Hello everyone.
I'm developing a simple C++ GUI toolkit based on wayland, and get stuck with a weird problem recently.

I created a simple project with minimal code to reproduce this issue for demonstration: https://github.com/zhanggyb/set-user-data

It's a CMake C++ project and use simple C++ structures to wrap wayland client objects.

My test environment:

    Fedora 25
    Gnome 3.22.2
    wayland-devel-1.12.0-1.fc25.x86_64
    libwayland-client-1.12.0-1.fc25.x86_64

Checkout, build and run:

    $ git clone https://github.com/zhanggyb/set-user-data
    $ mkdir build
    $ cd build/
    $ cmake ..
    $ ./demo

Expected result:

    display a rectangle surface on desktop

Actual result:

    assert error will be raised, and the const member variables around wayland object will be changed in the wl_output or wl_surface listener callbacks, see https://github.com/zhanggyb/set-user-data/blob/master/surface.cpp and https://github.com/zhanggyb/set-user-data/blob/master/output.cpp

    Uncomment these 2 lines can avoid assert error:

    https://github.com/zhanggyb/set-user-data/blob/master/main.cpp:163: output.SetUserData(this);
    https://github.com/zhanggyb/set-user-data/blob/master/main.cpp:164: surface.SetUserData(this);

I don't know why this happens, it seems the compositor changes the data in client, or did I use the xxxx_set_user_data() API in the wrong way?

Thanks in advance.