CMake: header-only library with generated files

Question

I have a library that needs to carry some constant data injected from the content of non-source files (in this case, OpenGL shader code). To achieve this, I'm using add_custom_command() to generate include files that I can then #include into my code to initialize const static variables.

This works perfectly with regular libraries (static or shared), but now I'd like to make my library header-only. The ability of C++ to let static methods return static data without running the risk of having that data duplicated in each translation unit ("magic statics") makes this possible.

The problem however is that CMake seems to assume that an INTERFACE library (which is the CMake feature that I'm using to create header-only libraries) does not need building - which, in this case, is wrong.

(I realize that there is no actual obligation for my library to be header-only. In this particular case, the reason I want this is that I would like the library, which is doing OpenGL, to remain independent of any specific binding library [such as GLEW or GLee or the newcomer glbinding]. By keeping my library header-only, I can leave that choice to the user - all he needs to do is #include the header of the binding library before mine.)

Does anyone see a way to have CMake trigger the header-generating custom commands, at the latest when the consumer project is being built?

EDIT: I just realized that I could have the "best of both worlds" as it were by keeping my library static but still keeping all my code except for the constant data in the header files. That way, there would still be no need to choose a specific OpenGL binding library. However, there are still advantages to having a library be header-only - simplicity of use for one - so I'm leaving my question open.

EDIT #2: Here is the relevant part of my CMakeLists.txt file (I only stripped the library dependencies - all header-only - from the end):

set(SHADER_FILES "src/vertex.glsl" "src/fragment.glsl")

add_library(libGPCGUIGLRenderer INTERFACE)
target_sources(libGPCGUIGLRenderer INTERFACE ${SHADER_FILES})

target_include_directories(libGPCGUIGLRenderer BEFORE
  INTERFACE
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
    $<INSTALL_INTERFACE:include>
)

# Embed shader files

source_group("Shader files" FILES ${SHADER_FILES})

set(GENERATED "${CMAKE_CURRENT_BINARY_DIR}/generated")
target_include_directories(libGPCGUIGLRenderer INTERFACE ${GENERATED})

# Find the GPC Bin2C utility
find_package(GPCBin2C REQUIRED)

# Add a custom target and a dependency for each shader file    
foreach(shader ${SHADER_FILES})
  get_filename_component(name "${shader}" NAME)
  set(shader_header "${GENERATED}/${name}.h")
  add_custom_command(
    OUTPUT ${shader_header}
    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/${shader}
    COMMAND GPCBin2C --input=${CMAKE_CURRENT_SOURCE_DIR}/${shader} --output=${shader_header}
  )
  target_sources(libGPCGUIGLRenderer INTERFACE ${shader_header})
endforeach()

Answer 1

Creating a static library with headers as the only sources worked for me. It is, of course, only a work-around.

• Creating a static library with only header files results in an empty library. Mine says !<arch> as the only content.
• CMake will automatically get the dependencies correct across sub-directories.
• Since all sources are headers, you need to tell CMake which linker language should be used.

Code:

set(OUTDIR "${CMAKE_CURRENT_BINARY_DIR}/generated_include")
add_custom_command(
    OUTPUT "${OUTDIR}/outfile.h"
    # Replace the next two lines with a proper generating script.
    COMMAND mkdir -p ${OUTDIR}
    COMMAND touch ${OUTDIR}/outfile.h
)

# Note, I am only adding header files to the library.
add_library(generated-headers STATIC 
    "${OUTDIR}/outfile.h"
)
set_target_properties(generated-headers
    PROPERTIES LINKER_LANGUAGE CXX)
target_include_directories(generated-headers PUBLIC ${OUTDIR})

Use in other directories like this:

# In any other directory of the same CMake project:
add_executable(main main.cpp)
target_link_libraries(main generated-headers)

Tested on CMake 3.2, 3.8 and 3.9. Using Ninja and Make generators.