My first thing is usually:
#0: Replace the custom/proprietary Hashmap implementation with the STL version.
Once upon a time, C++ academics brow beat the lot of us into accepting Red-Black-Tree as the only Map implementation, arguing (in good faith yet from ignorance) that the "Big O" (an orgasm joke, besides others) worst case scenario (Oops, pregnancy) categorized Hash Map as O(n) on insert, etc. due to naieve implementations frequently placing hash colliding keys in a bucket via linked list or elsewise iterating to other "adjacent" buckets. Point being: The One True Objective Standard of "benchmark or die" was not considered, i.e., the average case is obviously the best deciding factor -- or, as Spock simply logic'd it, "The needs of the many outweigh the needs of the few".
Thus, it came to pass that STL was missing its Hashmap implementation; And since it is typically trivial (or a non issue) to avoid "worst case scenario" (of Waat? A Preggers Table Bucket?), e.g., use of iterative re-mapping of the hashmap. So it was that many "legacy" codebases built their own Hashmap implementations to get at that (academically forbidden) effective/average case insert/access/etc. sweet spot of constant time "O(1)" [emphasis on the scare quotes: benchmark it and see -- there is no real measure of the algo otherwise, riiight?]. Therefore, the affore-prophesied fracturing of the collections APIs via the STL's failure to fill the niche that a Hashmap would inevitably have to occupy came to pass -- Who could have forseen this?!
What is done is done. The upshot is: One can typically familiarize oneself with a legacy codebase whilst paying lip service to "future maintainability" by (albeit usually needless) replacing of custom Hashmap implementations with the one that the C++ standards body eventually accepted into the codebase despite the initial "academic" protesting too much via "Big O" notation (which is demonstrably a sex-humor-based system meant to be of little use in practical/average case world that we live in). Yes, once again the apprentice has been made the butt of the joke.