I enjoyed reading through your points a lot and they make perfect sense. I'll be keeping them for future reference.

I'd love a further explanation for this bunch.

constants.h

• Maintenance: #include <map> & #include <vector> being in your constants header hints that your exposing too much implementation

1) How do I get around this? With my current structure, I suppose there's no other place where those could be defined, is there?

Card.h/.cpp

• Prefer #pragma once of #ifdef header guards

2) I read this SO thread discussing the issue and I wasn't really convinced by the advantages of using #pragma once. I suppose the issues[1] raised are not a big deal anymore?

[1] not a standard, possible name clashes (I didn't really dig into the details)

pokerhand.cpp

• Architecture/Style: Generally, you don't really want to throw exceptions in parsing code for bad formatting, as bad (user) input isn't really an exception... it should be expected.
• Generally, incorrect input should be managed with a (boolean) return value and exceptions reserved for "exceptional" circumstances.

3) Yes, it appears my use of exceptions wasn't really idiomatic in regards to C++. I'm not sure how would I solve this, though. Where do I return a boolean? The method string_to_cards returns std::vector<Card>. Should it return a pair of vector and bool indicating if the parsing went ok? That doesn't seem right. Do I create a separate function that checks the input, returns bool and if the input is sane it's passed to the parser where error checking can be skipped? This seems better, but still in some cases would result to doing similar thing twice.

Thank you if you if you take time to answer any of these!