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Restore a duplicate key's prior value when the callback rejects its new value #5466

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Merged
nlohmann merged 2 commits into from
Sep 22, 2026
Merged

Restore a duplicate key's prior value when the callback rejects its new value #5466

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9e483bf
Restore a duplicate key's prior value when the callback rejects its n…
nlohmann Sep 5, 2026
31b281f
Mark parser-callback test lambdas noexcept to fix GCC -Wnoexcept -Werror
nlohmann Sep 5, 2026
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108 changes: 94 additions & 14 deletions include/nlohmann/detail/input/json_sax.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -8,11 +8,11 @@

#pragma once

#include <algorithm> // min
#include <algorithm> // find_if, min
#include <cstddef>
#include <string> // string
#include <type_traits> // enable_if_t
#include <utility> // move
#include <utility> // move, pair
#include <vector> // vector

#include <nlohmann/detail/exceptions.hpp>
Expand Down Expand Up @@ -631,7 +631,17 @@ class json_sax_dom_callback_parser
// add discarded value at the given key and store the reference for later
if (keep && ref_stack.back())
{
object_element = &(ref_stack.back()->m_data.m_value.object->operator[](val) = discarded);
auto& obj = *ref_stack.back()->m_data.m_value.object;
const auto it = obj.find(val);
if (it != obj.end())
{
// this is a duplicate key (legal in JSON); remember its
// current value so it can be restored later if the new
// value is rejected by the callback, instead of being
// erased together with the discarded placeholder
duplicate_key_stash.emplace_back(&(it->second), it->second);
}
object_element = &(obj[val] = discarded);
}

return true;
Expand All @@ -643,13 +653,18 @@ class json_sax_dom_callback_parser
{
if (!callback(static_cast<int>(ref_stack.size()) - 1, parse_event_t::object_end, *ref_stack.back()))
{
// discard object
*ref_stack.back() = discarded;
// discard object, unless this slot holds a duplicate key's
// previous value pending restoration, in which case that
// value is restored instead of being discarded
if (!resolve_duplicate_key_stash(ref_stack.back(), true))
{
*ref_stack.back() = discarded;

#if JSON_DIAGNOSTIC_POSITIONS
// Set start/end positions for discarded object.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
// Set start/end positions for discarded object.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
#endif
}
}
else
{
Expand All @@ -663,6 +678,10 @@ class json_sax_dom_callback_parser
#endif

ref_stack.back()->set_parents();
// this object is finally, definitively kept; drop any
// pending duplicate-key stash entry for its slot since it
// can no longer be restored
resolve_duplicate_key_stash(ref_stack.back(), false);
}
}

Expand Down Expand Up @@ -743,16 +762,25 @@ class json_sax_dom_callback_parser
#endif

ref_stack.back()->set_parents();
// this array is finally, definitively kept; drop any
// pending duplicate-key stash entry for its slot since it
// can no longer be restored
resolve_duplicate_key_stash(ref_stack.back(), false);
}
else
{
// discard array
*ref_stack.back() = discarded;
// discard array, unless this slot holds a duplicate key's
// previous value pending restoration, in which case that
// value is restored instead of being discarded
if (!resolve_duplicate_key_stash(ref_stack.back(), true))
{
*ref_stack.back() = discarded;

#if JSON_DIAGNOSTIC_POSITIONS
// Set start/end positions for discarded array.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
// Set start/end positions for discarded array.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
#endif
}
}
}

Expand Down Expand Up @@ -869,6 +897,35 @@ class json_sax_dom_callback_parser
}
#endif

/// if there is a pending duplicate-key stash entry for this exact slot,
/// remove it from the stash; if restore_value is true, the stashed
/// previous value is moved back into the slot first (use this when the
/// new value at that slot was rejected); otherwise the stash entry is
/// simply dropped (use this when the new value was accepted, so it
/// correctly supersedes the old one and no restore should ever happen
/// for this slot again)
/// @return whether a matching stash entry was found (and processed)
bool resolve_duplicate_key_stash(BasicJsonType* slot, bool restore_value)
{
const auto it = std::find_if(duplicate_key_stash.begin(), duplicate_key_stash.end(),
[slot](const std::pair<BasicJsonType*, BasicJsonType>& entry)
{
return entry.first == slot;
});

if (it == duplicate_key_stash.end())
{
return false;
}

if (restore_value)
{
*slot = std::move(it->second);
}
duplicate_key_stash.erase(it);
return true;
}

/*!
@brief the key the value now being handled will be stored under

Expand All @@ -887,7 +944,9 @@ class json_sax_dom_callback_parser
}

/*!
@brief remove the discarded value the callback rejected from its parent
@brief remove the discarded value the callback rejected from its parent,
unless it is a duplicate key's slot with a stashed previous value, in
which case that previous value is restored instead

A rejected value can only ever be the one most recently added to @a parent:
the last element of an array, or the placeholder key() stored under @a key
Expand All @@ -902,7 +961,7 @@ class json_sax_dom_callback_parser
@param[in,out] parent the container to remove the rejected value from
@param[in] key the key the value was stored under; unused for arrays
*/
static void remove_discarded_value(BasicJsonType& parent, const string_t& key)
void remove_discarded_value(BasicJsonType& parent, const string_t& key)
{
if (parent.is_array())
{
Expand All @@ -918,7 +977,12 @@ class json_sax_dom_callback_parser
const auto it = object.find(key);
if (it != object.end() && it->second.is_discarded())
{
object.erase(it);
// a duplicate key's slot has a stashed previous value that
// must be restored instead of being erased
if (!resolve_duplicate_key_stash(&it->second, true))
{
object.erase(it);
}
}
}
}
Expand Down Expand Up @@ -1020,6 +1084,16 @@ class json_sax_dom_callback_parser

JSON_ASSERT(object_element);
*object_element = std::move(value);
if (!skip_callback)
{
// this scalar value finally, definitively replaces whatever was
// at this slot; drop any pending duplicate-key stash entry for
// it since it can no longer be restored (a container value at
// this slot is resolved later, in end_object()/end_array(),
// since skip_callback is true for the placeholder handling that
// happens here for those)
resolve_duplicate_key_stash(object_element, false);
}
return {true, object_element};
}

Expand All @@ -1039,6 +1113,12 @@ class json_sax_dom_callback_parser
std::vector<string_t> container_key_stack {}; // NOLINT(readability-redundant-member-init)
/// helper to hold the reference for the next object element
BasicJsonType* object_element = nullptr;
/// stash of (slot pointer, previous value) for object members that
/// already existed when key() was called again for the same key
/// (duplicate keys); used to restore the previous value if the new
/// value is later rejected by the callback, instead of erasing the
/// member entirely
std::vector<std::pair<BasicJsonType*, BasicJsonType>> duplicate_key_stash {};
/// whether a syntax error occurred
bool errored = false;
/// callback function
Expand Down
108 changes: 94 additions & 14 deletions single_include/nlohmann/json.hpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -7951,11 +7951,11 @@ NLOHMANN_JSON_NAMESPACE_END



#include <algorithm> // min
#include <algorithm> // find_if, min
#include <cstddef>
#include <string> // string
#include <type_traits> // enable_if_t
#include <utility> // move
#include <utility> // move, pair
#include <vector> // vector

// #include <nlohmann/detail/exceptions.hpp>
Expand Down Expand Up @@ -11404,7 +11404,17 @@ class json_sax_dom_callback_parser
// add discarded value at the given key and store the reference for later
if (keep && ref_stack.back())
{
object_element = &(ref_stack.back()->m_data.m_value.object->operator[](val) = discarded);
auto& obj = *ref_stack.back()->m_data.m_value.object;
const auto it = obj.find(val);
if (it != obj.end())
{
// this is a duplicate key (legal in JSON); remember its
// current value so it can be restored later if the new
// value is rejected by the callback, instead of being
// erased together with the discarded placeholder
duplicate_key_stash.emplace_back(&(it->second), it->second);
}
object_element = &(obj[val] = discarded);
}

return true;
Expand All @@ -11416,13 +11426,18 @@ class json_sax_dom_callback_parser
{
if (!callback(static_cast<int>(ref_stack.size()) - 1, parse_event_t::object_end, *ref_stack.back()))
{
// discard object
*ref_stack.back() = discarded;
// discard object, unless this slot holds a duplicate key's
// previous value pending restoration, in which case that
// value is restored instead of being discarded
if (!resolve_duplicate_key_stash(ref_stack.back(), true))
{
*ref_stack.back() = discarded;

#if JSON_DIAGNOSTIC_POSITIONS
// Set start/end positions for discarded object.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
// Set start/end positions for discarded object.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
#endif
}
}
else
{
Expand All @@ -11436,6 +11451,10 @@ class json_sax_dom_callback_parser
#endif

ref_stack.back()->set_parents();
// this object is finally, definitively kept; drop any
// pending duplicate-key stash entry for its slot since it
// can no longer be restored
resolve_duplicate_key_stash(ref_stack.back(), false);
}
}

Expand Down Expand Up @@ -11516,16 +11535,25 @@ class json_sax_dom_callback_parser
#endif

ref_stack.back()->set_parents();
// this array is finally, definitively kept; drop any
// pending duplicate-key stash entry for its slot since it
// can no longer be restored
resolve_duplicate_key_stash(ref_stack.back(), false);
}
else
{
// discard array
*ref_stack.back() = discarded;
// discard array, unless this slot holds a duplicate key's
// previous value pending restoration, in which case that
// value is restored instead of being discarded
if (!resolve_duplicate_key_stash(ref_stack.back(), true))
{
*ref_stack.back() = discarded;

#if JSON_DIAGNOSTIC_POSITIONS
// Set start/end positions for discarded array.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
// Set start/end positions for discarded array.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
#endif
}
}
}

Expand Down Expand Up @@ -11642,6 +11670,35 @@ class json_sax_dom_callback_parser
}
#endif

/// if there is a pending duplicate-key stash entry for this exact slot,
/// remove it from the stash; if restore_value is true, the stashed
/// previous value is moved back into the slot first (use this when the
/// new value at that slot was rejected); otherwise the stash entry is
/// simply dropped (use this when the new value was accepted, so it
/// correctly supersedes the old one and no restore should ever happen
/// for this slot again)
/// @return whether a matching stash entry was found (and processed)
bool resolve_duplicate_key_stash(BasicJsonType* slot, bool restore_value)
{
const auto it = std::find_if(duplicate_key_stash.begin(), duplicate_key_stash.end(),
[slot](const std::pair<BasicJsonType*, BasicJsonType>& entry)
{
return entry.first == slot;
});

if (it == duplicate_key_stash.end())
{
return false;
}

if (restore_value)
{
*slot = std::move(it->second);
}
duplicate_key_stash.erase(it);
return true;
}

/*!
@brief the key the value now being handled will be stored under

Expand All @@ -11660,7 +11717,9 @@ class json_sax_dom_callback_parser
}

/*!
@brief remove the discarded value the callback rejected from its parent
@brief remove the discarded value the callback rejected from its parent,
unless it is a duplicate key's slot with a stashed previous value, in
which case that previous value is restored instead

A rejected value can only ever be the one most recently added to @a parent:
the last element of an array, or the placeholder key() stored under @a key
Expand All @@ -11675,7 +11734,7 @@ class json_sax_dom_callback_parser
@param[in,out] parent the container to remove the rejected value from
@param[in] key the key the value was stored under; unused for arrays
*/
static void remove_discarded_value(BasicJsonType& parent, const string_t& key)
void remove_discarded_value(BasicJsonType& parent, const string_t& key)
{
if (parent.is_array())
{
Expand All @@ -11691,7 +11750,12 @@ class json_sax_dom_callback_parser
const auto it = object.find(key);
if (it != object.end() && it->second.is_discarded())
{
object.erase(it);
// a duplicate key's slot has a stashed previous value that
// must be restored instead of being erased
if (!resolve_duplicate_key_stash(&it->second, true))
{
object.erase(it);
}
}
}
}
Expand Down Expand Up @@ -11793,6 +11857,16 @@ class json_sax_dom_callback_parser

JSON_ASSERT(object_element);
*object_element = std::move(value);
if (!skip_callback)
{
// this scalar value finally, definitively replaces whatever was
// at this slot; drop any pending duplicate-key stash entry for
// it since it can no longer be restored (a container value at
// this slot is resolved later, in end_object()/end_array(),
// since skip_callback is true for the placeholder handling that
// happens here for those)
resolve_duplicate_key_stash(object_element, false);
}
return {true, object_element};
}

Expand All @@ -11812,6 +11886,12 @@ class json_sax_dom_callback_parser
std::vector<string_t> container_key_stack {}; // NOLINT(readability-redundant-member-init)
/// helper to hold the reference for the next object element
BasicJsonType* object_element = nullptr;
/// stash of (slot pointer, previous value) for object members that
/// already existed when key() was called again for the same key
/// (duplicate keys); used to restore the previous value if the new
/// value is later rejected by the callback, instead of erasing the
/// member entirely
std::vector<std::pair<BasicJsonType*, BasicJsonType>> duplicate_key_stash {};
/// whether a syntax error occurred
bool errored = false;
/// callback function
Expand Down
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