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Make convert_null_to() take the container type as a template argument

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Passing array_t or object_t instead of a value_t makes an invalid target
a compile error instead of a runtime assertion, and removes the branch.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-30 21:31:58 +02:00
parent 09d41a894b
commit 3a04f78507
2 changed files with 56 additions and 42 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/json.hpp
+28 -21
View File
@@ -2741,26 +2741,33 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
return (*j.m_data.m_value.array)[idx];
}
/// @brief convert a null value to an empty container of type @a t (array or object)
void convert_null_to(value_t t)
/// @brief convert a null value to an empty container of type @a Container
/// @tparam Container array_t or object_t; any other type does not compile
template<typename Container>
void convert_null_to()
{
JSON_ASSERT(is_null());
JSON_ASSERT(t == value_t::array || t == value_t::object);
// create the container before touching the type, so a throwing
// allocation leaves this value as a valid null rather than a type
// tag with a dangling/null pointer behind it
if (t == value_t::array)
{
m_data.m_value.array = create<array_t>();
}
else
{
m_data.m_value.object = create<object_t>();
}
m_data.m_type = t;
set_container(create<Container>());
assert_invariant();
}
/// @brief store a freshly created array and set the matching type
void set_container(array_t* array) noexcept
{
m_data.m_value.array = array;
m_data.m_type = value_t::array;
}
/// @brief store a freshly created object and set the matching type
void set_container(object_t* object) noexcept
{
m_data.m_value.object = object;
m_data.m_type = value_t::object;
}
public:
////////////////////
// element access //
@@ -2823,7 +2830,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty array
if (is_null())
{
convert_null_to(value_t::array);
convert_null_to<array_t>();
}
// operator[] only works for arrays
@@ -2881,7 +2888,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty object
if (is_null())
{
convert_null_to(value_t::object);
convert_null_to<object_t>();
}
// operator[] only works for objects
@@ -2932,7 +2939,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty object
if (is_null())
{
convert_null_to(value_t::object);
convert_null_to<object_t>();
}
// operator[] only works for objects
@@ -3804,7 +3811,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an array
if (is_null())
{
convert_null_to(value_t::array);
convert_null_to<array_t>();
}
// add the element to the array (move semantics)
@@ -3835,7 +3842,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an array
if (is_null())
{
convert_null_to(value_t::array);
convert_null_to<array_t>();
}
// add the element to the array
@@ -3865,7 +3872,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an object
if (is_null())
{
convert_null_to(value_t::object);
convert_null_to<object_t>();
}
// add the element to the object
@@ -3919,7 +3926,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an array
if (is_null())
{
convert_null_to(value_t::array);
convert_null_to<array_t>();
}
// add the element to the array (perfect forwarding)
@@ -3942,7 +3949,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an object
if (is_null())
{
convert_null_to(value_t::object);
convert_null_to<object_t>();
}
// add the element to the array (perfect forwarding)
@@ -4137,7 +4144,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty object
if (is_null())
{
convert_null_to(value_t::object);
convert_null_to<object_t>();
}
if (JSON_HEDLEY_UNLIKELY(!is_object()))
single_include/nlohmann/json.hpp
+28 -21
View File
@@ -28775,26 +28775,33 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
return (*j.m_data.m_value.array)[idx];
}
/// @brief convert a null value to an empty container of type @a t (array or object)
void convert_null_to(value_t t)
/// @brief convert a null value to an empty container of type @a Container
/// @tparam Container array_t or object_t; any other type does not compile
template<typename Container>
void convert_null_to()
{
JSON_ASSERT(is_null());
JSON_ASSERT(t == value_t::array || t == value_t::object);
// create the container before touching the type, so a throwing
// allocation leaves this value as a valid null rather than a type
// tag with a dangling/null pointer behind it
if (t == value_t::array)
{
m_data.m_value.array = create<array_t>();
}
else
{
m_data.m_value.object = create<object_t>();
}
m_data.m_type = t;
set_container(create<Container>());
assert_invariant();
}
/// @brief store a freshly created array and set the matching type
void set_container(array_t* array) noexcept
{
m_data.m_value.array = array;
m_data.m_type = value_t::array;
}
/// @brief store a freshly created object and set the matching type
void set_container(object_t* object) noexcept
{
m_data.m_value.object = object;
m_data.m_type = value_t::object;
}
public:
////////////////////
// element access //
@@ -28857,7 +28864,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty array
if (is_null())
{
convert_null_to(value_t::array);
convert_null_to<array_t>();
}
// operator[] only works for arrays
@@ -28915,7 +28922,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty object
if (is_null())
{
convert_null_to(value_t::object);
convert_null_to<object_t>();
}
// operator[] only works for objects
@@ -28966,7 +28973,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty object
if (is_null())
{
convert_null_to(value_t::object);
convert_null_to<object_t>();
}
// operator[] only works for objects
@@ -29838,7 +29845,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an array
if (is_null())
{
convert_null_to(value_t::array);
convert_null_to<array_t>();
}
// add the element to the array (move semantics)
@@ -29869,7 +29876,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an array
if (is_null())
{
convert_null_to(value_t::array);
convert_null_to<array_t>();
}
// add the element to the array
@@ -29899,7 +29906,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an object
if (is_null())
{
convert_null_to(value_t::object);
convert_null_to<object_t>();
}
// add the element to the object
@@ -29953,7 +29960,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an array
if (is_null())
{
convert_null_to(value_t::array);
convert_null_to<array_t>();
}
// add the element to the array (perfect forwarding)
@@ -29976,7 +29983,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// transform a null object into an object
if (is_null())
{
convert_null_to(value_t::object);
convert_null_to<object_t>();
}
// add the element to the array (perfect forwarding)
@@ -30171,7 +30178,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// implicitly convert a null value to an empty object
if (is_null())
{
convert_null_to(value_t::object);
convert_null_to<object_t>();
}
if (JSON_HEDLEY_UNLIKELY(!is_object()))