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This adds another piece of C++20, the std::atomic specializations for std::shared_ptr and std::weak_ptr. The new _Sp_atomic type mimics the structure of shared_ptr<T> and weak_ptr<T>, holding a T* pointer (the one returned by get() on a shared_ptr/weak ptr) and a _Sp_counted_base<>* pointer to the ref-counted control block. For _Sp_atomic the low bit of the control block pointer is used as a lock bit, to ensure only one thread will access the object at a time. The pointer is actually stored as a uintptr_t to avoid accidental dereferences of the pointer when unlocked (which would be a race) or when locked (which would dereference the wrong pointer value due to the low bit being set). To get a raw pointer to the control block, the lock must be acquired. Converting between a _Sp_atomic and a shared_ptr or weak_ptr requires manually adjusting the T* and _Sp_counted_base<>* members of the shared/weak ptr, instead of going through the public API. This must be done carefully to ensure that any change in the number of owners is reflected in a ref-count update. Co-authored-by: Thomas Rodgers <trodgers@redhat.com> Signed-off-by: Thomas Rodgers <trodgers@redhat.com> libstdc++-v3/ChangeLog: * include/bits/shared_ptr_atomic.h (__cpp_lib_atomic_shared_ptr): New macro. (_Sp_atomic): New class template. (atomic<shared_ptr<T>>, atomic<weak_ptr<T>>): New partial specializations. * include/bits/shared_ptr_base.h (__shared_count, __weak_count) (__shared_ptr, __weak_ptr): Declare _Sp_atomic as a friend. * include/std/version (__cpp_lib_atomic_shared_ptr): New macro. * testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc: New test. * testsuite/20_util/weak_ptr/atomic_weak_ptr.cc: New test. |
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| assign | ||
| atomic | ||
| casts | ||
| comparison | ||
| cons | ||
| creation | ||
| dest | ||
| hash | ||
| misc | ||
| modifiers | ||
| observers | ||
| requirements | ||
| thread | ||