spring/spring — comment on commit 1a11f11198e051020268cceaf027a26ee30126ed
File:  line 0
By jk3064:

http://www.cplusplus.com/reference/stl/set/

Set
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Internally, the __elements in a set are always sorted from lower to higher__ following a specific strict weak ordering criterion set on container construction.
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Therefore, the main characteristics of set as an associative container are:
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__Elements follow a strict weak ordering at all times__. Unordered associative arrays, like unordered_set, are available in implementations following TR1.