What are smart pointers and when to use each?

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**std::unique_ptr:** Single owner, zero overhead ```cpp auto p = std::make_unique(); auto p2 = std::move(p); // p is null ``` **std::shared_ptr:** Shared ownership with ref counting ```cpp auto p1 = std::make_shared(); auto p2 = p1; // ref count = 2 ``` **std::weak_ptr:** Non-owning reference (breaks cycles) ```cpp std::weak_ptr wp = sp; if (auto locked = wp.lock()) { /* use */ } ``` **When to use:** - `unique_ptr`: default choice for ownership - `shared_ptr`: shared ownership, complex graphs - `weak_ptr`: cache, observer, breaking cycles - Raw pointers: non-owning references

C++: What are smart pointers and when to use each?