My project can find one library header, but not another one - c++

Question

I have two C++ libraries, which I am using on Ubuntu. One of them, let's call it foo, I installed through apt-get, e.g. sudo apt-get install libfoo-dev. The other, let's call it bar, I installed by downloading the source files, and running make install. After these installations, I then have header files from foo in locations such as /usr/include/foo/foo.h, and header files from bar in locations such as /usr/local/include/bar/bar.h. From my knowledge, foo is a dependency of bar.
I then created my own C++ project, and included the line #include "bar/bar.h". But when compiling my project, I get an error saying error: foo.h: No such file or directory. If I click on the error in my debugger, it opens the file bar.h, and highlights the line #include <foo.h>. So, my project is able to find bar.h, but not foo.h. I do not mention either foo or bar in my CMakeLists.txt file.
So my questions are:
How does my project know how to find bar.h, when I have not told it where to find it in CMakeLists.txt?
What do I need to do to get my project to find foo.h and compile properly?
Thank you!
Edit: Here is my CMakeLists.txt file (the foo and bar libraries are none of the ones mentioned here):
cmake_minimum_required(VERSION 2.8.1)
set(CMAKE_CXX_FLAGS "-std=c++11 -O3")
project(Grasping_Simulator)
find_package(OpenGL REQUIRED)
find_package(GLEW REQUIRED)
find_package(GLUT REQUIRED)
find_package(Eigen3 REQUIRED)
find_package(Boost REQUIRED)
find_package(OpenCV REQUIRED)
include_directories(${OPENGL_INCLUDE_DIR} ${GLEW_INCLUDE_DIR} ${GLUT_INCLUDE_DIR} ${EIGEN3_INCLUDE_DIR} ${BOOST_INCLUDE_DIRS} ${OPENCV_INCLUDE_DIRS})
file(GLOB SRCS *.cpp *.h)
add_executable(${PROJECT_NAME} ${SRCS})
target_link_libraries(${PROJECT_NAME} ${GLEW_LIBRARY} ${GLUT_LIBRARY} ${OpenCV_LIBS} pthread GL boost_system)

Answer 1

You need to add the option -I/usr/include/foo/ to your compilation command.
By default, locations such as /usr/include/ and /usr/local/include are already in the include search path. However, your library bar does something naughty by using #include <foo.h>. Really, it should be using things like #include <foo/foo.h> (by the way if you can make this change that would be cleaner). That would allow the compiler to search all its include paths, including the path /usr/include/, and try appending /foo/foo.h - which would succeed. As it stands, there is nothing in the default include path which would work merely by appending /foo.h, so it fails to find it.
EDIT: Given your CMake code above, most likely you need to append a variable that contains the value /usr/include/foo to the include_directories line to achieve the desired effect (this being the inclusion of -I/usr/include/foo/ on the compilation line).