C++ include library in Cmake project - c++

Question

I am attempting to integrate a C++ library I downloaded into a new project using CMake.
I downloaded the library ("downloaded_library"), created the build folder, and inside of it ran cmake .. and make. These both ran successfully, and than I navigated to the build folder and ran ./example to run the example file that came with the library. This also was successful, and so I hoped to add it to another project.
I added this working project into the 'libraries' folder of the following directory structure:
project
-libraries
-downloaded_library
-build
-include
-downloaded_lib
-downloaded_lib.h
-src
-examples
-example.cpp
-CMakeLists.txt
-src
-main.cpp
-build
-CMakeLists.txt
I hope to run the same code that ran in example.cpp in main.cpp, but I have been unable to get the import working. I navigate to the build folder and cmake .. runs successfully, but 'make' fails with an import error (fatal error on the include line, can't find the header file). This makes sense, as I didn't expect the same include line to work (I copy and pasted example.cpp to main.cpp), but I'm not sure what to make that.
fatal error: downloaded_lib/downloaded_lib.h: No such file or directory
1 | #include "downloaded_lib/downloaded_lib.h"
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
In this situation, what should my CMakeLists.txt contain, and what should my #include contain to be able to use the contents of this library in main.cpp in the same way I can in example.cpp?
EDIT---
I changed the package names above for simplicity sake, but the repository of 'downloaded_library' is the following: https://github.com/siposcsaba89/socketcan-cpp.
My top level CMakeLists.txt looks like this:
# Minimum version of CMake required to build this project
cmake_minimum_required(VERSION 3.0)
# Name of the project
project(projectname)
# Add all the source files needed to build the executable
add_executable(${PROJECT_NAME} src/main.cpp)
add_subdirectory(libraries/downloaded_library)
# Link the executable and the library together
target_link_libraries(${PROJECT_NAME} downloaded_library)
Edit 2-- (Here I will use the original package name, socketcan-cpp).
Top level CMakeLists.txt:
# Minimum version of CMake required to build this project
cmake_minimum_required(VERSION 3.0)
# Name of the project
project(socketcan_demo)
# Add all the source files needed to build the executable
add_executable(${PROJECT_NAME} src/main.cpp)
add_subdirectory("libraries/socketcan-cpp")
target_include_directories(${PROJECT_NAME} PUBLIC "libraries/socketcan-cpp/include")
When running make I get this error:
fatal error: socketcan_cpp/socketcan_cpp_export.h: No such file or directory
4 | #include <socketcan_cpp/socketcan_cpp_export.h>
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
So the top level CMakeLists is able to find the header file located in include, but that header file contains the line: #include <socketcan_cpp/socketcan_cpp_export.h> which references a file in a gen directory inside the build directory created by the library's CMakeLists.txt (https://github.com/siposcsaba89/socketcan-cpp/blob/master/CMakeLists.txt). And my top level package is unable to find it.

Answer 1

In order to include the header files from the downloaded library, you can add the following to the CMakeLists.txt file:
find_library(downloaded_lib
NAMES downloaded_lib
HINTS "path to downloaded lib file")
if(NOT downloaded_lib)
message(FATAL_ERROR "downloaded_lib not found!")
endif()
target_include_directories(${PROJECT_EXECUTABLE} PUBLIC "path to download_lib.h")
target_link_libraries(${PROJECT_EXECUTABLE} ${downloaded_lib})
This includes the directory where the header file is located in addition to the library file.