atomic-shared-ptr

atomic_shared_ptr<T> -- thread-safe atomic operations over a std::shared_ptr<T>, with the same member API as std::atomic (load/store/exchange/ compare_exchange_weak/_strong/is_lock_free). Extracted from Xapiand.

#include "atomic_shared_ptr.h"

atomic_shared_ptr<Node> head{std::make_shared<Node>()};
auto cur = head.load();
auto next = std::make_shared<Node>();
while (!head.compare_exchange_weak(cur, next)) { /* retry */ }

Self-retiring

C++20 (P0718) adds a std::atomic<std::shared_ptr<T>> specialization that does exactly this. This header retires itself gracefully:

• Where the standard library implements it (feature-test macro __cpp_lib_atomic_shared_ptr), atomic_shared_ptr<T> is a plain alias for std::atomic<std::shared_ptr<T>> and this header adds nothing.
• Where it is not yet implemented (notably Apple clang's libc++ as of clang 17, which lacks the specialization but still provides the pre-C++20 free std::atomic_*(std::shared_ptr*) functions), it falls back to a small wrapper over those functions, exposing the same std::atomic<std::shared_ptr> member API.

Both branches are strict drop-ins for std::atomic<std::shared_ptr> (non-copyable, non-movable, no implicit conversion), so a consumer that sticks to that member API builds unchanged on either, and the wrapper disappears automatically once the toolchain gains P0718. Header-only.

Dependencies

None — header-only, standard library only.

Build / test

cmake -B build && cmake --build build && ctest --test-dir build   # incl. an 8-thread smoke test

Use (FetchContent)

FetchContent_Declare(atomic-shared-ptr GIT_REPOSITORY https://github.com/Kronuz/atomic-shared-ptr.git GIT_TAG main)
FetchContent_MakeAvailable(atomic-shared-ptr)
target_link_libraries(your_app PRIVATE atomic-shared-ptr::atomic-shared-ptr)