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nlohmann merged 1 commit into from
Sep 30, 2026
Merged

Share one nesting depth limit between all bounded descents #5637

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8 changes: 7 additions & 1 deletion include/nlohmann/detail/recursion_depth_limit.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -19,11 +19,17 @@ namespace detail
/*!
@brief the number of nesting levels an operation recurses into

Operations that walk a value (serializing, hashing, merging, ...) recurse once
Operations that walk a value (copying, comparing, serializing, hashing, merging,
...) recurse once
per nesting level, which is fastest, but a value nested deeply enough would
exhaust the call stack. So they recurse only this many levels deep and finish
whatever lies below with an explicit stack. All of them share this limit.

Most of them pass the depth down as an argument. The copy constructor and the
comparison operators cannot, as their signatures are fixed, so they count it
in basic_json::nesting_depth() instead, a byte per thread; the limit must
therefore stay below 255.

@sa https://github.com/nlohmann/json/issues/5387
*/
constexpr std::size_t recursion_depth_limit() noexcept
Expand Down
18 changes: 7 additions & 11 deletions include/nlohmann/json.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -898,12 +898,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
}

#ifndef JSON_NO_THREAD_LOCAL
/// the number of levels an operation descends into before it finishes the
/// value below it without the call stack
static constexpr std::uint8_t nesting_depth_limit()
{
return 128;
}
// nesting_depth() is a byte and may exceed the limit by one level
static_assert(detail::recursion_depth_limit() < 255, "the nesting depth count must fit in a byte");

/*!
@brief how many levels the operation going on in this thread has descended into
Expand Down Expand Up @@ -945,7 +941,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
static_cast<void>(may_descend);
return true;
#else
return !may_descend || nesting_depth() >= nesting_depth_limit();
return !may_descend || nesting_depth() >= detail::recursion_depth_limit();
#endif
}

Expand All @@ -969,7 +965,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
#ifdef JSON_NO_THREAD_LOCAL
: m_okay(false)
#else
: m_okay(nesting_depth() < nesting_depth_limit())
: m_okay(nesting_depth() < detail::recursion_depth_limit())
#endif
{
#ifndef JSON_NO_THREAD_LOCAL
Expand Down Expand Up @@ -1173,7 +1169,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec

The values whose copy has not been created yet are kept on an explicit
worklist rather than on the call stack. This is only reached for values
nested deeper than @ref nesting_depth_limit levels, which is why it copies
nested deeper than @ref detail::recursion_depth_limit levels, which is why it copies
every container by hand instead of letting the container do it: the fast
ways of doing so would descend into the elements and defeat the purpose.
*/
Expand Down Expand Up @@ -1239,7 +1235,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec

Copying a container copies its elements, so a value nested deeply enough
used to exhaust the call stack. The descent is bounded here: the first
@ref nesting_depth_limit levels are copied by the containers themselves, just
@ref detail::recursion_depth_limit levels are copied by the containers themselves, just
as they always were, and anything below that is copied without the call
stack by @ref copy_iteratively. Copying a value can therefore no longer
exhaust the stack, however deeply it is nested, just like destroying one
Expand Down Expand Up @@ -1377,7 +1373,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/*!
@brief compare @a lhs and @a rhs without descending into them

Reached once a comparison has descended @ref nesting_depth_limit levels, so
Reached once a comparison has descended @ref detail::recursion_depth_limit levels, so
that comparing values cannot exhaust the call stack however deeply they are
nested. The two values are walked in lockstep on an explicit stack and
compared lexicographically, element by element in the order the containers
Expand Down
26 changes: 14 additions & 12 deletions single_include/nlohmann/json.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7281,11 +7281,17 @@ namespace detail
/*!
@brief the number of nesting levels an operation recurses into

Operations that walk a value (serializing, hashing, merging, ...) recurse once
Operations that walk a value (copying, comparing, serializing, hashing, merging,
...) recurse once
per nesting level, which is fastest, but a value nested deeply enough would
exhaust the call stack. So they recurse only this many levels deep and finish
whatever lies below with an explicit stack. All of them share this limit.

Most of them pass the depth down as an argument. The copy constructor and the
comparison operators cannot, as their signatures are fixed, so they count it
in basic_json::nesting_depth() instead, a byte per thread; the limit must
therefore stay below 255.

@sa https://github.com/nlohmann/json/issues/5387
*/
constexpr std::size_t recursion_depth_limit() noexcept
Expand Down Expand Up @@ -26979,12 +26985,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
}

#ifndef JSON_NO_THREAD_LOCAL
/// the number of levels an operation descends into before it finishes the
/// value below it without the call stack
static constexpr std::uint8_t nesting_depth_limit()
{
return 128;
}
// nesting_depth() is a byte and may exceed the limit by one level
static_assert(detail::recursion_depth_limit() < 255, "the nesting depth count must fit in a byte");

/*!
@brief how many levels the operation going on in this thread has descended into
Expand Down Expand Up @@ -27026,7 +27028,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
static_cast<void>(may_descend);
return true;
#else
return !may_descend || nesting_depth() >= nesting_depth_limit();
return !may_descend || nesting_depth() >= detail::recursion_depth_limit();
#endif
}

Expand All @@ -27050,7 +27052,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
#ifdef JSON_NO_THREAD_LOCAL
: m_okay(false)
#else
: m_okay(nesting_depth() < nesting_depth_limit())
: m_okay(nesting_depth() < detail::recursion_depth_limit())
#endif
{
#ifndef JSON_NO_THREAD_LOCAL
Expand Down Expand Up @@ -27254,7 +27256,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec

The values whose copy has not been created yet are kept on an explicit
worklist rather than on the call stack. This is only reached for values
nested deeper than @ref nesting_depth_limit levels, which is why it copies
nested deeper than @ref detail::recursion_depth_limit levels, which is why it copies
every container by hand instead of letting the container do it: the fast
ways of doing so would descend into the elements and defeat the purpose.
*/
Expand Down Expand Up @@ -27320,7 +27322,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec

Copying a container copies its elements, so a value nested deeply enough
used to exhaust the call stack. The descent is bounded here: the first
@ref nesting_depth_limit levels are copied by the containers themselves, just
@ref detail::recursion_depth_limit levels are copied by the containers themselves, just
as they always were, and anything below that is copied without the call
stack by @ref copy_iteratively. Copying a value can therefore no longer
exhaust the stack, however deeply it is nested, just like destroying one
Expand Down Expand Up @@ -27458,7 +27460,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/*!
@brief compare @a lhs and @a rhs without descending into them

Reached once a comparison has descended @ref nesting_depth_limit levels, so
Reached once a comparison has descended @ref detail::recursion_depth_limit levels, so
that comparing values cannot exhaust the call stack however deeply they are
nested. The two values are walked in lockstep on an explicit stack and
compared lexicographically, element by element in the order the containers
Expand Down
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