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Copy the diff frame's members instead of holding a reference to it

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The loop in diff_iteratively held a reference to the top frame, which
enter() invalidates when it pushes and the end of the loop invalidates
when it pops. Nothing used it afterwards, but a later change could. As in
the other iterative walks, the members the loop reads are now copied out
as constants and the ones it advances are changed through stack.back().
The frame as a whole is not copied: it holds the common keys and the
"add" operations of an object.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-27 14:00:04 +02:00
parent aa5084d713
commit 2038838eea
2 changed files with 46 additions and 38 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/json.hpp
+23 -19
View File
@@ -6391,23 +6391,26 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
enter(source, target);
while (!stack.empty())
{
// invalidated when enter() pushes a frame and by the pop_back()
// at the end, so not used after either
diff_frame& frame = stack.back();
const std::size_t path_length = frame.path_length;
// the frame is copied out member by member and changed through
// stack.back(): enter() may push a frame and the end of the loop
// pops it, either of which would invalidate a reference to it
const basic_json* const s = stack.back().source;
const basic_json* const t = stack.back().target;
const std::size_t path_length = stack.back().path_length;
const std::size_t depth = stack.size();
if (frame.source->is_array())
if (s->is_array())
{
const auto& source_array = *frame.source->m_data.m_value.array;
const auto& target_array = *frame.target->m_data.m_value.array;
const auto& source_array = *s->m_data.m_value.array;
const auto& target_array = *t->m_data.m_value.array;
// first pass: traverse common elements
if (frame.index < source_array.size() && frame.index < target_array.size())
const std::size_t i = stack.back().index;
if (i < source_array.size() && i < target_array.size())
{
const std::size_t i = frame.index++;
++stack.back().index;
detail::concat_into(current_path, '/', detail::to_string<string_t>(i));
enter(source_array[i], target_array[i]); // may push, which invalidates `frame`
enter(source_array[i], target_array[i]);
if (stack.size() == depth)
{
current_path.resize(path_length);
@@ -6417,20 +6420,21 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// We now reached the end of at least one array
// in a second pass, traverse the remaining elements
diff_array_tails(result, *frame.source, *frame.target, current_path, frame.index);
diff_array_tails(result, *s, *t, current_path, i);
}
else
{
if (frame.member != frame.source->cend())
const const_iterator it = stack.back().member;
if (it != s->cend())
{
const const_iterator it = frame.member;
++frame.member;
if (frame.next_common < frame.common_keys.size() && it.key() == frame.common_keys[frame.next_common])
++stack.back().member;
const std::size_t next_common = stack.back().next_common;
if (next_common < stack.back().common_keys.size() && it.key() == stack.back().common_keys[next_common])
{
++frame.next_common;
const basic_json& target_value = (*frame.target)[it.key()];
++stack.back().next_common;
const basic_json& target_value = (*t)[it.key()];
detail::concat_into(current_path, '/', detail::escape(it.key()));
enter(it.value(), target_value); // may push, which invalidates `frame`
enter(it.value(), target_value);
if (stack.size() == depth)
{
current_path.resize(path_length);
@@ -6446,7 +6450,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// append the "add" ops for brand-new keys collected when the
// object was entered
result.insert(result.end(), frame.added_ops.begin(), frame.added_ops.end());
result.insert(result.end(), stack.back().added_ops.begin(), stack.back().added_ops.end());
}
// this array or object is done: continue with the one it is in
single_include/nlohmann/json.hpp
+23 -19
View File
@@ -31349,23 +31349,26 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
enter(source, target);
while (!stack.empty())
{
// invalidated when enter() pushes a frame and by the pop_back()
// at the end, so not used after either
diff_frame& frame = stack.back();
const std::size_t path_length = frame.path_length;
// the frame is copied out member by member and changed through
// stack.back(): enter() may push a frame and the end of the loop
// pops it, either of which would invalidate a reference to it
const basic_json* const s = stack.back().source;
const basic_json* const t = stack.back().target;
const std::size_t path_length = stack.back().path_length;
const std::size_t depth = stack.size();
if (frame.source->is_array())
if (s->is_array())
{
const auto& source_array = *frame.source->m_data.m_value.array;
const auto& target_array = *frame.target->m_data.m_value.array;
const auto& source_array = *s->m_data.m_value.array;
const auto& target_array = *t->m_data.m_value.array;
// first pass: traverse common elements
if (frame.index < source_array.size() && frame.index < target_array.size())
const std::size_t i = stack.back().index;
if (i < source_array.size() && i < target_array.size())
{
const std::size_t i = frame.index++;
++stack.back().index;
detail::concat_into(current_path, '/', detail::to_string<string_t>(i));
enter(source_array[i], target_array[i]); // may push, which invalidates `frame`
enter(source_array[i], target_array[i]);
if (stack.size() == depth)
{
current_path.resize(path_length);
@@ -31375,20 +31378,21 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// We now reached the end of at least one array
// in a second pass, traverse the remaining elements
diff_array_tails(result, *frame.source, *frame.target, current_path, frame.index);
diff_array_tails(result, *s, *t, current_path, i);
}
else
{
if (frame.member != frame.source->cend())
const const_iterator it = stack.back().member;
if (it != s->cend())
{
const const_iterator it = frame.member;
++frame.member;
if (frame.next_common < frame.common_keys.size() && it.key() == frame.common_keys[frame.next_common])
++stack.back().member;
const std::size_t next_common = stack.back().next_common;
if (next_common < stack.back().common_keys.size() && it.key() == stack.back().common_keys[next_common])
{
++frame.next_common;
const basic_json& target_value = (*frame.target)[it.key()];
++stack.back().next_common;
const basic_json& target_value = (*t)[it.key()];
detail::concat_into(current_path, '/', detail::escape(it.key()));
enter(it.value(), target_value); // may push, which invalidates `frame`
enter(it.value(), target_value);
if (stack.size() == depth)
{
current_path.resize(path_length);
@@ -31404,7 +31408,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// append the "add" ops for brand-new keys collected when the
// object was entered
result.insert(result.end(), frame.added_ops.begin(), frame.added_ops.end());
result.insert(result.end(), stack.back().added_ops.begin(), stack.back().added_ops.end());
}
// this array or object is done: continue with the one it is in