[fnord123]

That's the most used example of a memory leak but it is not the definition of a memory leak.

If you put data into a hash map and forget the key, you leaked.

[saghm]

This is also why valgrind classifies the leaks it reports with stuff like "still reachable" or "possibly still in use" (I might be remembering the exact phrasing incorrectly). It would be pretty hard to programmatically determine whether the memory that's still kept around was intended to be kept around or not, which is why valgrind supports generating "suppressions" (and specifying them in subsequent runs to be ignored).

[titzer]

This is the use case for weak maps, which both Java and JavaScript have. In the latter case, the map is not iterable, so one cannot observe JavaScript GC (through WeakMap at least).