Include upstream folders for header-only libraries

Code
1

I’m writing two C++ project, the upstream one providing some header-only libraries. I wonder what is the right (i.e. modern-CMake) way to “export” the headers folder from upstream and use it in downstream. I know I can export some custom variables but I’d like to stick with a streamlined solution if it exists.

2

If those are two different projects, then I’d make a proper installable CMake package for the upstream project, and then add it to the downstream project with the usual:

find_package(UpstreamHeaders CONFIG REQUIRED)
target_link_libraries(${PROJECT_NAME}
    PRIVATE
        upstream::headers # or UpstreamHeaders, or whichever you choose
)
3

What I don’t understand is how to define the upstream::headers target in the upstream project. I guess that in your example it is an empty library which is used just to include the upstream header folder in downstream targets linking it, right? IF yes, then I cannot create something similar in my upstream project since defining no source file I get this error:

CMake Error at src/common/CMakeLists.txt:7 (add_library):
No SOURCES given to target: KM3NeTCommon

Or did I misunderstood your suggestion?

4

Got it:

add_library(EmptyLib INTERFACE)
target_include_directories(EmptyLib INTERFACE $<BUILD_INTERFACE:${CMAKE_SOURCE_DIR}/include>)

Thanks for your help!

5

I didn’t get what you mean either.

A header-only library is declared with:

add_library(UpstreamHeaders INTERFACE)

and it does not need to have sources. It will need to have target_include_directories() with INTERFACE scope though (and the corresponding generator expressions for BUILD_INTERFACE and INSTALL_INTERFACE), but that you seem to have figured out.

To make it “namespaced”, one usually does the following:

add_library(upstream::headers ALIAS UpstreamHeaders)

and also(?) provide NAMESPACE argument for the install() command that exports target(s).

6

Sorry for not being clear, my question was basically how to define a header-only library, thanks again for your help.

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