Copy-and-Swap Idiom

Writing a correct copy assignment operator for a class that manages a resource is harder than it looks. The naive version has at least three problems: it doesn't handle self-assignment, it leaves the object in a broken state if an allocation fails partway through, and it duplicates almost all of the logic already in the copy constructor and destructor. The copy-and-swap idiom solves all three at once by delegating to the copy constructor and a swap function. This chapter covers the problem in detail, the implementation step by step, why it gives the strong exception guarantee, how the same operator can also serve as the move assignment operator, the role of a non-member swap and argument-dependent lookup, the tradeoffs, and where modern C++ pushes you toward the rule of zero instead.