问题
I wrote a program that have to read a big file with operator>> , count some stuff and insert datas (string and int) in a multimap. The problem is that the program crashes and it seems to be a memory problem (it works well with small files). I think that it reaches the memory allocation limit of Windows.
What I tried : I made a 32-bit and an 64-bit versions. As you know, Windows has a memory allocation limit that is not the same for 32bits and 64bits builds. With both it crashes. I looked the performance tab of the task manager to observe the memory consumption. 32-bit : the program crashes every time it reaches 5.3 GB. 64-bit : the memory consumption of the program continually increases until it reaches the whole RAM of the system and crashes.
I tried with a map instead of a multimap : the program never crashes and stay stable at roughly 3.5 GB (32-bit and 64-bit builds) all along the program execution.
So, do you know if there is a difference between map and multimap in terms of memory usage or memory management ? Or is it just because map do not allows duplicate keys ?
回答1:
Of course there is. Did you read descriptions?
std::map:
std::map is a sorted associative container that contains key-value pairs with unique keys. Keys are sorted by using the comparison function Compare.
std::map::insert:
Return value: 1-2) Returns a pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool denoting whether the insertion took place.
You cannot have two same keys, so you overwrite values under the same key, you don't allocate any new space.
std::multimap:
Multimap is an associative container that contains a sorted list of key-value pairs.
std::multimap::insert cannot "fail", it always return an iterator because, it always adds new association. So adding items under the same key in a loop will allocate new memory.
Similar is true for every multi-container (set, unordered map, etc.). They are semantically different.
来源：https://stackoverflow.com/questions/29229492/map-and-multimap-memory-usage-management-differences