Undefined reference to VTable when using cmake

gahjouyooj

When I attempt to compile a program, it causes a linker error, and the usual fixes aren't working.

CMakeFiles/aoeu.dir/src/reader.cc.obj:reader.cc:(.rdata$.refptr._ZTV6reader[.refptr._ZTV6reader]+0x0): undefined reference to `vtable for reader'

When I try to include reader.moc, I get an include error. When I move everything to main.cc, and compile it there, it works. I do intend to use slots and stuff in my class, so I do need to include Q_OBJECT.

I'm using MSYS2, g++ and ld.

CMakeLists.txt

# Standards and versions  
cmake_minimum_required(VERSION 3.30.0)
set(CMAKE_CXX_STANDARD_REQUIRED 17)

# Metadata 
project(aoeu VERSION 0.0.0 LANGUAGES CXX)
set(CMAKE_INCLUDE_CURRENT_DIR TRUE)
set(CMAKE_AUTOMOC ON)

# Packages 

find_package(Qt6 COMPONENTS Widgets REQUIRED)

file(GLOB_RECURSE SOURCES ${PROJECT_SOURCE_DIR}/src/*.cc)

add_executable(${PROJECT_NAME} ${SOURCES})

# Pch files
target_precompile_headers(${PROJECT_NAME} PRIVATE <QtWidgets>)

target_include_directories(${PROJECT_NAME} PRIVATE "${PROJECT_SOURCE_DIR}/include")

target_link_libraries(${PROJECT_NAME} PRIVATE Qt6::Widgets)

main.cc

#include <QtWidgets>

#include "reader.hh"

int main(int argc, char **argv) {
  QApplication app(argc, argv);

  QWidget window;
  window.show();

  // Placeholder for actual text previewing 
  reader textBox("boeu", &window);

  return app.exec();
}

reader.hh

#if !defined(READER_HH)
#define READER_HH

#include <QPlainTextEdit>

class reader : public QPlainTextEdit {
  Q_OBJECT

public:
  reader(QString text, QWidget *parent = nullptr);

  virtual ~reader();
};

// `fatal error: reader.moc: No such file or directory` if uncommented 
// #include "reader.moc"

#endif

reader.cc

#include "reader.hh"

reader::reader(QString text, QWidget *parent) : QPlainTextEdit(text, parent) {
  setReadOnly(true);
}

reader::~reader() {}

ChrisW67

The root cause of this problem is the separation of C++ source files from header files. The separation hides reader.hh from AUTOMOC and results in no generated code for this class.
Placing all of these sources and includes in the same directory links correctly.

You can repair this by adding the header files to the add_executable() calls beside the SOURCES. There may be other/better ways to achieve this.

gahjouyooj

@ChrisW67 said in Undefined reference to VTable when using cmake:

The root cause of this problem is the separation of C++ source files from header files. The separation hides reader.hh from AUTOMOC and results in no generated code for this class.
Placing all of these sources and includes in the same directory links correctly.

You can repair this by adding the header files to the add_executable() calls beside the SOURCES. There may be other/better ways to achieve this.

Sorry, this has not helped the issue. When including reader.moc, it still fails to run the moc. In addition, when removing the reader.moc include, I had another linker error.

multiple definition of `reader::staticMetaObject'; CMakeFiles/aoeu.dir/aoeu_autogen/mocs_compilation.cpp.obj:U:/repos/novelreader/build/aoeu_autogen/UVLADIE3JM/moc_reader.cpp:63: first defined here

Christian Ehrlicher

You have to include the header, not the generated moc file

gahjouyooj

I see. I've managed to fix the issues.

Changes I've made to solve it:

Added .hh files to sources.
Removed #include "reader.moc".

Just to clarify, when do you need to use #include "file.moc"? It's been suggested as a solution to the vtable include error by many people.

Christian Ehrlicher

You only have to include the moc file in the source file when you declared a qobject derived class in the source instead the header.

gahjouyooj

I see, thanks for explaining.