Note that there's a great C99/C++ single header library, tinyobjloader, that provides robust (in my experience) and feature-full OBJ loading, including triangulation of n-gons and full material parsing.

https://github.com/tinyobjloader/tinyobjloader

It's fairly mature and handles many of the parsing footguns you'll inevitably run into trying to write your own OBJ parser.