<memory>: Add different control block types for allocate_shared_for_overwrite by frederick-vs-ja · Pull Request #4274 · microsoft/STL (original) (raw)

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allocate_shared_for_overwrite creates allocated non-array objects by default-initialization, so these objects should be destroyed by plain destructor calls, not Ator::destroy. Currently the standard wording is unclear on make_shared_for_overwrite and allocate_shared_for_overwrite, so I submitted LWG-4024.

It's a bit unfortunate that the existing control block types can't be reused because the _Destroy functions can't recognize _For_overwrite_tag.

Unblocks one libcxx test:

• std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.create/allocate_shared_for_overwrite.pass.cpp

Drive-by changes:

• completing visualizers of control blocks for array types.

frederick-vs-ja changed the title<memory>: Add different control block types for allocator_shared_for_overwrite <memory>: Add different control block types for allocate_shared_for_overwrite

Dec 17, 2023

Do these control blocks need new visualizers?

Do these control blocks need new visualizers?

I'm now trying to add visualizer for _Ref_count_obj_alloc_for_overwrite.

But visualization of control block types for array types still seems missing and I have no idea how to handle them. Previous PRs #309, #1315, and the previous issue #2787 didn't mention these stuffs.

Edit: Completed these visualizers. However, it's unfortunate that the control blocks for std::make_shared<TrivialType[]> can't be completely visualized since the size of array is not stored.

Test program

#include #include #include

int main() { // make_shared for arrays of trivial types { auto p = std::make_shared<int[8]>(); p[1] = 42; } { auto p = std::make_shared<int[]>(8); p[1] = 42; } { auto p = std::make_shared_for_overwrite<int[8]>(); p[1] = 42; } { auto p = std::make_shared_for_overwrite<int[]>(8); p[1] = 42; } // make_shared for arrays of non-trivial types { auto p = std::make_sharedstd::string[8]("s"); p[1] = "42"; } { auto p = std::make_sharedstd::string[](8, "s"); p[1] = "42"; } { auto p = std::make_shared_for_overwritestd::string[8](); p[1] = "42"; } { auto p = std::make_shared_for_overwritestd::string[](8); p[1] = "42"; } // allocate_shared(_for_overwrite) { auto p = std::allocate_shared<int[8]>(std::allocator{}); p[1] = 42;

    auto q = std::allocate_shared<int[8]>(std::pmr::polymorphic_allocator<int>{});
    q[2]   = -42;
}
{
    auto p = std::allocate_shared<int[]>(std::allocator<int>{}, 8);
    p[1]   = 42;

    auto q = std::allocate_shared<int[]>(std::pmr::polymorphic_allocator<int>{}, 8);
    q[2]   = -42;
}
{
    auto p = std::allocate_shared_for_overwrite<int>(std::allocator<int>{});
    *p     = 42;

    auto q = std::allocate_shared_for_overwrite<int>(std::pmr::polymorphic_allocator<int>{});
    *q     = -42;
}
{
    auto p = std::allocate_shared_for_overwrite<int[8]>(std::allocator<int>{});
    p[1]   = 42;

    auto q = std::allocate_shared_for_overwrite<int[8]>(std::pmr::polymorphic_allocator<int>{});
    q[2]   = -42;
}
{
    auto p = std::allocate_shared_for_overwrite<int[]>(std::allocator<int>{}, 8);
    p[1]   = 42;

    auto q = std::allocate_shared_for_overwrite<int[]>(std::pmr::polymorphic_allocator<int>{}, 8);
    q[2]   = -42;
}

}

Thanks! 😻 This investigation, fix, and visualizers must have been a lot of work, I really appreciate it. I pushed a couple of tiny commits and I think this is ready to go.

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

Thanks for destroying this bug! 🐞 💥 😹

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