I profiled bitcoind's memory usage and noticed that the tx hashes as keys in mapNextTx are taking a large amount of space, but they're redundant since the corresponding value has a pointer to the CTransaction that has the hash in it. On a system with 64-bit pointers, replacing these hashes with pointers or references would, once the mempool warms up, save about 7% of total memory usage.

I explored different ways of doing this.

I ruled out std::reference_wrapper (possibly the most straightforward way of using references as keys in a map), because it's a big footgun and also the syntax for using it would be weird; you'd have to static_cast<COutpoint&>(it->first) to get the "value" of a key from an iterator.

The simplest approach I found was to use a map<const COutPoint*, CInpoint, DereferencingComparator<const COutPoint*>> -- but this also required strange syntax: map.find(&outpoint) is misleading, since the outpoints will be compared as values.

The implementation here is a refinement of the DereferencingComparator approach: I created refmap as a simple proxy to a map that has pointers as keys and uses the DereferencingComparator; refmap passes through all methods except those that normally take a comparison value, which it takes by const reference and gives the address to the underlying map. This allows a map to use pointers as keys, and:

• compare keys by pointed-to value
• insert keys or return them from the map as pointers (which makes clear to the map user the constraint on the lifetime of the pointed-to key object)
• perform value lookups by reference as a normal map would (making it clear that keys are compared by value)