GObject - Class Implementation

Class Implementation

Each GObject class is implemented by at least two structures: the class structure and the instance structure.

The class structure

The class structure corresponds to the vtable of a C++ class. It must begin with the class structure of the superclass. Following that, it will hold a set of function pointers — one for each virtual method of the class. Class-specific variables can be used to emulate class members.

The instance structure

The instance structure, which will exist in one copy per object instance, must begin with the instance structure of the superclass (this ensures that all instances begin with a pointer to the class structure, since all fundamental instantiable types share this property). After the data belonging to the superclass, the structure can hold any instance-specific variables, corresponding to C++ member variables.

Since a C structure cannot have access modifiers like “public”, “protected”, or “private”, a common technique is to include a pointer to the private data — conventionally called _priv — in the instance structure. The private structure can be declared in the public header file, but defined only in the implementation file, with the effect that the private data is opaque to users, but transparent to the implementor. If the private structure is registered with GType, it will be automatically allocated by the object system. Indeed, it is not even necessary to include the _priv pointer, if one is willing to use the incantation G_TYPE_INSTANCE_GET_PRIVATE every time the private data is needed.

The main drawback of the GObject framework is its verbosity. Large amounts of boilerplate code, such as manual definitions of type casting macros and obscure type registration incantations, are necessary to create a new class. The GObject Builder, or GOB2, is a tool that attempts to remedy this problem by offering a template syntax reminiscent of Java. Code written using GOB2 is pre-processed into vanilla C code prior to compilation. Another compiler-to-C for the GObject type system is Vala, which uses a C#-style syntax.