In C++, which of the following is not a derived container under STL?

Question

Accepted Answer

The correct option is (2):
The correct option is (2), **deque** (Double-Ended Queue). In the C++ Standard Template Library (STL), containers are fundamentally divided into sequence containers, associative containers, and container adaptors. The `deque` is a direct and fundamental **sequence container**, alongside `vector` and `list`. It is an independent data structure that manages its own memory (typically as a dynamic array of blocks) and provides full functionality, including constant-time insertion and deletion at both the front and back, and random access to elements. Because it is a foundational, standalone container and not built by restricting or wrapping another container, it is not considered a "derived container" or adaptor.

The incorrect option is (1):
The incorrect option is (1), **queue**. In the STL, `std::queue` is classified as a **container adaptor**, not a fundamental container. An adaptor is a wrapper that provides a restricted interface to an underlying sequence container to impose a specific data structure model. The `queue` adaptor typically uses `std::deque` as its default base container and enforces a First-In, First-Out (FIFO) access policy. Its limited interface (like `push` at the back and `pop` from the front) is "derived" from the full capabilities of its base container.

The incorrect option is (3):
The incorrect option is (3), **priority queue**. Similar to `queue`, `std::priority_queue` is an **container adaptor**. This adaptor typically wraps a sequence container like `std::vector` and imposes an ordering (heap structure) based on the elements' values. It "derives" its functionality by adapting the base container's storage and applying heap management algorithms to ensure the element with the highest priority is always accessible, making it an adapted, or conceptually derived, container.

The incorrect option is (4):
The incorrect option is (4), **stack**. The `std::stack` is also a **container adaptor** in the STL. It is not an independent data structure but rather a layer built on top of an existing sequence container, which is often `std::deque` or `std::list`. The adaptor restricts access to the underlying container, only allowing operations at one end (the top) to enforce a Last-In, First-Out (LIFO) policy. Since it is constructed by adapting another base container, it is considered a derived container adaptor.