add_library() for header-only c++ target... again

Usage
1

Hello!

I’m trying to make a header-only target in my c++ project.

Environment: MSVC 2019, CMake 3.20 bundled with MSVC, c++ 20, WIndows 10

Here is a sample structure of my project:

/proj/common/CMakeLists.txt
/proj/common/Common.h
/proj/src/main.cpp
/proj/dll/CMakeLists.txt
/proj/dll/dll.cpp
/proj/CMakeLists.txt

Both main.cpp and dll.cpp use Common.h: #include <Common.h>.
/proj/common/CMakeLists.txt contents are like said in most examples:

add_library(common_headers INTERFACE)

target_include_directories(common_headers 
    INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}> 
              $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
)

Now, when I’m referring common_headers target in /proj/CMakeLists.txt like this:

add_executable( main WIN32 src/main.cpp)
target_link_libraries(main common_headers dll)

everything compiling fine. No erros.
But, when I try to reference the targets a different way (pointet by a CMake docs btw):

add_executable( main WIN32 src/main.cpp)
target_link_libraries(main 
   INTERFACE common_headers 
   PRIVATE dll
)

I get a compiler error in main.cpp “Unknown header Common.h”
Could someone please explain finally a correct way of doing a header-only targets in CMake? Because literally half of an examples on this topic I found in google does not work in my case.

2

You specified the wrong keyword for the compilation of target main.
The INTERFACE keyword means that consumers of the main target will be allowed to use the common_headers target, but it is not available for the main target itself. To ensure the main target can use the common_headers target, the keyword PRIVATE or PUBLIC must be used (PUBLIC is a synthetic way to specify the target for both PRIVATE and INTERFACE).

In your case, I think you can use PRIVATE keyword for both targets:

add_executable( main WIN32 src/main.cpp)
target_link_libraries(main PRIVATE common_headers dll)
1 Like
3

Thank you!

The usage specifics of these keywords was not obvious for me.