What are smart pointers in C++ and when should you use each type?

Smart pointers automate memory management using RAII. unique_ptr: exclusive ownership, non-copyable, movable. Use for single owner resources. shared_ptr: shared ownership via reference counting, copied increments count, deletion when count reaches zero. Use when multiple owners needed. weak_ptr: non-owning reference to shared_ptr, doesn't affect count, check validity with lock(). Use to break circular references. make_unique and make_shared preferred for creation. Smart pointers prevent memory leaks and dangling pointers. Avoid raw pointers for ownership; use for non-owning references.