Unresolved symbol in user DLL "public: static struct QMetaObject"

roderick.huang

I search Qt's community and stackoverflow, found there are many related posts, but these are not same problem what I am.

I write a user-defined widget inheite from QOpenGLWidget, and create a DLL for it using CMake : qt-open-glpoint-cloud.

The whole error message is:

error LNK2001: 无法解析的外部符号 "public: static struct QMetaObject const test::gl_painter::GLCloudWidget2D::staticMetaObject" (?staticMetaObject@GLCloudWidget2D@gl_painter@test@@2UQMetaObject@@B)

The CMake script list bellow:

cmake_minimum_required(VERSION 3.10 FATAL_ERROR)

set(CMAKE_INCLUDE_CURRENT_DIR ON)
set(CMAKE_AUTOUIC ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)

file(GLOB SRCS src/*.cc)
file(GLOB HEADERS ${PROJECT_SOURCE_DIR}/include/gl_widget_2d/*.h)

set(target_name "gl_widget_2d")
add_library(${target_name} SHARED ${SRCS} ${HEADERS})
target_link_libraries(${target_name} PRIVATE Qt5::Core Qt5::Widgets Qt5::OpenGL)
target_compile_definitions(${target_name} PUBLIC -DGL_WIDGET_2D_LIB)

# set_target_properties(${target_name} PROPERTIES CXX_VISIBILITY_PRESET "hidden")
set(target_gl_widget_2d ${target_name} PARENT_SCOPE)

The header file of the user-defined widget:

#pragma once

// #include <memory>
//#include <vector>

#include <QBrush>
#include <QColor>
#include <QFont>
#include <QList>
#include <QOpenGLWidget>
#include <QPen>
#include <QWidget>

#include "gl_widget_2d_exp_def.h"

namespace test::gl_painter {

class CloudWidget2DPaintInterface;
class CloudWidget2DPaintDash;
class CloudWidget2DPaintRect;
class CloudWidget2DPaintHorizontalDashLine;

class GL_WIDGET_2D_API GLCloudWidget2D : public QOpenGLWidget {
  friend class CloudWidget2DPaintHorizontalDashLine;
  friend class CloudWidget2DPaintPoints;
  friend class CloudWidget2DPaintDash;
  friend class CloudWidget2DPaintRect;

  Q_OBJECT

 public:
  GLCloudWidget2D(QWidget* parent);
  ~GLCloudWidget2D() override;
  void setBackColor(QColor color);

  void addPaint(CloudWidget2DPaintInterface* paint);
  const QList<QColor>& colors() const;
  QList<QColor>& colors();

  void setRightToLeft(bool value);
  void setBottomToTop(bool value);

 public Q_SLOTS:
  void animate();

 protected:
  void paintEvent(QPaintEvent* event) override;
  void mousePressEvent(QMouseEvent* event) override;
  void mouseMoveEvent(QMouseEvent* event) override;
  void resizeEvent(QResizeEvent* event) override;

 Q_SIGNALS:
  void signalPaintHit(int pid, int eid, double x, double y);
  void signalSizeChanged(QSize);

 protected:
  QList<CloudWidget2DPaintInterface*> paints_;
  QList<QColor> colors_;

 protected:
  bool right_to_left_{false}, bottom_to_top_{false};

 protected:
  QBrush background_;
  QBrush circleBrush;
  QFont textFont;
  QPen circlePen;
  QPen textPen;
};

}  // namespace test::gl_painter

The exported symbol can be found using Dependencies, and show as below:

We can see that other user-defined class and it's member function is exported as __cdecl, but the static struct QMetaObject ... does'nt, which is defined in Q_OBJECT.

So how to let this symbol mark as exported ?

Christian Ehrlicher

@roderick-huang said in Unresolved symbol in user DLL "public: static struct QMetaObject":

PUBLIC DGL_WIDGET_2D_LIB

This does not look right.

https://doc.qt.io/qt-6/sharedlibrary.html

roderick.huang

@Christian-Ehrlicher Thanks. But it's correct.

The following two styles are the same:

target_compile_definitions(${target_name} PUBLIC -DGL_WIDGET_2D_LIB)

target_compile_definitions(${target_name} PUBLIC GL_WIDGET_2D_LIB)

The build log message of mine:

[build] [11/15  26% :: 8.792] "D:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.41.34120\bin\Hostx64\x64\cl.exe"   /TP -DGL_WIDGET_2D_LIB -DNDEBUG -DQT_CORE_LIB -DQT_GUI_LIB -DQT_MESSAGELOGCONTEXT -DQT_NO_DEBUG -DQT_OPENGL_LIB -DQT_WIDGETS_LIB -DSPDLOG_ACTIVE_LEVEL=SPDLOG_LEVEL_WARN -DSPDLOG_WCHAR_TO_UTF8_SUPPORT -Dgl_widget_2d_EXPORTS -ID:\work\web\qtOpenGLPointCloud2\build\src\gl_widget_2d -ID:\work\web\qtOpenGLPointCloud2\src\gl_widget_2d -ID:\work\web\qtOpenGLPointCloud2\build\src\gl_widget_2d\gl_widget_2d_autogen\include -ID:\work\web\qtOpenGLPointCloud2\system -ID:\work\web\qtOpenGLPointCloud2\3rd\include -ID:\work\web\qtOpenGLPointCloud2\include -external:IC:\Qt\5.15.2\msvc2019_64\include -external:IC:\Qt\5.15.2\msvc2019_64\include\QtCore -external:IC:\Qt\5.15.2\msvc2019_64\.\mkspecs\win32-msvc -external:IC:\Qt\5.15.2\msvc2019_64\include\QtWidgets -external:IC:\Qt\5.15.2\msvc2019_64\include\QtGui -external:IC:\Qt\5.15.2\msvc2019_64\include\QtANGLE -external:IC:\Qt\5.15.2\msvc2019_64\include\QtOpenGL -external:W0 /DWIN32 /D_WINDOWS /W4 /GR /EHsc /O2 /Ox /Ot /Qpar /Gy /Gw /GF /MD /O2 /Ob2 /DNDEBUG -std:c++20 /utf-8 /showIncludes /Fosrc\gl_widget_2d\CMakeFiles\gl_widget_2d.dir\src\gl_cloud_widget_2d.cc.obj /Fdsrc\gl_widget_2d\CMakeFiles\gl_widget_2d.dir\ /FS -c D:\work\web\qtOpenGLPointCloud2\src\gl_widget_2d\src\gl_cloud_widget_2d.cc

Christian Ehrlicher

Please read the documentation I gave you so you can see when this define must be there and when not. You're doing it wrong.

roderick.huang

@Christian-Ehrlicher Thanks. It's my mistake. The compile option should be defined as PRIVATE, which means the test exe should use Q_DECL_IMPORT to link the DLL's symbols -- I'm not clear about the difference between PRIVATE and PUBLIC in CMake script.

target_compile_definitions(${target_name} PRIVATE -DGL_WIDGET_2D_LIB)

But I'm not understand why no link errors for other symbols exported in the DLL, and why the symbol static struct QMetaObject const test::gl_painter::GLCloudWidget2D::staticMetaObject which not marked as __cdecl can not link ?

Christian Ehrlicher

@roderick-huang said in Unresolved symbol in user DLL "public: static struct QMetaObject":

I'm not clear about the difference between PRIVATE and PUBLIC in CMake script.

See https://cmake.org/cmake/help/latest/manual/cmake-buildsystem.7.html#target-command-scope

But I'm not understand why no link errors for other symbols exported in the DLL,

They would be appear later I would guess.