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Test that destroy() performs no allocation, even under memory pressure

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Update the #4842 regression test: it used to check that destroying a
nested array/object made at least one allocation through the provided
allocator (the old flattening stack). Now that destroy() does not
allocate at all, assert the opposite: zero allocations, deallocations
only.

Rework the #5135 regression test to use a dedicated failing/counting
allocator instead of overriding the process-wide ::operator new and
::operator delete, which affected every allocation in the whole
unit-regression2 binary rather than just the values under test. Keep
the original small repro as one case, and add deep (100000 levels) and
wide-and-deep nested array/object/ordered_json cases, all destroyed
while every further allocation is made to fail: the destructor must
complete without allocating, without throwing, and without leaking.

Co-authored-by: Michael Sam <9461037+michaelsam94@users.noreply.github.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-10-04 22:22:17 +02:00
co-authored by Michael Sam
parent d0278f8983
commit 77fccaf0e7
2 changed files with 173 additions and 41 deletions
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tests/src/unit-allocator.cpp
+23 -7
View File
@@ -606,6 +606,7 @@ TEST_CASE("bad my_allocator::construct")
namespace
{
std::size_t counting_allocator_allocations = 0;
std::size_t counting_allocator_deallocations = 0;
template<class T>
struct counting_allocator : std::allocator<T>
@@ -618,6 +619,12 @@ struct counting_allocator : std::allocator<T>
return std::allocator<T>::allocate(n);
}
void deallocate(T* p, std::size_t n)
{
++counting_allocator_deallocations;
std::allocator<T>::deallocate(p, n);
}
template <class U>
struct rebind
{
@@ -626,9 +633,15 @@ struct counting_allocator : std::allocator<T>
};
} // namespace
TEST_CASE("destructor uses the provided allocator")
TEST_CASE("destructor performs no allocation, only deallocation")
{
// see https://github.com/nlohmann/json/issues/4842
// see https://github.com/nlohmann/json/issues/4842 and
// https://github.com/nlohmann/json/issues/5135: destroying nested
// arrays/objects used to allocate a temporary stack (first with
// std::allocator, later - after #4842 - with the provided allocator).
// Since that stack could itself throw bad_alloc from inside the
// noexcept destructor (#5135), destroy() no longer allocates anything:
// it only ever frees what is already there.
using counting_json = nlohmann::basic_json<std::map,
std::vector,
std::string,
@@ -641,17 +654,20 @@ TEST_CASE("destructor uses the provided allocator")
SECTION("array")
{
auto* j = new counting_json({1, {2, {3, 4}}, 5}); // NOLINT(cppcoreguidelines-owning-memory)
const auto before = counting_allocator_allocations;
const auto allocations_before = counting_allocator_allocations;
const auto deallocations_before = counting_allocator_deallocations;
delete j; // NOLINT(cppcoreguidelines-owning-memory)
// the stack used to destroy the children is allocated with the provided allocator
CHECK(counting_allocator_allocations > before);
CHECK(counting_allocator_allocations == allocations_before);
CHECK(counting_allocator_deallocations > deallocations_before);
}
SECTION("object")
{
auto* j = new counting_json({{"a", {{"b", {1, 2}}}}, {"c", 3}}); // NOLINT(cppcoreguidelines-owning-memory)
const auto before = counting_allocator_allocations;
const auto allocations_before = counting_allocator_allocations;
const auto deallocations_before = counting_allocator_deallocations;
delete j; // NOLINT(cppcoreguidelines-owning-memory)
CHECK(counting_allocator_allocations > before);
CHECK(counting_allocator_allocations == allocations_before);
CHECK(counting_allocator_deallocations > deallocations_before);
}
}