Question

I use a python script (gen_instantiations.py) to generate a cpp file (autogen_instantiations.cpp) which is included in another cpp file (foo.cpp)
So I want CMake to regenerate this file whenever foo.hpp or gen_instantiations.py changes.
Following instructions from the CMake FAQ, here’s what I did

add_custom_command(
  COMMAND "./gen_instantiations.py" 
  OUTPUT "autogen_instantiations.cpp"
  WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}" 
  DEPENDS gen_instantiations.py foo.hpp
)
add_custom_target(instantiations ALL DEPENDS autogen_instantiations.cpp)

add_library(foo
   foo.cpp  
 )
 add_dependencies(foo instantiations)

But this runs runs the script every single time. What am I doing wrong?

Answer 1

the better approach is to not #include generated autogen_instantiations.cpp, but add it to the foo library as one more source file, so cmake could see that foo depends on it and call your generator if smth has changed

set(GENERATED_SOURCES autogen_instantiations.cpp)
add_custom_command(
    OUTPUT ${GENERATED_SOURCES}
    COMMAND "./gen_instantiations.py"
    WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
    DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/gen_instantiations.py ${CMAKE_CURRENT_SOURCE_DIR}/foo.hpp
  )

add_library(
    foo
    foo.cpp
    ${GENERATED_SOURCES}
  )