header-only llibrary using meson

Can anyone help me understand the following: I am writng a header-only library 'foo' with the following structure:

Project_Root

1. meson.build
2. include/foo.hpp
3. foo/meson.build
4. foo/bar.hpp

Here are the details:

[file: meson.build]

project('foobar', 'cpp',
  version: '0.1.0',
  default_options: ['cpp_std=c++20']
)

project_headers = []

subdir('include')

install_headers(project_headers, preserve_path: true)

[file: include/meson.build]

project_headers += files('foo.hpp')

subdir('foo')

[file: include/foo/meson.build]

project_headers += files('bar.hpp')

My question is why during installation, both foo.hpp and bar.hpp are installed in the same directory even though subdirectory-structure should remain preserved by using preserve_path: true ?

When using preserve_path meson extracts the directory from the file name. Here's the code from install_headers():

...
    if kwargs['preserve_path']:
        for file in source_files:
            dirname = os.path.dirname(file.fname)
            dirs[dirname].append(file)
    else:
        dirs[''].extend(source_files)
...

https://github.com/mesonbuild/meson/blob/e2c454b86e5fcb2dbbabe33199dc4b877dcdb95d/mesonbuild/interpreter/interpreter.py#L2228

In your case the fname for bar.hpp will be bar.hpp so will have no path. The subdirectory is stored by files() but is held in a different attribute. I cannot comment on why it is implemented in this way. I am not a Meson developer.

The example in the Meson release notes shows the path being included with the file names:

https://mesonbuild.com/Release-notes-for-0-63-0.html#added-preserve_path-arg-to-install_headers