[/tmp/root] : ls -R
.:
header  src

./header:
class

./header/class:
a.h  other.h

./src:
foo.c
[/tmp/root] : cat ./header/class/a.h 
#include "class/other.h"
[/tmp/root] : cat ./header/class/other.h 
foo!
[/tmp/root] : gcc -E -I /tmp/root/header src/foo.c 
# 1 "src/foo.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "src/foo.c"
# 1 "/tmp/root/header/class/a.h" 1
# 1 "/tmp/root/header/class/other.h" 1
foo!
# 1 "/tmp/root/header/class/a.h" 2
# 1 "src/foo.c" 2
[/tmp/root] :