MSVC 2019: DLLImport static data member not allowed

fem_dev

I was developing my Qt C++ main application and my plugins in a Linux Ubuntu 20.04 machine and both projects works well in this system.

Now, I copied both Qt projects to my Windows 10 machine.
The main app works great!

But the plugin project gives me this compile time error below:

moc_plugin.cpp: 81: error: C2491: 'Plugin :: staticMetaObject': ‡ definition ‡ ‡ dllimport static data member not allowed

Besides that, I got the same warning 43 times too:

moc_plugin.cpp: 107: warning: C4273: 'Plugin :: qt_metacall':  inconsistent dll link

Fiinally, I got too a lot of this warnings below (one for each plugin method):

warning: 'Plugin::myMethod' redeclared without 'dllimport' attribute: 'dllexport' attribute added

Here is my Interface (Plugin_API class):

#ifndef PLUGIN_QT_API_H
#define PLUGIN_QT_API_H

#include <QtPlugin>
#include <QString>

#include "third-party/highfive/H5Easy.hpp"
#include "third-party/nlohmann/json.hpp"

#include "message_types.h"

#define API_VERSION_MAJOR 1
#define API_VERSION_MINOR 0
#define API_VERSION_FIX 0

using JSON = nlohmann::json;

class Plugin_API : public QObject
{
    Q_OBJECT

private:

public:
    QString _pluginName;
    QString _pluginType;
    QString _authorName;
    QString _authorEmail;
    QString _company;
    size_t _majorVersion;
    size_t _minorVersion;
    size_t _fixVersion;
    JSON _inputArgumentsTypes;
    JSON _outputArgumentsTypes;
    JSON* _project;
    QString _filename;
    H5Easy::File *_outputFile;
    bool _enableWriteResultsToFile;

     Plugin_API() :
         _majorVersion(0),
         _minorVersion(0),
         _fixVersion(0),
         _enableWriteResultsToFile(true)
     {}

    virtual ~Plugin_API() = default;

    virtual const QString getName(void) const = 0;

    virtual const QString getType(void) const = 0;

    virtual const JSON getMetadata(void) const = 0;

    virtual const JSON* getProject(void) const = 0;

    // ===== SETTER METHODS ===== //
    virtual void setProject(JSON* project) = 0;

    virtual void setPluginName(const QString& pluginName) = 0;

    // ===== OTHER METHODS ===== //
    virtual void sendStatusMsg(const QString& msg) = 0;

    virtual void sendWarningMsg(const QString& msg) = 0;

    virtual void sendErrorMsg(const QString& msg) = 0;

    virtual void setMetadata(void) = 0;

    virtual void initInputParameters(void) = 0;

    virtual void setInputArgumentsTypes(const JSON& input) = 0;

    virtual void setOutputArgumentsTypes(const JSON& output) = 0;

signals:
    void sendMsg(QString source, MSG_TYPE msgType, QString msg);
    void savePluginSettings(JSON settings);
};

Q_DECLARE_INTERFACE(Plugin_API, "com.rotortest.plugin")

#endif // PLUGIN_QT_API_H

Here is the message_types.h:

#ifndef MESSAGE_TYPES_H
#define MESSAGE_TYPES_H

enum MSG_TYPE {
    ERROR_MSG = -2,
    WARNING_MSG = -1,
    STATUS_MSG = 0
};

#endif // MESSAGE_TYPES_H

Here is the plugin_global.h:

#ifndef PLUGIN_GLOBAL_H
#define PLUGIN_GLOBAL_H

#include <QtCore/qglobal.h>

#if defined(PLUGIN_LIBRARY)
#  define PLUGIN_EXPORT Q_DECL_EXPORT
#else
#  define PLUGIN_EXPORT Q_DECL_IMPORT
#endif

#endif // PLUGIN_GLOBAL_H

Here is the plugin.h:

#include <vector>

#include "dialog.h"
#include "plugin_global.h"
#include "plugin_qt_api.h"

class PLUGIN_EXPORT Plugin : public Plugin_API
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "com.rotortest.plugin")
    Q_INTERFACES(Plugin_API)

private:
    Dialog _ui;

public:

    explicit Plugin(QObject* parent = nullptr);

    ~Plugin();

   all_methods override;

public slots:
    void receivedUi(JSON& ui);
};

How can I solve it and compile the plugin project on my Windows machine?

My systems are:

System A (Working good!)

Ubuntu 20.04 x64
Qt Creator IDE 4.12.4
Qt 5.15.0
GCC 9.3

System B (NOT compiling the plugin project)

Windows 10 x64
Qt Creator IDE 4.12.4
Qt 5.15.0
MSVC 2019

hskoglund

Hi, it might be something missing in the .pro file for the plugin project, it should have a line like this:

...
DEFINES += PLUGIN_LIBRARY
...

kshegunov

Interfaces don't have members, and they for sure don't derive from QObject. There's a pretty decent example in the documentation, you should follow it.

fem_dev

@hskoglund said in MSVC 2019: DLLImport static data member not allowed:

DEFINES += PLUGIN_LIBRARY

Thank you for your quick response...
First: It works and solved my error and all warnings in the MSVC compilation!

But I don't get your point with this define line.
Why this line fixed my problem?

hskoglund

While I haven't built any Qt plugins, but if they are built the same way normal .dlls are, then the error you get " ...import ... not allowed" when compiling your plugin, could be due to that missing #define.

If you look in the plugin_global.h file, the error maybe can occur if
#define PLUGIN_EXPORT Q_DECL_EXPORT
gets #defined rather than
#define PLUGIN_EXPORT Q_DECL_IMPORT

(i..e the plugin should be exporting its functions, not importing them)

fem_dev

@kshegunov said in MSVC 2019: DLLImport static data member not allowed:

There's a pretty decent example in the documentation, you should follow it.

Thank you for your attention.
Could you please give me links about this information?

I follow this Qt Wiki tutorial below:
https://wiki.qt.io/How_to_create_a_library_with_Qt_and_use_it_in_an_application

And now I saw this Qt 5.15.0 little explanation here:
https://doc.qt.io/qt-5/sharedlibrary.html

Like you said before, may be I'm doing something wrong and I would like to do it right!

I derived my interface from QObject because I saw it on a Udemy Qt online course...
I was thinking that this was "the right way" to add the same set of signals and slots for all developed plugins.

But I would like to do that in the best possible way.
Please @kshegunov , if is there a better way, feel free to comment about it.

Thank you!

fem_dev

@hskoglund said in MSVC 2019: DLLImport static data member not allowed:

missing #define.

#ifndef PLUGIN_GLOBAL_H
#define PLUGIN_GLOBAL_H

#include <QtCore/qglobal.h>

#if defined(PLUGIN_LIBRARY) // HERE IS WHY YOU NEED TO PASS THIS DEFINE AT COMPILATION TIME
#  define PLUGIN_EXPORT Q_DECL_EXPORT
#else
#  define PLUGIN_EXPORT Q_DECL_IMPORT
#endif

#endif // PLUGIN_GLOBAL_H

Brilhant @hskoglund ! Thank you so much!

jsulm

@fem_dev said in MSVC 2019: DLLImport static data member not allowed:

I follow this Qt Wiki tutorial below:
https://wiki.qt.io/How_to_create_a_library_with_Qt_and_use_it_in_an_application

This is for normal libraries, not plug-ins.
If you want to implement a plug-in why not following https://doc.qt.io/qt-5/plugins-howto.html ?
There are links to examples...

fem_dev

@fem_dev said in MSVC 2019: DLLImport static data member not allowed:

But I would like to do that in the best possible way.

In this case that I want to give the same set of signals and slots to all my develop plugins should I:

Remove all variable members, QObject and signals and slots from the Plugin_API class (interface)
create a new class (Base_Plugin class)
In this Base_Plugin add all signals and slots
In this Base_Plugin add all variable members that are commum to all plugins
For each Plugin derive like:

class PLUGIN_EXPORT Plugin : public QObject, public Base_Plugin, public Plugin_API
{
...
    all_methods override;
...
}`

Is this the right way?
@hskoglund @kshegunov @jsulm

jsulm

@fem_dev See "The Low-Level API: Extending Qt Applications" in the link I posted.
There is no need for two classes Base_Plugin and Plugin_API.
Implement one interface class and derive your Plugin from it and QObject as shown in the example.

fem_dev

@jsulm said in MSVC 2019: DLLImport static data member not allowed:

There is no need for two classes Base_Plugin and Plugin_API.

Thank you for the informations.
Doubt:
If the interface doesn't derive from QObject, how can I use signals and slots between my main app and a plugin?

jsulm

@fem_dev said in MSVC 2019: DLLImport static data member not allowed:

If the interface doesn't derive from QObject, how can I use signals and slots between my main app and a plugin?

If the interface class does not derive from QObject it can still create and return instances of QObject derived classes.