Unable to include C++ header file while compiling

edit

My answer is just guessing, so i point out certain potential concerns related to your implementation, in numbered points.

1. You should paste a bigger piece of error message.

   If you have only two files in your package, most likely you have #include "example_ros_class.h" in the example_ros_class.cpp resulting in fatal error: example_ros_class.h: No such file or directory

2. Your CMakeLists.txt contains a lot of mistakes

A fixed file would look like this

cmake_minimum_required(VERSION 2.8.3)

project(eg_ros_class)

find_package(catkin REQUIRED COMPONENTS
  roscpp
  std_msgs
)

catkin_package(
  # (A) INCLUDE_DIRS include
  LIBRARIES eg_ros_class
  CATKIN_DEPENDS roscpp std_msgs
  # (B) DEPENDS system_lib
)

##BUILD##
include_directories(
  include
  # (C) src/eg_ros_class
  ${catkin_INCLUDE_DIRS}
  # (D) ${Boost_INCLUDE_DIRS}
  # (E) ${GSTREAMER_INCLUDE_DIRS}
)

add_library(eg_ros_class src/example_ros_class.cpp)

add_executable(eg_ros_class_exe src/example_ros_class.cpp)
# (F) target_link_libraries(eg_ros_class ${catkin_LIBRARIES}, )
target_link_libraries(eg_ros_class ${catkin_LIBRARIES})

add_executable(implementation_exe src/implementation.cpp)
target_link_libraries(implementation_exe ${catkin_LIBRARIES})
add_dependencies(implementation_exe eg_ros_class_generate_messages_cpp)

##Install##
# (G)
#catkin_package(INCLUDE_DIRS include
#               LIBRARIES eg_ros_class)
# install(TARGETS eg_ros_class
#   ARCHIVE DESTINATION ${CATKIN_PACKAGE_LIB_DESTINATION}
#   LIBRARY DESTINATION ${CATKIN_PACKAGE_LIB_DESTINATION}
#   RUNTIME DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
# )
# install(DIRECTORY include/${PROJECT_NAME}/
#   DESTINATION ${CATKIN_PACKAGE_INCLUDE_DESTINATION}
#   FILES_MATCHING PATTERN "*.h"
#   PATTERN ".svn" EXCLUDE
# )E_BIN_DESTINATION}
# )
# install(DIRECTORY include/${PROJECT_NAME}/
#   DESTINATION ${CATKIN_PACKAGE_INCLUDE_DESTINATION}
#   FILES_MATCHING PATTERN "*.h"
#   PATTERN ".svn" EXCLUDE
# )

2 A. You wrote that your project contains only two files, so include directory is empty. In this case this line is not needed.

2 B. What is a system_lib? It's probably the name used as an example in the template CMakeLists.txt.

2 C. In this line you should include only headers

2 D and 2E. It depends on if you using Boost or GStreamer library. Are you using?

2 F. Freaky comma before parenthesis. Fixed in the next line: target_link_libraries(eg_ros_class ${catkin_LIBRARIES})

2 G. You don't need use catkin_package again