Extending Ranges and Views to Non-Iterable Container Adaptors
1. Motivation
std::stack, std::queue, and std::priority_queue are container adaptors: thin
wrappers around an underlying sequence container (vector, deque, list) that
expose a deliberately restricted interface — push, pop, top/front, empty,
size. None of them expose begin()/end(). This is not an oversight; it is a
design decision inherited from the original SGI STL, whose documentation states
outright that priority_queue "does not allow iteration through its elements."
The rationale is that the adaptor's whole contract is "you may only ever see the
current top/front element," and exposing arbitrary iteration would leak the
underlying container's storage order (vector's heap layout, in the case of
priority_queue), which is not part of the abstraction.
The consequence is that these three types cannot participate in the modern
iteration and composition idioms C++ has accumulated since C++11 — range-based
for, and since C++20, the <ranges> view/pipe machinery — because both require
some notion of begin()/end(). This document describes pop_range, a small
header-only library that closes that gap by giving these adaptors a well-defined,
honestly-named destructive view: iterating it doesn't just read the adaptor, it
drains it, one pop() per increment.