GObject inheritance (simple example)

Igor_Morgado · October 16, 2019, 1:49am

I’m trying to build a simple example of GObject usage. I came with the following code

gist.github.com

https://gist.github.com/igormorgado/2525b56c2e4e8bab75dcf8848d2a2bc3

A_readme

 How to use G_DECLARE_DERIVATIVE_TYPE and/or G_DECLARE_FINAL_TYPE in this code to reduce the "boilerplate code"?

Right now, errors building the code:

pet-dog.c: In function ‘pet_dog_print’:
pet-dog.c:17:36: error: invalid type argument of ‘->’ (have ‘PetBaseClass’ {aka ‘struct _PetBaseClass’})
     PET_DOG_GET_CLASS(self)->parent->print(self);
                                    ^~
pet-dog.c: In function ‘pet_dog_class_init’:
pet-dog.c:86:27: warning: assignment to ‘void (*)(PetBase *)’ {aka ‘void (*)(struct _PetBase *)’} from incompatible pointer type ‘void (*)(PetDog *)’ {aka ‘void (*)(struct _PetDog *)’} [-Wincompatible-pointer-types]

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Makefile

all:
	gcc -ggdb -g `pkg-config --cflags --libs glib-2.0`  `pkg-config --cflags --libs gobject-2.0` -I. -g -o pets main.c pet-base.c pet-dog.c

clean:
	rm -rf pets

main.c

#include <stdio.h>
#include "pet-dog.h"

int
main(int argc, char *argv[])
{
    PetBase  * generic_pet = pet_base_new ();

    /*
     * Is there any better way to set properties

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The code do not compile because it complains about invalid type argument.

What can I do to fix the code?

Besides that, any other tips are welcome.

Cheers.

baedert · October 16, 2019, 6:18am

For the first error:
The way to chain up to the parent class in GObject is to get the parent class and call the vfunc, i.e. PET_BASE_CLASS (pet_dog_parent_class)->print(PET_BASE (self)); (but of course that entire function is useless in this case since it just chains up and does nothing else).

The second error:
You’re assigning a function that takes a PetDog * argument to a function pointer that expects a Pet Base *) argument. Just change the function prototype and if you need a PetDog* inside the function, cast as appropriate.

system · October 30, 2019, 6:18am

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