which header to include

[ZeeGeek <zeegeek gmail com>]

I'm quite new to programming, I don't know which specific header file
to include when I want to use a certain class, for example, I'm about
to declare a UTF-8 string with Glib::ustring, then which header file
should I include? I've checked the API Reference, but it seems that it
doesn't contain which header it belongs to on the page of that class.
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