There is a facility to get temp files, isn't it ?

This will leave a ton of garbage behind itself.

Remove extra line.

The fact you need to check this all over the place indicate this should probably be pushed down further. Write and Erase should probably take a destination as parameter.

Either use std::move or put the EncodeLegacyAddr call inline.

dito

Either EraseDstPair should use that one or the other way around. The same code is duplicated 4 times, witht he same problem 4 times.

dito

It's unfortunate that this is required. I assume this is because CTxDestination is a typedef instead of a class.

It's probably too much to change this, but worth adding a comment to mention why this include is needed.

Unless this require to access private data, this can be a static method in the cpp file.

This is when you should stop and pause. What about something more along the lines of

bool buildShitToWrite(const CTxDestination &address, const std::string &key, std::pair<whatever...> &shit) {
    if (!IsValidDestination(dst)) { return false; }
    return {key, EncodeLegacyAddr(address, Params())};
}

bool CWalletDB::WritePurpose(const CTxDestination &address,
                             const std::string &strPurpose) {
    std::pair<whatever> shit;
    if (!buildShitToWrite(address, "purpose", shit)) { return false; }
    nWalletDBUpdateCounter++;
    return Write(shit, strPurpose);
}

In C++ this kind of thing tend to work better as policies: https://en.wikipedia.org/wiki/Policy-based_design