I'm looking for suggestions regarding in-memory key-value store engines or libraries, that have C++ interfaces or that are written in C++.
I'm looking for solutions that can scale without any problems to about 100mill key-value pairs and that are compatible/compilable on linux and win32/64
How about std::map?
http://cplusplus.com/reference/stl/map/
If you really need to store such amount of pairs in memory consider this Sparse Hash. It has special implementation which is optimized for low memory consumption.
std::map is fine given that size of key and value is small and the available memory is large ( for about 100million pairs). If its not the case, and you want to run a program over the key-value pairs, consider using a standard MapReduce API. Map Reduce is specifically meant to be used on distributed systems and process large data specially key-value pairs. Also there are nice C++ APIs for Map Reduce. http://en.wikipedia.org/wiki/MapReduce
Try Tokyo Cabinet, it supports hashtables and B+trees:
Try FastDB, though you may get more than you ask for. Tokyo cabinet also seems to support in-memory databases. (Or, backed by file mapped by mmap. With modern operating systems, there's no much difference between "in-ram" database and something mmap'd as the OS caching makes also the latter very efficient).
A hash map (also called unordered map) is the best bet for that many pairs. You can find an implementation in Boost and TR1.
Edit: Some people have questioned the size- if he's got, say, a 64bit server, there's plenty of space for 100million kv pairs.
Oracle Berkeley_db is what you need.
来源：https://stackoverflow.com/questions/3122652/c-in-memory-key-value-stores