Developer(s) | The GNOME Project |
---|---|
Initial release | 11 March 2002 |
Written in | C |
Operating system | Cross-platform |
Available in | Multilingual [ which? ] |
Type | Software library |
License | GNU LGPL |
Website | developer |
The GLib Object System, or GObject, is a free software library providing a portable object system and transparent cross-language interoperability. GObject is designed for use both directly in C programs to provide object-oriented C-based APIs and through bindings to other languages to provide transparent cross-language interoperability, e.g. PyGObject.
Depending only on GLib and libc, GObject is a cornerstone of GNOME and is used throughout GTK, Pango, ATK, and most higher-level GNOME libraries like GStreamer and applications. Prior to GTK+ 2.0, code similar to GObject was part of the GTK codebase. (The name “GObject” was not yet in use — the common baseclass was called GtkObject
.)
At the release of GTK+ 2.0, the object system was extracted into a separate library due to its general utility. In the process, most non-GUI-specific parts of the GtkObject
class were moved up into GObject
, the new common baseclass. Having existed as a separate library since March 11, 2002 (the release date of GTK+ 2.0), the GObject library is now used by many non-GUI programs such as command-line and server applications.
Though GObject has its own separate set of documentation [1] and is usually compiled into its own shared library file, the source code for GObject resides in the GLib source tree and is distributed along with GLib. For this reason, GObject uses the GLib version numbers and is typically packaged together with GLib (for example, Debian puts GObject in its libglib2.0
package family).
At the most basic level of the GObject framework lies a generic and dynamic type system called GType. The GType system holds a runtime description of all objects allowing glue code to facilitate multiple language bindings. The type system can handle any singly inherited class structure, in addition to non-classed types such as opaque pointers, strings, and variously sized integers and floating point numbers.
The type system knows how to copy, assign, and destroy values belonging to any of the registered types. This is trivial for types like integers, but many complex objects are reference-counted, while some are complex but not reference-counted. When the type system “copies” a reference-counted object, it will typically just increase its reference count, whereas when copying a complex, non-reference-counted object (such as a string), it will typically create an actual copy by allocating memory.
This basic functionality is used for implementing GValue
, a type of generic container that can hold values of any type known by the type system. Such containers are particularly useful when interacting with dynamically typed language environments in which all native values reside in such type-tagged containers.
Types that do not have any associated classes are called non-classed. These types, together with all types that correspond to some form of root class, are known as fundamental types: the types from which all other types are derived. These make up a relatively closed set, but although the average user is not expected to create their own fundamental types, the possibility does exist and has been exploited to create custom class hierarchies — i.e., class hierarchies not based on the GObject
class.
As of GLib 2.9.2, [2] the non-classed built-in fundamental types are:
void
(G_TYPE_NONE
);char
, int
, long
, and 64-bit integers (G_TYPE_CHAR
, G_TYPE_UCHAR
, G_TYPE_INT
, G_TYPE_UINT
, G_TYPE_LONG
, G_TYPE_ULONG
, G_TYPE_INT64
, and G_TYPE_UINT64
);G_TYPE_BOOLEAN
);enum
type, but differing in that the latter is only used for bit fields (G_TYPE_ENUM
and G_TYPE_FLAGS
);float
and double
(G_TYPE_FLOAT
and G_TYPE_DOUBLE
);char *
(G_TYPE_STRING
);void *
(G_TYPE_POINTER
).The classed built-in fundamental types are:
GObject
, the root of the standard class inheritance tree (G_TYPE_OBJECT
)G_TYPE_INTERFACE
)G_TYPE_BOXED
)G_TYPE_PARAM
).Types that can be instantiated automatically by the type system are called instantiable. An important characteristic of these types is that the first bytes of any instance always contain a pointer to the class structure (a form of virtual table) associated to the type of the instance. For this reason, any instantiable type must be classed. Contrapositively, any non-classed type (such as integer or string) must be non-instantiable. On the other hand, most classed types are instantiable, but some, such as interface types, are not.
The types that are derived from the built-in GObject fundamental types fall roughly into four categories:
enum
type) that one wishes to use in some way that is related to the object system — for example, as the type of an object property — should be registered with the type system. Typically, the initialization code that takes care of registering these types is generated by an automated tool called glib-mkenums
[3] and stored in a separate file.background-color
property, whose values should be instances of a structure that looks like structcolor{intr,g,b;}
. To avoid having to subclass GObject
, we can create a boxed type to represent this structure, and provide functions for copying and freeing. GObject ships with a handful of boxed types wrapping simple GLib data types. Another use for boxed types is as a way to wrap foreign objects in a tagged container that the type system can identify and will know how to copy and free.G_TYPE_POINTER
), it is often a good idea to create a derived pointer type, documenting the fact that the pointers should reference a particular kind of object, even though nothing else is said about it.GObject
. There are also interfaces, which, unlike classic Java-style interfaces, can contain implemented methods. GObject interfaces can thus be described as mixins.The GObject messaging system consists of two complementary parts: closures and signals.
Each GObject class is implemented by at least two structures: the class structure and the instance structure.
Defining a class in the GObject framework is complex, requiring large amounts of boilerplate code, such as manual definitions of type casting macros and obscure type registration incantations. Also, since a C structure cannot have access modifiers like “public”, “protected”, or “private”, workarounds must be used to provide encapsulation. One approach 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.
To address some of these complexities, several higher-level languages exist that source-to-source compiles to GObject in C. The Vala programming language uses a C#-style syntax and is pre-processed into vanilla C code. The GObject Builder, or GOB2, offers a template syntax reminiscent of Java.
The combination of C and GObject is used in many successful free software projects, such as the GNOME desktop, the GTK toolkit and the GIMP image manipulation program.
Though many GObject applications are written entirely in C, the GObject system maps well into the native object systems of many other languages, like C++, Java, Ruby, Python, Common Lisp, and .NET/Mono. As a result, it is usually relatively painless to create language bindings for well-written libraries that use the GObject framework.
Writing GObject code in C in the first place, however, is relatively verbose. The library takes a good deal of time to learn, and programmers with experience in high-level object-oriented languages often find it somewhat tedious to work with GObject in C. For example, creating a subclass (even just a subclass of GObject
) can require writing and/or copying large amounts of boilerplate code. [5] However, using Vala, a language that is designed primarily to work with GObject and which converts to C, is likely to make working with GObject or writing GObject based libraries nicer.
Although they are not really first-class objects (there are no actual metatypes in GType), metaobjects like classes and interfaces are created by GObject applications at runtime, and provide good support for introspection. The introspective capabilities are used by language bindings and user interface design applications like Glade to allow doing things like loading a shared library that provides a GObject class—usually some kind of widget, in the case of Glade—and then obtain a list of all properties of the class, complete with type information and documentation strings.
This section possibly contains original research .(December 2011) |
Since GObject provides a mostly complete object system for C[ citation needed ], it can be seen as an alternative to C-derived languages such as C++ and Objective-C. (Though both also offer many other features beyond just their respective object systems.) An easily observed difference between C++ and GObject is that GObject (like Java) does not support multiple inheritance. [6]
GObject's use of GLib's g_malloc() memory allocation function will cause the program to exit unconditionally upon memory exhaustion, unlike the C library's malloc(), C++'s new, and other common memory allocators which allow a program to cope with or even fully recover from out-of-memory situations without simply crashing. [7] This tends to work against including GObject in software where resilience in the face of limited memory is important, or where very many or very large objects are commonly handled. The g_try_new() can be used when a memory allocation is more likely to fail (for a large object for example), but this cannot grant that the allocation will not fail elsewhere in the code. [8]
Another important difference is that while C++ and Objective-C are separate languages, GObject is strictly a library and as such does not introduce any new syntax or compiler intelligence. For example, when writing GObject-based C code, it is frequently necessary to perform explicit upcasting.[ citation needed ] Hence, “C with GObject”, also called "glib-flavored C", considered as a language separate from plain C, is a strict superset of plain C — like Objective C, but unlike C++.
On platforms where there is no standard ABI that works across all C++ compilers (which is not usually the case, since either the Itanium ABI or the Microsoft ABI are usually followed), a library compiled with one C++ compiler is not always able to call a library compiled with a different one.[ citation needed ] If such compatibility is required, the C++ methods must be exported as plain C functions, partly defeating the purpose of the C++ object system.[ citation needed ] The problem occurs in part because different C++ compilers use different kinds of name mangling to ensure the uniqueness of all exported symbols. (This is necessary because, for example, two different classes may have identically named member functions, one function name may be overloaded multiple times, or identically named functions may appear in different namespaces, but in object code these overlaps are not allowed.)[ citation needed ] In contrast, since C does not support any form of overloading or namespacing, authors of C libraries will typically use explicit prefixes to ensure the global uniqueness of their exported names. [ citation needed ] Hence, despite being object-oriented, a GObject-based library written in C will always use the same external symbol names regardless of which compiler is used.
Perhaps the most profound difference is GObject's emphasis on signals (called events in other languages).[ citation needed ] This emphasis derives from the fact that GObject was specifically designed to meet the needs of a GUI toolkit. Whilst there are signal libraries for most object-oriented languages out there, in the case of GObject it is built into the object system. Because of this, a typical GObject application will tend to use signals to a much larger extent than a non-GObject application would, making GObject components much more encapsulated and reusable than the ones using plain C++ or Java.[ citation needed ][ according to whom? ] If using glibmm/gtkmm, the official C++ wrappers to Glib/GTK respectively, the sibling project libsigc++ allows easy use of underlying GObject signals using standard C++. Of course, other implementations of signals are available on almost all platforms, although sometimes an extra library is needed, such as Boost.Signals2 for C++.
In computer science, reference counting is a programming technique of storing the number of references, pointers, or handles to a resource, such as an object, a block of memory, disk space, and others.
In programming languages, a closure, also lexical closure or function closure, is a technique for implementing lexically scoped name binding in a language with first-class functions. Operationally, a closure is a record storing a function together with an environment. The environment is a mapping associating each free variable of the function with the value or reference to which the name was bound when the closure was created. Unlike a plain function, a closure allows the function to access those captured variables through the closure's copies of their values or references, even when the function is invoked outside their scope.
This article compares two programming languages: C# with Java. While the focus of this article is mainly the languages and their features, such a comparison will necessarily also consider some features of platforms and libraries.
Glade Interface Designer is a graphical user interface builder for GTK, with additional components for GNOME. In its third version, Glade is programming language–independent, and does not produce code for events, but rather an XML file that is then used with an appropriate binding.
In computer science, dynamic dispatch is the process of selecting which implementation of a polymorphic operation to call at run time. It is commonly employed in, and considered a prime characteristic of, object-oriented programming (OOP) languages and systems.
PyGTK is a set of Python wrappers for the GTK graphical user interface library. PyGTK is free software and licensed under the LGPL. It is analogous to PyQt/PySide and wxPython, the Python wrappers for Qt and wxWidgets, respectively. Its original author is GNOME developer James Henstridge. There are six people in the core development team, with various other people who have submitted patches and bug reports. PyGTK has been selected as the environment of choice for applications running on One Laptop Per Child systems.
In computer programming, a virtual method table (VMT), virtual function table, virtual call table, dispatch table, vtable, or vftable is a mechanism used in a programming language to support dynamic dispatch.
GLib is a bundle of three low-level system libraries written in C and developed mainly by GNOME. GLib's code was separated from GTK, so it can be used by software other than GNOME and has been developed in parallel ever since.
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C# is a general-purpose high-level programming language supporting multiple paradigms. C# encompasses static typing, strong typing, lexically scoped, imperative, declarative, functional, generic, object-oriented (class-based), and component-oriented programming disciplines.
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Vala is an object-oriented programming language with a self-hosting compiler that generates C code and uses the GObject system.
java-gnome is a set of language bindings for the Java programming language for use in the GNOME desktop environment. It is part of the official GNOME language bindings suite and provides a set of libraries allowing developers to write computer programs for GNOME using the Java programming language and the GTK cross-platform widget toolkit.
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Seed is a JavaScript interpreter and a library of the GNOME project to create standalone applications in JavaScript. It uses the JavaScript engine JavaScriptCore of the WebKit project. It is possible to easily create modules in C.
Objective-C is a high-level general-purpose, object-oriented programming language that adds Smalltalk-style message passing (messaging) to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was selected by NeXT for its NeXTSTEP operating system. Due to Apple macOS’s direct lineage from NeXTSTEP, Objective-C was the standard language used, supported, and promoted by Apple for developing macOS and iOS applications from 1997, when Apple purchased NeXT until the introduction of the Swift language in 2014.
Swift is a high-level general-purpose, multi-paradigm, compiled programming language created by Chris Lattner in 2010 for Apple Inc. and maintained by the open-source community. Swift compiles to machine code and uses an LLVM-based compiler. Swift was first released in June 2014 and the Swift toolchain has shipped in Xcode since Xcode version 6, released in September 2014.
Nim is a general-purpose, multi-paradigm, statically typed, compiled high-level system programming language, designed and developed by a team around Andreas Rumpf. Nim is designed to be "efficient, expressive, and elegant", supporting metaprogramming, functional, message passing, procedural, and object-oriented programming styles by providing several features such as compile time code generation, algebraic data types, a foreign function interface (FFI) with C, C++, Objective-C, and JavaScript, and supporting compiling to those same languages as intermediate representations.