MSVC 2019: DLLImport static data member not allowed

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.