GdkGLTexture.build creates and returns a GLTexture, but casts the return value to a Texture (the base class of GLTexture) before returning it. Why?
The relevant sources (gdk_gl_texture_builder_build, gdk_gl_texture_new_from_builder) show that the created object is indeed always a GLTexture; there’s no other type that it could return.
Moreover, it would be extremely surprising if it returned anything else. The name is crystal clear: it’s a GLTexture builder.
In the C API, it’s common for GTK to return base classes as that avoids additional casts when using the returned instance; there are more consumers of GdkTexture than GdkGLTexture.
You can see this pattern in play in most of the GTK widget API, where constructors return GtkWidget instead of the constructor’s type.
This is less of an issue on higher level languages.
I’m programming in C++. The glibmm wrapper doesn’t translate the return type to GLTexture, unlike most other constructors, and this seems like an oversight - probably now frozen in due to ABI.
Still, there’s nothing to stop the docs calling it a GLTexture. But they also refer to it as Texture, in contrast to other widget constructors:
Description: Builds a new GdkTexture with the values set up in the builder.
Return value: A newly built GdkTexture
So you can see how this can lead to the confusion.
gtkmm usually use the data types corresponding to the types used in gtk.
For instance when gdk_gl_texture_builder_build() returns GdkTexture*,
Gdk::GLTextureBuilder::build() returns Glib::RefPtr<Gdk::Texture>.
Provided the returned object is really a GdkGLTexture, it will be wrapped in a
Gdk::GLTexture object. You can dynamic-cast it, if you want to use GLTexture API.
auto texture = my_gl_texture_builder->build();
auto gl_texture = std::dynamic_pointer_cast<Gdk::GLTexture>(texture);
if (gl_texture) // If you want to be extra safe.
// Your code here
