Here's the code: https://github.com/Nyapp/Cell/blob/master/src/engine_classes.h
The TTF_CloseFont() that I'm having trouble with is in Text::~Text(). When I close the game, I get a segfault. What am I doing wrong?

You are probably deleting/closing the font twice. Segmentation faults generally occur upon trying to dereference invalid pointers, or similar.

Set a breakpoint in the destructor and verify that your font is a valid address.

If you have TTF_CloseFont() somewhere else, put breakpoints there as well.

Check you aren't deleting text that has already been deleted too.

Classes should obey the Rule of Three. Anytime you add a destructor to a class, think about whether to handle copying instances of that class, or else make the class as noncopyable.