suppose i have directory tree like this :

.
├── .catkin_tools
│   ├── CATKIN_IGNORE
│   ├── profiles
│   │   └── default
│   ├── README
│   └── VERSION
└── src
    ├── pkg_a
    │   ├── CMakeLists.txt
    │   ├── mutiply.cpp
    │   ├── package.xml
    │   └── sumfunc.cpp
    └── pkg_b
        ├── CMakeLists.txt
        ├── main.cpp
        └── package.xml

6 directories, 10 files

how can i edit CMakeLists.txt So that i can use those file in pkg_a inside pkg_b/main.cpp like this :

#include <pkg_a/mutiply.cpp>
#include <pkg_a/sumfunc.cpp>
#include <pkg_a/someheader.h>// or maybe this one

int main(void) {
    int sum_result = sum(5,5);
    int muli_result = muti(5,5);
    return 0;
}

NOTE : i already take a look at this recommend but i not quite understand it.

Edit :

after @Weasfas Answer i have create func_terminal.h file in pkg_a and have this tree file :

└── src
    ├── pkg_a
    │   ├── CMakeLists.txt
    │   ├── func_terminal.h
    │   ├── mutiply.cpp
    │   ├── package.xml
    │   └── sumfunc.cpp
    └── pkg_b
        ├── CMakeLists.txt
        ├── main.cpp
        └── package.xml

and add some command in cmakeList.txt in pkg_b : add find package

find_package(catkin REQUIRED COMPONENTS roscpp pkg_a)

add include_directories

include_directories(
  include
  ${catkin_INCLUDE_DIRS}

)

add catkin package

catkin_package(
  CATKIN_DEPENDS pkg_a
)

and in building

add_executable(main_node main.cpp) 
target_link_libraries(main_node ${catkin_LIBRARIES}) 
add_dependencies(main_node ${catkin_EXPORTED_TARGETS})

then edit package.xml by add these line

  <build_depend>pkg_a</build_depend>
  <build_export_depend>pkg_a</build_export_depend>
  <exec_depend>pkg_a</exec_depend>

but i will get an error

fatal error: pkg_a/func_terminal.h: No such file or directory

from catkin build if i try to add this line #include <pkg_a/func_terminal.h> in pkg_b/main.cpp file

Edit #2(RECAP):

I'll explain the solution i have got thank to @Weasfas steb by steb here : first i need to create a proper catkin_work_space for example in my case i use these command :

mkdir tryCatkin && mkdir tryCatkin/src && cd tryCatkin && catkin_make

then i can either use catkin_create_pkg command or just create out of of nowhere by following catkin package format which will create directory tree like this:

├── src
│   ├── CMakeLists.txt -> /opt/ros/kinetic/share/catkin/cmake/toplevel.cmake
│   ├── pkg_a
│   │   ├── CMakeLists.txt
│   │   ├── include
│   │   │   └── pkg_a
│   │   │       └── fuction_terminal.hpp
│   │   ├── package.xml
│   │   └── src
│   │       └── function_terminal.cpp
│   └── pkg_b
│       ├── CMakeLists.txt
│       ├── include
│       │   └── pkg_b
│       ├── package.xml
│       └── src
│           └── main.cpp

in pkg_b/src/main.cpp :

#include <iostream>
#include <pkg_a/fuction_terminal.hpp>

int main(void) {
    int sum_result = sumation(5,5); ///sumation is function in func_terminal.hpp
    std::cout << "test :  " << sum_result;

    return 0;
}

then setup those package.xml and CmakeList.txt which i follow from @Weasfas instruction. first in pkg_a/Cmakelist.txt :

cmake_minimum_required(VERSION 2.8.3)
project(pkg_a)

find_package(catkin REQUIRED COMPONENTS)

catkin_package(
  INCLUDE_DIRS include
  LIBRARIES SumFunc
)

include_directories(
  include
  ${catkin_INCLUDE_DIRS}
)

add_library(SumFunc
   src/function_terminal.cpp
)

pkg_b/CmakeList.txt :

cmake_minimum_required(VERSION 2.8.3)
project(pkg_b)


find_package(catkin REQUIRED COMPONENTS
  pkg_a
)
catkin_package()

include_directories(
  ${catkin_INCLUDE_DIRS}
)

add_executable(main_node src/main.cpp)

target_link_libraries(main_node
  ${catkin_LIBRARIES}
)

and then in pkg_b/package.xml add this line:

  <depend>pkg_a</depend>