Does std::multiset guarantee insertion order?

Question

I have a std::multiset which stores elements of class A. I have provided my own implementation of operator< for this class. My question is if I insert two equivalent objects into this multiset is their order guaranteed? For example, first I insert a object a1 into the set and then I insert an equivalent object a2 into this set. Can I expect the a1 to come before a2 when I iterate through the set? If no, is there any way to achieve this using multiset?

Answer 1

In C++03 you are not guaranteed that insert and erase preserve relative ordering. However, this is changed in C++0x:

n3092, §23.2.4/4: An associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. The set and map classes support unique keys; the multiset and multimap classes support equivalent keys. For multiset and multimap, insert and erase preserve the relative ordering of equivalent elements. _{Emphasis mine.}

This is discussed in this defect report. This page is the collection of comments on the issue, it's well-written and quite fleshed-out. (I very much recommend reading this one over the previous "overview" link.)

From that comment page you'll find a comparison of current implementations, so you can check if the implementations you intend to use follow what you expect.

I can't think of a way to force the ordering you want off the top of my head. :/

Answer 2

Taking into account that a1 and a2 would compare equal in your example, and what is actually stored in the std::multiset are copies of a1 and a2, I don't really know how would you know which is which.

If you can tell the difference, maybe class A was not well designed in the first place. So std::multiset does not guarantee such a thing.

Answer 3

std::multimap does not guarante this. If you can express your operator< using an integer via a function e.g. int A::orderingInt(), you could use a

std::multiset<MyCustom> myset;

with

class MyCustom : public std::vector<A> {}

with overloaded

bool operator<(const MyCustom& a, const MyCustom& b) {
   // theoretically empty MyCustom should not occure
   return a[0].orderingInt() < b[0].orderingInt();
}

Of course adding and iteration would be different now:

A a;
myset[a.orderingInt()].push_back(a);

// groups with "small" elements first
for(std::multiset<MyCustom>::iterator it=myset.begin(); it!=myset.end(); it++) {
    // those elements are "equal"
    for(std::vector<A>::iterator jt=it->begin(); jt->end(); jt++) { 
         // use A& a = *jt;
    }
}