Custom Exception Classes

The standard exceptions (std::runtime_error, std::invalid_argument, and the rest) cover generic failures, but they can't tell a caller anything specific about your domain. When an order can't be processed because the cart is empty, or a checkout fails because three units were requested and only one is on the shelf, your callers need a precise type they can catch and react to. Custom exception classes give you that precision: domain-specific error types that carry the context a caller actually needs.