How to determine which header files to include?

Is there an easier/better way to ensure cross-compiler compatibility?

This is always going to be a bit of a chore if you have a huge codebase and haven't been doing this so far, but once you've gone through fixing your includes, you can stick to a simple procedure:

When you write new code that uses a standard feature, like std::stoi, plug that name into Google, go to the cppreference.com article for it, then look at the top to see which header it's defined in.

Then include that, if it's not already included. Job done!

(You could use the standard for this, but that's not as accessible.)

Do not be tempted to sack it all off in favour of cheap, unportable hacks like <bits/stdc++.h>!

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tl;dr: documentation