jhenstridge / gobject-cpp-reflection

GObject oriented experiments with C++ Static Reflection

Tracking GObject inheritance with C++ Reflection

This repository contains a few utility functions that make use of the draft C++ Static Reflection standard. Sadly this feature hasn't been included in the C++17 standard, but an implementation for Clang is available as the reflexpr branch of the following repository:

https://github.com/matus-chochlik/clang

is_base_of

The is_base_of templated constant can be used to determine whether one struct type is the "GObject style" base of another struct type.

That is, is_base_of<Base, Subclass> is true if either Base and Subclass are the same type or if is_base_of<Base, T> is true, where T is the type of the first data member of Subclass.

is_base_of is a constant expression, so it's value is determined at compile time.

gobj::is_base_of<GtkBox, GtkBox>    // == true
gobj::is_base_of<GtkWidget, GtkBox> // == true
gobj::is_base_of<GtkObject, GtkBox> // == true
gobj::is_base_of<GObject, GtkBox>   // == true
gobj::is_base_of<GtkGrid, GtkBox>   // == false

gobj_cast

The gobj_cast helper function can be used to cast a pointer to a GObject subclass to a pointer to any of its base classes. It is built on top of the is_base_of machinery.

For correct casts, the function call will be optimised away:

using gobj::gobj_cast;
...
GtkBox *box = ...;
auto widget = gobj_cast<GtkWidget>(box);

For invalid upcasts, it will produce a compile time error with a somewhat readable error message:

In file included from test.cpp:3:
./gobj_cast.h:19:5: error: static_assert failed "Could not verify that target
      type is a base of source type"
    static_assert(is_base_of<Target, Source>,
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:11:17: note: in instantiation of function template specialization
      'gobj::gobj_cast<_GtkGrid, _GtkBox>' requested here
    auto grid = gobj_cast<GtkGrid>(box);
                ^
1 error generated.