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    <p>Despite what others have written here, I don't think that "index" or "position" has meaning with respect to a set. In mathematical terms, a set exposes only its members and maybe its cardinality. The only meaningful operations involve testing whether an item is a member of the set, and combining or subtracting sets to yield new sets.</p> <p>Some people talk about sets <em>as data structures</em> in looser terms, by facets of being "ordered" or "unordered", and whether they permit duplicates or enforce uniqueness. The former facet distinguishes an array with an <em>O(n)</em> insertion guard, where an attempt to insert an item first scans the existing members to see if the new item exists and, if not, inserts the new item at the end, and a hash table, that <em>might</em> retain such order only within a bucket's chain. A tree such as the Red-Black Tree used by <code>std::set</code> is somewhere in between; its traversal order is deterministic with respect to the <a href="http://en.wikipedia.org/wiki/Strict_weak_ordering" rel="nofollow noreferrer"><em>strict weak order</em></a> imposed by the comparator predicate, but, unlike the array sketched above, it doesn't retain <em>insertion order</em>.</p> <p>The other facet — whether the set permits duplicate elements — is meaningless in mathematics, and is more accurately described as a <em>bag</em>. Such a structure acknowledges the difference between identity and value-based "sameness."</p> <p>Your problem may involve caring about some position; it's not clear what that position means, but I expect you're going to need some data structure separate from <code>std::set</code> to model this properly. Perhaps a <code>std::map</code> mapping from your set of elements to each position would do. That would not guarantee that the positions are unique.</p> <p>It may also help clarify the problem to think how you'd model it as <em>relations</em>, such as in a relational database. What comprises the key? What portions of the entities can vary independently?</p>
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    1. POindex or position in std::set
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    1. COI am building up a "set" of unique strings from a database, then those strings must be represented as numbers. The distance() function suffices. It is possible (probable?) that set is not a good choice, I made it because it seemed an efficient way to guarantee uniqueness of the strings. The STL is new territory for me.
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    2. COIf you've found a design that solves your problem, all is well. If you find yourself warming to the STL concepts, you might enjoy the book *Elements of Programming* by Stepanov and McJones (http://www.elementsofprogramming.com/).
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