Merge remote-tracking branch 'origin/develop' into HEAD

include/nlohmann/detail/input/json_sax.hpp

@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include <algorithm> // min
 #include <cstddef>
 #include <string> // string
 #include <type_traits> // enable_if_t
@@ -17,6 +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
 
@@ -150,6 +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
 
@@ -305,6 +330,11 @@ class json_sax_dom_parser
             JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
         }
 
+        if (len != detail::unknown_size())
+        {
+            reserve_array(*ref_stack.back()->m_data.m_value.array, len, priority_tag<1> {});
+        }
+
         return true;
     }
 
@@ -683,6 +713,11 @@ class json_sax_dom_callback_parser
             {
                 JSON_THROW(out_of_range::create(408, concat("excessive array size: ", std::to_string(len)), ref_stack.back()));
             }
+
+            if (len != detail::unknown_size())
+            {
+                reserve_array(*ref_stack.back()->m_data.m_value.array, len, priority_tag<1> {});
+            }
         }
 
         return true;