How Can I Access the Underlying Container of an STL Container Adaptor?

Accessing Underlying Container of STL Container Adaptors

Container adaptors in the C Standard Template Library (STL), such as stack, queue, and priority_queue, provide a convenient interface on top of underlying containers like deque or vector. However, obtaining a reference to the underlying container, as needed for customized operations, can be challenging.

Is There a Standard Way?

No, there is no standard method to directly access the underlying container from these adaptors. The _Get_container() method mentioned is not part of the C standard and may vary depending on the implementation.

Common Workaround

One common workaround involves using template specialization, as follows:

<code class="cpp">template <class Type>
struct ContainerAdapter {
  static Type& container(const Type& adapter) {
    return adapter; // Assumes underlying container is of the same type
  }
};</code>

By specializing this template for each specific adaptor type, we can access the underlying container. For example:

<code class="cpp">template <>
struct ContainerAdapter<std::stack<int>> {
  static std::deque<int>& container(const std::stack<int>& stack) {
    return stack.*&stack::_deque; // Internal implementation hack
  }
};</code>

Custom Implementation for priority_queue

In the case of priority_queue, a similar approach can be implemented using a struct template:

<code class="cpp">template <class T, class S, class C>
struct PriorityQueueAdapter {
  static S& container(priority_queue<T, S, C>& pq) {
    // Use a type trick to access private member
    struct Inner : private priority_queue<T, S, C> {
      static S& container(priority_queue<T, S, C>& pq) {
        return pq.*&Inner::c;
      }
    };
    return Inner::container(pq);
  }
};</code>

Official Documentation

The official documentation for the C Standard Library can be found on the ISO/IEC website: https://www.iso.org/standard/74983.html

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