Qt 5 cmake fails with undefined reference to vtable on hello world with inc & src as subdirs

Question

Update 2

After messing around a bit (and some editing of the generated Makefiles), it looks like what is happening is that moc is not properly processing MainWindow.h (included by main.cpp and MainWindow.cpp unless it is in the same folder as the source files which included it.

Moc runs on MainWindow.cpp, doesn't process the include and thus doesn't see the Q_OBJECT macro so proceeds to produce an empty output file. I'm not sure whether moc usually processes includes or if it just scans the directory, but either way, headers that need mocing but are in other directories are not being processed!

Update

The problem appears to be related to the output produced by moc. In the first case (the one that compiles), hello-world_automoc.cpp and moc_MainWindow.cpp are generated. hello-world_automoc.cpp looks like

/* This file is autogenerated, do not edit*/
#include "moc_MainWindow.cpp"

In the second case, a hello-world_automoc.cpp is produced that looks like

/* This file is autogenerated, do not edit*/
enum some_compilers { need_more_than_nothing };

and there is no moc_MainWindow.cpp at all. If I manually call moc in from cmake instead of using automoc in the broken case, I do get moc_MainWindow.cpp but it is empty.

Original Status

Firstly, no, I haven't forgotten to set(CMAKE_AUTOMOC ON). Also note that MainWindow's destructor is declared and implemented.

When my directory structure looks like:

CMakeLists.txt
|__ main.cpp
|__ MainWindow.cpp
|__ MainWindow.h
|__ MainWindow.ui

compiling works just fine.

However, when it looks like:

helloworld/
|__ CMakeLists.txt
|__ src/
|   |__ CMakeLists.txt
|   |__ main.cpp
|   |__ MainWindow.cpp
|
|__ inc/
|   |__ MainWindow.h
|
|__ gui/
    |__ MainWindow.ui

I get linking errors:

Linking CXX executable hello-world
CMakeFiles/hello-world.dir/MainWindow.cpp.o: In function `MainWindow::MainWindow()':
MainWindow.cpp:(.text+0x3b): undefined reference to `vtable for MainWindow'
MainWindow.cpp:(.text+0x4d): undefined reference to `vtable for MainWindow'
CMakeFiles/hello-world.dir/MainWindow.cpp.o: In function `MainWindow::~MainWindow()':
MainWindow.cpp:(.text+0xaf): undefined reference to `vtable for MainWindow'
MainWindow.cpp:(.text+0xc1): undefined reference to `vtable for MainWindow'
collect2: error: ld returned 1 exit status
make[2]: *** [src/hello-world] Error 1
make[1]: *** [src/CMakeFiles/hello-world.dir/all] Error 2

I'd really like to have my sources and headers in proper subdirectories, but I'm not quite sure how to fix this.

This is actually the simplest identifiable case of an error from a much larger project, so I'm really not all that keen to flatten the project directories just because I'm adding a Qt GUI to it.

Answer 1

As noted, moc is not processing MainWindow.h in your example. One way to force this to happen is to call qt_wrap_cpp() on it directly (instead of on MainWindow.cpp) and then include the resulting file in the call to add_executable().

Your top level CMakeLists.txt might look like:

cmake_minimum_required(VERSION 2.8.9)

#set(CMAKE_AUTOMOC ON)

set(CMAKE_PREFIX_PATH "/opt/Qt/5.1.1/gcc_64")
set(CMAKE_INCLUDE_CURRENT_DIR ON)

project(hello-world)

find_package(Qt5Widgets REQUIRED)

set(HW_HEADER_DIR ${CMAKE_CURRENT_SOURCE_DIR}/inc)
set(HW_GUI_DIR ${CMAKE_CURRENT_SOURCE_DIR}/gui)

include_directories(${HW_HEADER_DIR})

subdirs(src)

and your src level one like:

qt5_wrap_cpp(hello-world_SRC ${HW_HEADER_DIR}/MainWindow.h)
qt5_wrap_ui(hello-world_UI ${HW_GUI_DIR}/MainWindow.ui)

add_executable(hello-world MainWindow.cpp main.cpp
               ${hello-world_UI} ${hello-world_SRC})
qt5_use_modules(hello-world Widgets)

Addendum:

• This works in your example with and without AUTOMOC enabled. I don't know for sure if having it on will cause issues in the future. If you don't enable it you'll have to manually moc any other stuff... although it might all behave like MainWindow in which case you'll be manually mocing the headers regardless.
• You don't have to set the directory variables in the top level CMakeLists.txt but I find it cleaner than doing qt5_wrap_cpp(hello-world_SRC ../inc/MainWindow.h)
• There might be a better way of doing this.
• For anybody else with similar issues, so far this solution has held up in the larger project that I initially encountered this in. I will update accordingly if it fails.