reported positions are unchanged.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-08-01 07:33:11 +02:00

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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*/)>;
/*!
@brief syntax analysis
This class implements a recursive descent parser.
*/
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)
: callback(std::move(cb))
, m_lexer(std::move(adapter), ignore_comments)
, 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);
sax_parse_internal(&sdp);
if (!strict)
{
// the caller keeps using the input: position it right after
// the value by giving back the character that terminated it
m_lexer.restore_pending_unget();
}
// in strict mode, input must be completely read
else if (get_token() != token_type::end_of_input)
{
sdp.parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(),
exception_message(token_type::end_of_input, "value"), nullptr));
}
// in case of an error, return a discarded value
if (sdp.is_errored())
{
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> sdp(result, allow_exceptions, &m_lexer);
sax_parse_internal(&sdp);
if (!strict)
{
// see above
m_lexer.restore_pending_unget();
}
// in strict mode, input must be completely read
else if (get_token() != token_type::end_of_input)
{
sdp.parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr));
}
// in case of an error, return a discarded value
if (sdp.is_errored())
{
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 && !strict)
{
// the caller keeps using the input: position it right after the
// value by giving back the character that terminated it
m_lexer.restore_pending_unget();
}
// strict mode: next byte must be EOF
else if (result && strict && (get_token() != token_type::end_of_input))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr));
}
return result;
}
private:
template<typename SAX>
JSON_HEDLEY_NON_NULL(2)
bool sax_parse_internal(SAX* sax)
{
// stack to remember the hierarchy of structured values we are parsing
// true = array; false = object
std::vector<bool> states;
// value to avoid a goto (see comment where set to true)
bool skip_to_state_evaluation = false;
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 sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::value_string, "object key"), nullptr));
}
if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string())))
{
return false;
}
// parse separator (:)
if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr));
}
// remember we are now inside an object
states.push_back(false);
// 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(true);
// 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(406, concat("number overflow parsing '", m_lexer.get_token_string(), '\''), nullptr));
}
if (JSON_HEDLEY_UNLIKELY(!sax->number_float(res, m_lexer.get_string())))
{
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())))
{
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 sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::uninitialized, "value"), nullptr));
}
case token_type::end_of_input:
{
if (JSON_HEDLEY_UNLIKELY(m_lexer.get_position().chars_read_total == 1))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(),
"attempting to parse an empty input; check that your input string or stream contains the expected JSON", nullptr));
}
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::literal_or_value, "value"), nullptr));
}
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 sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::literal_or_value, "value"), nullptr));
}
}
}
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()) // array
{
// comma -> next value
// or end of array (ignore_trailing_commas = true)
if (get_token() == 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 false, we
// are effectively jumping to the beginning of this if.
JSON_ASSERT(!states.empty());
states.pop_back();
skip_to_state_evaluation = true;
continue;
}
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_array, "array"), nullptr));
}
// states.back() is false -> object
// comma -> next value
// or end of object (ignore_trailing_commas = true)
if (get_token() == 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 sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::value_string, "object key"), nullptr));
}
if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string())))
{
return false;
}
// parse separator (:)
if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr));
}
// 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 false, we
// are effectively jumping to the beginning of this if.
JSON_ASSERT(!states.empty());
states.pop_back();
skip_to_state_evaluation = true;
continue;
}
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_object, "object"), nullptr));
}
}
/// get next token from lexer
token_type get_token()
{
return last_token = m_lexer.scan();
}
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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