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Create a value before giving it its type

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Several functions set the type of a value before creating the string,
array, object, or binary value it stands for. When that creation threw -
std::bad_alloc from the allocator, say - the value was left behind with
the new type but nothing behind it:

- json::binary() returned no value, but its destructor asserted that a
  binary value has a binary array, aborting debug builds.
- operator[], push_back, emplace_back, emplace, and update turned a null
  value into an array or object that did not exist; any later access
  dereferenced a null pointer.
- The to_json conversions destroyed the old value first. A failed
  creation of the new one left a pointer to the destroyed old value,
  which was then used and freed again (heap-use-after-free).

The value is now created first and the type set after it, and to_json
destroys the old value only once the new one exists, so a failed
allocation leaves the value unchanged.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-09-25 22:19:39 +02:00
1 parent f422b753cc
commit 2c108d0b56
4 files changed
+239 -80
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include/nlohmann/json.hpp
+15 -15
View File
@@ -1687,8 +1687,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
if (is_an_object)
{
// the initializer list is a list of pairs -> create an object
m_data.m_type = value_t::object;
m_data.m_value = value_t::object;
m_data.m_type = value_t::object;
for (auto& element_ref : init)
{
@@ -1710,8 +1710,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
}
#endif
// the initializer list describes an array -> create an array
m_data.m_type = value_t::array;
m_data.m_value.array = create<array_t>(init.begin(), init.end());
m_data.m_type = value_t::array;
}
set_parents();
@@ -1724,8 +1724,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
static basic_json binary(const typename binary_t::container_type& init)
{
auto res = basic_json();
res.m_data.m_type = value_t::binary;
res.m_data.m_value = init;
res.m_data.m_type = value_t::binary;
return res;
}
@@ -1735,8 +1735,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
static basic_json binary(const typename binary_t::container_type& init, typename binary_t::subtype_type subtype)
{
auto res = basic_json();
res.m_data.m_type = value_t::binary;
res.m_data.m_value = binary_t(init, subtype);
res.m_data.m_type = value_t::binary;
return res;
}
@@ -1746,8 +1746,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
static basic_json binary(typename binary_t::container_type&& init)
{
auto res = basic_json();
res.m_data.m_type = value_t::binary;
res.m_data.m_value = std::move(init);
res.m_data.m_type = value_t::binary;
return res;
}
@@ -1757,8 +1757,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
static basic_json binary(typename binary_t::container_type&& init, typename binary_t::subtype_type subtype)
{
auto res = basic_json();
res.m_data.m_type = value_t::binary;
res.m_data.m_value = binary_t(std::move(init), subtype);
res.m_data.m_type = value_t::binary;
return res;
}
@@ -2815,8 +2815,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty array
if (is_null())
{
m_data.m_type = value_t::array;
m_data.m_value.array = create<array_t>();
m_data.m_type = value_t::array;
assert_invariant();
}
@@ -2875,8 +2875,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty object
if (is_null())
{
m_data.m_type = value_t::object;
m_data.m_value.object = create<object_t>();
m_data.m_type = value_t::object;
assert_invariant();
}
@@ -2928,8 +2928,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty object
if (is_null())
{
m_data.m_type = value_t::object;
m_data.m_value.object = create<object_t>();
m_data.m_type = value_t::object;
assert_invariant();
}
@@ -3864,8 +3864,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an array
if (is_null())
{
m_data.m_type = value_t::array;
m_data.m_value = value_t::array;
m_data.m_type = value_t::array;
assert_invariant();
}
@@ -3897,8 +3897,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an array
if (is_null())
{
m_data.m_type = value_t::array;
m_data.m_value = value_t::array;
m_data.m_type = value_t::array;
assert_invariant();
}
@@ -3929,8 +3929,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an object
if (is_null())
{
m_data.m_type = value_t::object;
m_data.m_value = value_t::object;
m_data.m_type = value_t::object;
assert_invariant();
}
@@ -3985,8 +3985,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an array
if (is_null())
{
m_data.m_type = value_t::array;
m_data.m_value = value_t::array;
m_data.m_type = value_t::array;
assert_invariant();
}
@@ -4010,8 +4010,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an object
if (is_null())
{
m_data.m_type = value_t::object;
m_data.m_value = value_t::object;
m_data.m_type = value_t::object;
assert_invariant();
}
@@ -4192,8 +4192,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty object
if (is_null())
{
m_data.m_type = value_t::object;
m_data.m_value.object = create<object_t>();
m_data.m_type = value_t::object;
assert_invariant();
}