411 REFERENCE · Wikibooks
C++ Language/Std/Stl/CollectionClasses/SortedCollections/Map/Indexing
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A map can be indexed-by-key (a lookup operation).
Use either the C-array-operator, the at() function, or the find() function.
These differ in how they deal with a non-existent key.
Additional information about map indexing (includes interactive examples)