How to add defines to moc via cmake

Stewart 0
Viewed 21

I have a header like this:

#ifdef WITH_QT
#include <QObject>
#endif

namespace foo {
#ifdef WITH_QT
    Q_NAMESPACE
#endif

    enum class Letters {A, B, C };
#ifdef WITH_QT
    Q_ENUM_NS(Letters)
#endif
}

And a cmake file:

cmake_minimum_required(VERSION 3.13)

find_package(Qt5 COMPONENTS Core)
qt5_wrap_cpp(moc_source foo.h)

add_library(foo STATIC ${moc_source})
target_compile_definitions(foo PUBLIC WITH_QT)

make VERBOSE=1 shows (abbreviated):

/usr/bin/moc foo.h
foo.h:0: Note: No relevant classes found. No output generated.
qt cmake
1 Answers

moc needs to be run with

/usr/bin/moc -DWITH_QT foo.h

CMake isn't forwarding your compile definitions to moc. That prevents those macros from existing, and inhibits moc from finding anything useful to generate.

A few solutions:

  1. This will add WITH_QT globally:
add_definitions(-DWITH_QT)
  1. This more refined method will tell qt5_wrap_cpp to import the compile definitions of a specific target. This is better as it avoids contaminating the compile definitions of the rest of your project.
qt5_wrap_cpp(moc_source foo.h TARGET foo)

See https://doc.qt.io/qt-5/qtcore-cmake-qt5-wrap-cpp.html for qt5_wrap_cpp() usage.

  1. Use AUTOMOC by changing your CMakeLists to:
cmake_minimum_required(VERSION 3.13)

find_package(Qt5 COMPONENTS Core)
set(CMAKE_AUTOMOC ON)

add_library(foo STATIC foo.h)
target_compile_definitions(foo PUBLIC WITH_QT)

All three solutions will give you an archive with the missing symbol:

$ nm -gC libfoo.a 

moc_foo.cpp.o:
                 U _GLOBAL_OFFSET_TABLE_
                 U qt_version_tag
0000000000000000 D foo::staticMetaObject
Stewart0
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