How can I access the underlying container in STL container adaptors like stack, queue, and priority_queue?

Accessing the Underlying Container in STL Container Adaptors

Accessing the underlying container in STL container adaptors, such as stack, queue, and priority_queue, has long been a subject of curiosity. While there may not be an official standard method, a clever technique known as "container hacking" can be employed.

Container Hacking

This approach involves creating a helper class derived from the container adaptor of interest. This derived class exposes a static Container() method that returns a reference to the underlying container.

Consider the following code snippet:

<code class="cpp">template <class T, class S, class C>
S& Container(priority_queue<T, S, C>& q) {
    struct HackedQueue : private priority_queue<T, S, C> {
        static S& Container(priority_queue<T, S, C>& q) {
            return q.*&HackedQueue::c;
        }
    };
    return HackedQueue::Container(q);
}

int main() {
    priority_queue<SomeClass> pq;
    vector<SomeClass>& tasks = Container(pq);
    return 0;
}</code>

Here, the HackedQueue struct, which inherits from priority_queue, exposes the Container() method. This method grants access to the underlying vector that stores the elements in the priority queue.

Non-Standard Approaches

In Visual Studio 2008, certain container adaptors provide a non-standard _Get_container() method to access the underlying container. However, this method is not part of the official standard and may not be available in all implementations.

Official Documentation

For authoritative documentation on the C standard library, refer to the C Standard Library Reference at https://www.cppreference.com/. This website provides comprehensive information on the syntax, semantics, and usage of all standard library components.

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