How does gcc know where boost is installed I installed boost using `sudo apt-get install` on Ubuntu. Later I did not have to give gcc the `I` flag alongwith the path to the boost libraries to use boost ! Nor did I do any `pkg-config --cflags --libs` of any sort. What is the mystery here ? How is gcc able to know where boost is and how is it linking to it automatically ?

Boost is a mostly header-only library, so there is no library to link with (most of the time).

As for the headers, Ubuntu place them in `/usr/include/`, which is one of the include paths GCC use by default. So any `#include ` will work out of the box on Ubuntu.