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Destroy the first levels of a value by recursion

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The pointer-reversing walk in destroy_container() avoids the call stack but
moves a value around for every container it visits. Recurse for the first
recursion_depth_limit() levels instead - no allocation, no thread-local, a
bounded frame - and hand anything deeper to the walk. Destroy twitter
0.49 -> 0.36 ms, citm 0.80 -> 0.74 ms (3.12.0: 0.47 / 1.00).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-10-15 11:47:26 +02:00
1 parent 8b2d29f28b
commit 82250cbe02
2 files changed
+130 -28
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include/nlohmann/json.hpp
+55 -4
View File
@@ -878,7 +878,58 @@ public:
return;
}
// Destroy the tree without recursing per nesting level and
// This json_value is not itself a basic_json, so the top-level
// container is first moved into a local stand-in ("cur"); a
// default-constructed basic_json has a null pointer in its
// m_value (see data::m_value's initializer), so swapping it
// with *this leaves this union's own pointer null, and it is
// never looked at or freed a second time.
basic_json cur;
cur.m_data.m_type = t;
using std::swap;
swap(cur.m_data.m_value, *this);
destroy_bounded(cur, 0);
}
// Destroy the array/object v, whose children are destroyed first.
// The first recursion_depth_limit() levels are destroyed by plain
// recursion, which is the fastest way and cannot throw; anything
// nested deeper is handed to destroy_walk(), which uses no call stack.
static void destroy_bounded(basic_json& v, std::size_t depth) noexcept
{
if (JSON_HEDLEY_UNLIKELY(depth >= detail::recursion_depth_limit()))
{
destroy_walk(v);
return;
}
if (v.m_data.m_type == value_t::array)
{
for (auto& child : *v.m_data.m_value.array)
{
if (child.m_data.m_type == value_t::array || child.m_data.m_type == value_t::object)
{
destroy_bounded(child, depth + 1);
}
}
}
else
{
for (auto& child : *v.m_data.m_value.object)
{
if (child.second.m_data.m_type == value_t::array || child.second.m_data.m_type == value_t::object)
{
destroy_bounded(child.second, depth + 1);
}
}
}
// only leaves and already destroyed (null) children remain
free_container(v);
}
// Destroy the tree below top without recursing per nesting level and
// without any heap allocation: a heap-allocated flattening
// stack (the previous implementation) can itself throw
// bad_alloc, which would escape this noexcept destructor and
@@ -902,10 +953,10 @@ public:
// pointer in its m_value (see data::m_value's initializer),
// so swapping it with *this leaves this union's own pointer
// null, and it is never looked at or freed a second time.
static void destroy_walk(basic_json& top) noexcept
{
basic_json cur;
cur.m_data.m_type = t;
using std::swap;
swap(cur.m_data.m_value, *this);
take(cur, top);
basic_json prev; // value_t::null: no parent
single_include/nlohmann/json.hpp
Generated
+75 -24
View File
@@ -30571,35 +30571,86 @@ public:
return;
}
// Destroy the tree without recursing per nesting level and
// without any heap allocation: a heap-allocated flattening
// stack (the previous implementation) can itself throw
// bad_alloc, which would escape this noexcept destructor and
// terminate the program (#5135).
//
// Instead, walk down a chain of children (always the one
// walk_child() picks), reversing links as we go: cur is the
// container currently being emptied, and prev is its parent
// (value_t::null when there is none). Each parent's
// walk_child() slot doubles as storage for that parent's own
// parent link while we are below it, so no extra memory is
// needed; the parent is not modified meanwhile, so
// walk_child() finds that same slot again on the way up. We
// only ever remove a child once it is a scalar or an empty
// array/object, which neither allocates nor recurses more
// than one level deep.
//
// This json_value is not itself a basic_json, so the
// top-level container is first moved into a local stand-in
// ("cur"); a default-constructed basic_json has a null
// pointer in its m_value (see data::m_value's initializer),
// so swapping it with *this leaves this union's own pointer
// null, and it is never looked at or freed a second time.
// This json_value is not itself a basic_json, so the top-level
// container is first moved into a local stand-in ("cur"); a
// default-constructed basic_json has a null pointer in its
// m_value (see data::m_value's initializer), so swapping it
// with *this leaves this union's own pointer null, and it is
// never looked at or freed a second time.
basic_json cur;
cur.m_data.m_type = t;
using std::swap;
swap(cur.m_data.m_value, *this);
destroy_bounded(cur, 0);
}
// Destroy the array/object v, whose children are destroyed first.
// The first recursion_depth_limit() levels are destroyed by plain
// recursion, which is the fastest way and cannot throw; anything
// nested deeper is handed to destroy_walk(), which uses no call stack.
static void destroy_bounded(basic_json& v, std::size_t depth) noexcept
{
if (JSON_HEDLEY_UNLIKELY(depth >= detail::recursion_depth_limit()))
{
destroy_walk(v);
return;
}
if (v.m_data.m_type == value_t::array)
{
for (auto& child : *v.m_data.m_value.array)
{
if (child.m_data.m_type == value_t::array || child.m_data.m_type == value_t::object)
{
destroy_bounded(child, depth + 1);
}
}
}
else
{
for (auto& child : *v.m_data.m_value.object)
{
if (child.second.m_data.m_type == value_t::array || child.second.m_data.m_type == value_t::object)
{
destroy_bounded(child.second, depth + 1);
}
}
}
// only leaves and already destroyed (null) children remain
free_container(v);
}
// Destroy the tree below top without recursing per nesting level and
// without any heap allocation: a heap-allocated flattening
// stack (the previous implementation) can itself throw
// bad_alloc, which would escape this noexcept destructor and
// terminate the program (#5135).
//
// Instead, walk down a chain of children (always the one
// walk_child() picks), reversing links as we go: cur is the
// container currently being emptied, and prev is its parent
// (value_t::null when there is none). Each parent's
// walk_child() slot doubles as storage for that parent's own
// parent link while we are below it, so no extra memory is
// needed; the parent is not modified meanwhile, so
// walk_child() finds that same slot again on the way up. We
// only ever remove a child once it is a scalar or an empty
// array/object, which neither allocates nor recurses more
// than one level deep.
//
// This json_value is not itself a basic_json, so the
// top-level container is first moved into a local stand-in
// ("cur"); a default-constructed basic_json has a null
// pointer in its m_value (see data::m_value's initializer),
// so swapping it with *this leaves this union's own pointer
// null, and it is never looked at or freed a second time.
static void destroy_walk(basic_json& top) noexcept
{
basic_json cur;
take(cur, top);
basic_json prev; // value_t::null: no parent
// true right after cur was entered: its children have not been