amorfo77/json
1
0
Fork 0
mirror of https://github.com/nlohmann/json.git synced 2026-09-13 19:57:58 +00:00
Code Issues Packages Projects Releases Wiki Activity

Only reserve array capacity if the array type supports it

Browse Source 
PR #5476 added an unconditional `reserve()` call to the `start_array()`
implementations of both `json_sax_dom_parser` and
`json_sax_dom_callback_parser`. `ArrayType` is a template parameter of
`basic_json`, however, and is not required to have a `reserve()` member
function: instantiating either parser for, e.g., `basic_json<std::map,
std::deque>` fails to compile, which breaks every use of `parse()` and
the binary readers for such a type.

Move the capped reservation into a `reserve_array()` helper that selects
a no-op overload through `priority_tag` when `ArrayType` has no
`reserve()`, mirroring `from_json_object_reserve()`. `std::vector` array
types keep reserving as before, and the 16384-element cap is unchanged.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Niels Lohmann
2026-09-13 18:02:01 +02:00
co-authored by Claude Opus 5
parent c0b2878a44
commit ee211df64a
3 changed files with 99 additions and 24 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/input/json_sax.hpp
+26 -12
View File
@@ -18,6 +18,7 @@
#include <nlohmann/detail/exceptions.hpp>
#include <nlohmann/detail/input/lexer.hpp>
#include <nlohmann/detail/macro_scope.hpp>
#include <nlohmann/detail/meta/cpp_future.hpp>
#include <nlohmann/detail/string_concat.hpp>
NLOHMANN_JSON_NAMESPACE_BEGIN
@@ -151,6 +152,29 @@ constexpr std::size_t unknown_size()
return (std::numeric_limits<std::size_t>::max)();
}
/*!
@brief reserve capacity for @a len elements in array @a arr
Reserving upfront avoids repeated reallocations while the elements are added,
but the reservation is capped so a bogus/hostile length (which is not bounded
by max_size(), unlike e.g. std::vector) cannot trigger an oversized allocation
for a small or truncated input.
The overload below is selected for array types without reserve() (e.g.,
std::deque), which are then left untouched.
*/
template<typename ArrayType>
auto reserve_array(ArrayType& arr, std::size_t len, priority_tag<1> /*unused*/)
-> decltype(arr.reserve(len), void())
{
constexpr std::size_t reserve_cap = 16384;
arr.reserve((std::min)(len, reserve_cap));
}
template<typename ArrayType>
inline void reserve_array(ArrayType& /*arr*/, std::size_t /*len*/, priority_tag<0> /*unused*/)
{}
/*!
@brief SAX implementation to create a JSON value from SAX events
@@ -308,12 +332,7 @@ class json_sax_dom_parser
if (len != detail::unknown_size())
{
// reserve upfront to avoid repeated reallocations while adding elements,
// but cap the reservation so a bogus/hostile length (which is not bounded
// by max_size(), unlike e.g. std::vector) cannot trigger an oversized
// allocation for a small or truncated input
constexpr std::size_t reserve_cap = 16384;
ref_stack.back()->m_data.m_value.array->reserve((std::min)(len, reserve_cap));
reserve_array(*ref_stack.back()->m_data.m_value.array, len, priority_tag<1> {});
}
return true;
@@ -697,12 +716,7 @@ class json_sax_dom_callback_parser
if (len != detail::unknown_size())
{
// reserve upfront to avoid repeated reallocations while adding elements,
// but cap the reservation so a bogus/hostile length (which is not bounded
// by max_size(), unlike e.g. std::vector) cannot trigger an oversized
// allocation for a small or truncated input
constexpr std::size_t reserve_cap = 16384;
ref_stack.back()->m_data.m_value.array->reserve((std::min)(len, reserve_cap));
reserve_array(*ref_stack.back()->m_data.m_value.array, len, priority_tag<1> {});
}
}