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qtgccheadercmakeprecompiled

CMake, Qt, gcc and precompiled headers

I'm (once again) struggling with the creation of precompiled headers in conjunction with gcc and Qt on the Apple platform.

When now creating my precompiled header I use a code section (based on good old "PCHSupport_26.cmake") to extract the compile flags as follows:

STRING(TOUPPER "CMAKE_CXX_FLAGS_${CMAKE_BUILD_TYPE}" _flags_var_name)
SET(_args ${CMAKE_CXX_FLAGS} ${${_flags_var_name}})
GET_DIRECTORY_PROPERTY(DIRINC INCLUDE_DIRECTORIES )
FOREACH(_item ${DIRINC})
   LIST(APPEND _args "-I${_item}")
ENDFOREACH(_item)
GET_DIRECTORY_PROPERTY(_defines_global COMPILE_DEFINITIONS)
LIST(APPEND defines ${_defines_global})
STRING(TOUPPER "COMPILE_DEFINITIONS_${CMAKE_BUILD_TYPE}" _defines_for_build_name) GET_DIRECTORY_PROPERTY(defines_build ${_defines_for_build_name})
LIST(APPEND _defines ${_defines_build})
FOREACH(_item ${_defines})
   LIST(APPEND _args "-D${_item}")
ENDFOREACH(_item ${_defines})
LIST(APPEND _args -c ${CMAKE_CURRENT_SOURCE_DIR}/${PRECOMPILED_HEADER} -o ${_gch_filename})
SEPARATE_ARGUMENTS(_args)

Unfortunately the above compiler flags miss two important parameter that CMake does generate when using the build-in compiler rules: -DQT_DEBUG and when compiling with the generated precompiled header, I get errors as follows: file.h: not used because QT_DEBUG is defined.

I would need your help with the following:

  1. Is the above way to retrieve the compiler flags correct ?
  2. Is there a better, easier, simpler way to do this ?
  3. Why does -DQT_DEBUG not show up in COMPILE_DEFINITIONS or COMPILE_DEFINITIONS_${CMAKE_BUILD_TYPE}
Solution

  • If you are using XCode simply:

        SET_TARGET_PROPERTIES(${target} PROPERTIES XCODE_ATTRIBUTE_GCC_PRECOMPILE_PREFIX_HEADER YES)
    SET_TARGET_PROPERTIES(${target} PROPERTIES XCODE_ATTRIBUTE_GCC_PREFIX_HEADER "${target}/std.h")

    I'm trying myself to use precompiled headers with raw gcc on linux through CMake but I haven't still figured it out. It look like it's working but I don't see any speed improvements.

    Edit: I managed to use pch in gcc finally with the macro you can find here: http://www.mail-archive.com/cmake@cmake.org/msg04394.html