Built and tested on:

• Linux (GCC/clang)
• Windows (MSVC 32/64)
• MacOS (clang)
• WebAssembly (Emscripten)

This is a small header-only library, providing a unique-ownership smart pointer observable_unique_ptr that can be observed with non-owning pointers observer_ptr. It is a mixture of std::unique_ptr and std::shared_ptr: it borrows the unique-ownership semantic of std::unique_ptr (movable, non-copiable), but allows creating observer_ptr to monitor the lifetime of the pointed object (like std::weak_ptr for std::shared_ptr).

This is useful for cases where the shared-ownership of std::shared_ptr is not desirable, e.g., when lifetime must be carefully controlled and not be allowed to extend, yet non-owning/weak/observer references to the object may exist after the object has been deleted.

Note: Because of the unique ownership model, observer pointers cannot extend the lifetime of the pointed object, hence observable_unique_ptr/observer_ptr provides less thread-safety compared to std::shared_ptr/std::weak_ptr. This is also true of std::unique_ptr, and is a fundamental limitation of unique ownership. If this is an issue, you will need either to add your own explicit locking logic, or use std::shared_ptr/std::weak_ptr.

This is a header-only library requiring a C++17-compliant compiler. You have multiple ways to set it up:

• just include this repository as a submodule in your own git repository and use CMake add_subdirectory (or use CMake FetchContent), then link with target_link_libraries(<your-target> PUBLIC oup::oup).
• download the header and include it in your own sources.

From there, include the single header <oup/observable_unique_ptr.hpp>, and directly use the smart pointer in your own code:

#include <oup/observable_unique_ptr.hpp> #include <string> #include <iostream> #include <cassert> int main() { // Non-owning pointer that will outlive the object oup::observer_ptr<std::string> obs_ptr; { // Unique pointer that owns the object auto owner_ptr = oup::make_observable_unique<std::string>("hello"); // Make the observer pointer point to the object obs_ptr = owner_ptr; // Observer pointer is valid assert(!obs_ptr.expired()); // It can be used like a regular raw pointer assert(obs_ptr != nullptr); std::cout << *obs_ptr << std::endl; // The unique pointer cannot be copied auto tmp_copied = owner_ptr; // error! // ... but it can be moved auto tmp_moved = std::move(owner_ptr); // OK } // The unique pointer has gone out of scope, the object is deleted, // the observer pointer is now null. assert(obs_ptr.expired()); assert(obs_ptr == nullptr); return 0; }

The follownig limitations are features that were not implemented simply because of lack of motivation.

• observable_unique_ptr does not support pointers to arrays, but std::unique_ptr and std::shared_ptr both do.
• observable_unique_ptr does not support custom allocators, but std::shared_ptr does.

In this comparison spreadsheet, the raw pointer T* is assumed to never be owning, and used only to observe an existing object (which may or may not have been deleted). The stack and heap sizes were measured with gcc and libstdc++.

Labels:

• raw: T*
• unique: std::unique_ptr<T>
• weak: std::weak_ptr<T>
• shared: std::shared_ptr<T>
• observer: oup::observable_ptr<T>
• observable_unique: oup::observable_unique_ptr<T>

Notes:

• (1) If expired() returns true, the pointer is garanteed to remain nullptr forever, with no ace condition. If expired() returns false, the pointer could still expire on the next instant, which can lead to race conditions.
• (2) By construction, only one thread can own the pointer, therefore deletion is thread-safe.
• (3) Yes if using std::atomic<std::shared_ptr<T>> and std::atomic<std::weak_ptr<T>>.

An alternative implementation of an "observable unique pointer" can be found here. It does not compile out of the box with gcc unfortunately, but it does contain more features (like creating an observer pointer from a raw this) and lacks others (their make_observable always performs two allocations). Have a look to check if this better suits your needs.