Add header-only library to executable in CMake - c++

Question

I am trying to include the Windows GNU GSL headers-only library (downloaded from https://gnuwin32.sourceforge.net/packages/gsl.htm) to an example code in C++ but have lots of errors unfortunately.
Here is the structure of my repository:
folder/
gsl/
gsl_sf_bessel.h
gsl_mode.h
*.h # other header files
main.cpp
CMakeLists.txt
main.cpp is as such:
#include <stdio.h>
#include <gsl_sf_bessel.h>
int main (void)
{
double x = 5.0;
double y = gsl_sf_bessel_J0 (x);
printf ("J0(%g) = %.18e\n", x, y);
return 0;
}
and CMakeLists.txt:
cmake_minimum_required(VERSION 3.0.0)
project(demoproject VERSION 0.1.0)
add_executable(
demoexecutable
main.cpp
)
target_include_directories(
demoexecutable
PUBLIC
gsl/
)
The error I get when compiling is main.cpp is:
fatal error: gsl/gsl_mode.h: No such file or directory
[build] 26 | #include <gsl/gsl_mode.h>
It looks like it managed to find gsl_sf_bessel.h from gsl/ but gsl_sf_bessel.h needs in its turn gsl_mode.h which the compiler cannot find. Any ideas on how to solve this issue?
I tried different combinations in CMakeLists.txt of functions such as add_library, include_directories, target_link_libraries but nothing worked unfortunately.

Answer 1

Try adding ${CMAKE_CURRENT_SOURCE_DIR} as an include directory. That is the directory that contains gsl/gsl_mode.h. The gsl directory does not contain itself, so adding it as an include directory will not make that error go away.

Answer 2

I've adopted the following practice in my CMake projects:
1) Project's folder structure.
Have an include/${PROJECT_NAME} subfolder (i.e., include subfolder and, within that, another subfolder named after the project's name). For example, in your case:
demoproject_or_whatever
|- include
| \- demoproject
| \- gsl
| |- gsl_sf_bessel.h
| \- gsl_mode.h
|- src
\- test
2) src/CMakeLists.txt.
Set an include_dir var at the top of the file, then use it when setting the executable's target_include_directories.
set(include_dir ${PROJECT_SOURCE_DIR}/include/${PROJECT_NAME})
add_executable(${PROJECT_NAME} ${app_sources})
target_include_directories(${PROJECT_NAME} PUBLIC
"$<BUILD_INTERFACE:${include_dir}>"
"$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
)
3. Including header files from source files.
Use paths relative to include/${PROJECT_NAME}.
#include <gsl/gsl_sf_bessel.h>