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Create a value before giving it its type (#5585)

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* Create a value before giving it its type

Squashed onto develop from:
- Create a value before giving it its type
- Skip the failed-allocation test when exceptions are disabled
- Keep the created pointer rather than an uninitialized json_value
- Skip the vector<bool> failed-allocation check for VS 2015 with iterator debugging
- Test the remaining to_json overloads with a failing allocation
- Skip the to_json allocation-failure section on VS 2015 Debug
- Fix false GCC -Warray-bounds error with JSON_DIAGNOSTICS at -O3 (#5744)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Store the new value with a helper in all to_json constructors

Every external_constructor<>::construct now creates the new value first and
hands it to basic_json::replace_value(), which destroys the old value, stores
the new one before setting its type (as elsewhere in this PR), sets the
parents, and checks the invariant.

The std::vector<bool> and std::valarray overloads use array_t's range
constructor, which the other array overloads already rely on. Range views
keep their loop, as begin() and end() of a view may have different types.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann authored and GitHub committed 2026-10-06 22:59:34 +02:00
1 parent 0c2329d0e2
commit 21a69230bd
6 files changed
+392 -204
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single_include/nlohmann/json.hpp
+66 -102
View File
@@ -6193,7 +6193,6 @@ NLOHMANN_JSON_NAMESPACE_END
#include <optional> // optional
#endif
#include <algorithm> // copy
#include <iterator> // begin, end
#include <memory> // allocator_traits
#include <string> // basic_string, char_traits
@@ -6833,10 +6832,14 @@ namespace detail
//////////////////
/*
* Note all external_constructor<>::construct functions need to call
* j.m_data.m_value.destroy(j.m_data.m_type) to avoid a memory leak in case j contains an
* allocated value (e.g., a string). See bug issue
* Note all external_constructor<>::construct functions need to store the new
* value with j.replace_value(), which destroys the old one to avoid a memory
* leak in case j contains an allocated value (e.g., a string). See bug issue
* https://github.com/nlohmann/json/issues/2865 for more information.
*
* A value that has to be allocated is created before the old one is destroyed:
* were it the other way around, an exception while creating the new value would
* leave j with the type of the new value, but the pointer to the destroyed old one.
*/
template<value_t> struct external_constructor;
@@ -6847,10 +6850,7 @@ struct external_constructor<value_t::boolean>
template<typename BasicJsonType>
static void construct(BasicJsonType& j, typename BasicJsonType::boolean_t b) noexcept
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::boolean;
j.m_data.m_value = b;
j.assert_invariant();
j.replace_value(value_t::boolean, b);
}
};
@@ -6860,19 +6860,15 @@ struct external_constructor<value_t::string>
template<typename BasicJsonType>
static void construct(BasicJsonType& j, const typename BasicJsonType::string_t& s)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::string;
j.m_data.m_value = s;
j.assert_invariant();
const typename BasicJsonType::json_value value(s);
j.replace_value(value_t::string, value);
}
template<typename BasicJsonType>
static void construct(BasicJsonType& j, typename BasicJsonType::string_t&& s)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::string;
j.m_data.m_value = std::move(s);
j.assert_invariant();
const typename BasicJsonType::json_value value(std::move(s));
j.replace_value(value_t::string, value);
}
template < typename BasicJsonType, typename CompatibleStringType,
@@ -6880,10 +6876,8 @@ struct external_constructor<value_t::string>
int > = 0 >
static void construct(BasicJsonType& j, const CompatibleStringType& str)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::string;
j.m_data.m_value.string = j.template create<typename BasicJsonType::string_t>(str);
j.assert_invariant();
const typename BasicJsonType::json_value value(j.template create<typename BasicJsonType::string_t>(str));
j.replace_value(value_t::string, value);
}
};
@@ -6893,19 +6887,15 @@ struct external_constructor<value_t::binary>
template<typename BasicJsonType>
static void construct(BasicJsonType& j, const typename BasicJsonType::binary_t& b)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::binary;
j.m_data.m_value = typename BasicJsonType::binary_t(b);
j.assert_invariant();
const typename BasicJsonType::json_value value(b);
j.replace_value(value_t::binary, value);
}
template<typename BasicJsonType>
static void construct(BasicJsonType& j, typename BasicJsonType::binary_t&& b)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::binary;
j.m_data.m_value = typename BasicJsonType::binary_t(std::move(b));
j.assert_invariant();
const typename BasicJsonType::json_value value(std::move(b));
j.replace_value(value_t::binary, value);
}
};
@@ -6915,10 +6905,7 @@ struct external_constructor<value_t::number_float>
template<typename BasicJsonType>
static void construct(BasicJsonType& j, typename BasicJsonType::number_float_t val) noexcept
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::number_float;
j.m_data.m_value = val;
j.assert_invariant();
j.replace_value(value_t::number_float, val);
}
};
@@ -6928,10 +6915,7 @@ struct external_constructor<value_t::number_unsigned>
template<typename BasicJsonType>
static void construct(BasicJsonType& j, typename BasicJsonType::number_unsigned_t val) noexcept
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::number_unsigned;
j.m_data.m_value = val;
j.assert_invariant();
j.replace_value(value_t::number_unsigned, val);
}
};
@@ -6941,10 +6925,7 @@ struct external_constructor<value_t::number_integer>
template<typename BasicJsonType>
static void construct(BasicJsonType& j, typename BasicJsonType::number_integer_t val) noexcept
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::number_integer;
j.m_data.m_value = val;
j.assert_invariant();
j.replace_value(value_t::number_integer, val);
}
};
@@ -6954,21 +6935,15 @@ struct external_constructor<value_t::array>
template<typename BasicJsonType>
static void construct(BasicJsonType& j, const typename BasicJsonType::array_t& arr)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::array;
j.m_data.m_value = arr;
j.set_parents();
j.assert_invariant();
const typename BasicJsonType::json_value value(arr);
j.replace_value(value_t::array, value);
}
template<typename BasicJsonType>
static void construct(BasicJsonType& j, typename BasicJsonType::array_t&& arr)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::array;
j.m_data.m_value = std::move(arr);
j.set_parents();
j.assert_invariant();
const typename BasicJsonType::json_value value(std::move(arr));
j.replace_value(value_t::array, value);
}
template < typename BasicJsonType, typename CompatibleArrayType,
@@ -6982,39 +6957,23 @@ struct external_constructor<value_t::array>
using std::begin;
using std::end;
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::array;
j.m_data.m_value.array = j.template create<typename BasicJsonType::array_t>(begin(arr), end(arr));
j.set_parents();
j.assert_invariant();
const typename BasicJsonType::json_value value(j.template create<typename BasicJsonType::array_t>(begin(arr), end(arr)));
j.replace_value(value_t::array, value);
}
template<typename BasicJsonType>
static void construct(BasicJsonType& j, const std::vector<bool>& arr)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::array;
j.m_data.m_value = value_t::array;
j.m_data.m_value.array->reserve(arr.size());
for (const bool x : arr)
{
j.m_data.m_value.array->push_back(x);
j.set_parent(j.m_data.m_value.array->back());
}
j.assert_invariant();
const typename BasicJsonType::json_value value(j.template create<typename BasicJsonType::array_t>(arr.begin(), arr.end()));
j.replace_value(value_t::array, value);
}
template<typename BasicJsonType, typename T,
enable_if_t<std::is_convertible<T, BasicJsonType>::value, int> = 0>
static void construct(BasicJsonType& j, const std::valarray<T>& arr)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::array;
j.m_data.m_value = value_t::array;
j.m_data.m_value.array->resize(arr.size());
std::copy(std::begin(arr), std::end(arr), j.m_data.m_value.array->begin());
j.set_parents();
j.assert_invariant();
const typename BasicJsonType::json_value value(j.template create<typename BasicJsonType::array_t>(std::begin(arr), std::end(arr)));
j.replace_value(value_t::array, value);
}
#if JSON_HAS_RANGE_VIEW_CONVERSION
@@ -7022,18 +6981,15 @@ struct external_constructor<value_t::array>
enable_if_t<is_compatible_range_view<std::remove_cvref_t<CompatibleArrayType>>::value, int> = 0>
static void construct(BasicJsonType& j, CompatibleArrayType && arr)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::array;
j.m_data.m_value = value_t::array;
// no range constructor: a view's begin() and end() may have different
// types, and the view may only be iterable once
typename BasicJsonType::array_t elements;
for (auto&& x : std::forward<CompatibleArrayType>(arr))
{
j.m_data.m_value.array->push_back(x);
elements.push_back(x);
}
// set the parents only once all elements are in place: a push_back
// that reallocates moves the earlier elements, which does not keep
// their parent pointers
j.set_parents();
j.assert_invariant();
const typename BasicJsonType::json_value value(std::move(elements));
j.replace_value(value_t::array, value);
}
#endif
};
@@ -7044,21 +7000,15 @@ struct external_constructor<value_t::object>
template<typename BasicJsonType>
static void construct(BasicJsonType& j, const typename BasicJsonType::object_t& obj)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::object;
j.m_data.m_value = obj;
j.set_parents();
j.assert_invariant();
const typename BasicJsonType::json_value value(obj);
j.replace_value(value_t::object, value);
}
template<typename BasicJsonType>
static void construct(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
{
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::object;
j.m_data.m_value = std::move(obj);
j.set_parents();
j.assert_invariant();
const typename BasicJsonType::json_value value(std::move(obj));
j.replace_value(value_t::object, value);
}
template < typename BasicJsonType, typename CompatibleObjectType,
@@ -7068,11 +7018,8 @@ struct external_constructor<value_t::object>
using std::begin;
using std::end;
j.m_data.m_value.destroy(j.m_data.m_type);
j.m_data.m_type = value_t::object;
j.m_data.m_value.object = j.template create<typename BasicJsonType::object_t>(begin(obj), end(obj));
j.set_parents();
j.assert_invariant();
const typename BasicJsonType::json_value value(j.template create<typename BasicJsonType::object_t>(begin(obj), end(obj)));
j.replace_value(value_t::object, value);
}
};
@@ -27921,6 +27868,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// constructor for rvalue binary arrays (internal type)
json_value(binary_t&& value) : binary(create<binary_t>(std::move(value))) {}
/// constructors taking ownership of an already created value
explicit json_value(string_t* value) noexcept : string(value) {}
explicit json_value(object_t* value) noexcept : object(value) {}
explicit json_value(array_t* value) noexcept : array(value) {}
private:
// raw, allocation-free transfer of m_data from src to dst: no
// set_parents()/assert_invariant() (the former is O(#children) per
@@ -28251,6 +28203,18 @@ public:
#endif
}
/// @brief replace the stored value with an already created one
/// The new value must be created before calling this function: if its
/// creation throws, the current value is left untouched.
void replace_value(value_t t, const json_value& v) noexcept
{
m_data.m_value.destroy(m_data.m_type);
m_data.m_value = v;
m_data.m_type = t;
set_parents();
assert_invariant();
}
iterator set_parents(iterator it, std::ptrdiff_t count_set_parents)
{
#if JSON_DIAGNOSTICS
@@ -29510,8 +29474,8 @@ public:
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)
{
@@ -29533,8 +29497,8 @@ public:
}
#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();
@@ -29547,8 +29511,8 @@ public:
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;
}
@@ -29558,8 +29522,8 @@ public:
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;
}
@@ -29569,8 +29533,8 @@ public:
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;
}
@@ -29580,8 +29544,8 @@ public:
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;
}