Implementation of a locally ordered set or priority queue?

Question

I have a rather large set of objects that represent numbers and I want to select such numbers according to a custom ordering. This ordering includes several criteria such as the type of their representation (some numbers are represented by an interval), their integrality and ultimatively their value. These numbers are shared throughout the programs (shared pointers) and there is nothing I can do about this.

However, the elements properties can change at any time such that the order changes while I can't notify the container about this. For example, some operations require a refinement of a number that is represented by an interval and during this refinement, the exact value can be found. Thereby, the number changes from the interval representation to a rational number, possibly even an integer. This change, due to the shared instance, immediately propagates to the number in the container and breaks the ordering (and I don't even know which number changed). This totally breaks std::set .

So what I'd like to have is a container that tries to be sorted, but does not rely on this. Whenever an operation detects an incorrect ordering, this ordering should be corrected locally. For example insert would insert the element (using binary search) and always check if the ordering of the current element (wrt the neighbors) is correct.

I'd be willing to accept that "give me the smallest element" would then be only "give me a small element" and that find or remove would have linear complexity: I only need front , insert and remove_front to be particularly efficient.

Is there any implementation that does something like this? How would you implement this?

Answer 1

If you are looking for an algorithm in the standard library, you should take a look at:

std::make_heap
std::pop_heap
std::push_heap

In <algorithm> . They might fit your need, and even if they don't I'm quite sure you will find what you are looking for in some kind of heap structure. Which one will probably depend on how your code is structured, and how often you expect a value to change etc.

In short:

A heap is a data structure in which it is fast to find and extract the smallest (or largest) element. It is also for most heaps possible to create restructure the heap in linear time or better. You could start out from this page on Wikipedia if you want to learn more about heaps.