API v2¶

Note that v2 API is somewhat different than previous API, so some porting of existing code may be needed. See also later section for a rationale and discussion on changes.

The generated code provides a straight binding as specified by the annotations, so everything is pretty much where expected, such as methods within classes in turn within namespaces. For example, all GObject types are within namespace gi::repository::GObject. With that in mind, it should be easy to use and navigate in generated code, along with following comments:

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As customary, anything within a detail or internal namespace is not meant for public use and subject to change. The top-level gi namespace defines a few things that make up public API which is meant to be stable (though at this stage of maturity no full guarantee is provided).

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Some generated code may have _ (underscore) appended to it simply to avoid clashing with a reserved keyword (or a preprocessor definition). It has no special (reserved) meaning otherwise.

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However, anything with leading underscore (if encountered) should be considered as internal (and not meant for public API).

In overall, the generated code is very lightweight and clear, easily understood and with little runtime overhead, as also illustrated by the following overview of wrappers for various kinds of types. Note that almost all of them essentially wrap a pointer and therefore should be checked for validity prior to many uses as with any "smart pointer" (e.g. using provided operator bool()).

Objects. A GObject is a single pointer along with class code that manages a single refcount (including decrement upon destruction). The refcount it manages is either received/taken from a full transfer, or ref_sink'ed (in case of none/floating transfer, see also discussion in subsequent section on the intricacies of the latter and theoretical edge cases).

Boxed Types. Similarly, but with a minor twist, wrappers for a boxed GType MyBox come in 2 kinds; an owning MyBox and a non-owning MyBox_Ref. In both cases, the wrapper is again a single pointer with some suitable/applicable helper methods. The former essentially acts a "unique ptr" (with g_boxed_free deleter) whereas the latter acts as a "naked ptr/reference" (without any ownership or cleanup). Obviously, for the latter case, all the usual caution regarding dangling references (etc) applies. The latter are used for transfer none cases and the former in transfer full situations. In case a safe "reference" needs to be kept around (e.g. in some member), then a _Ref can be .copy_()'d (which uses g_boxed_copy) to an owning wrapper. The above semantics also imply that the owning wrapper is move-only (and again .copy_() yields a copy). However, there are quite some cases where a boxed copy is based on a refcount (which also preserves the box identity/pointer). Those cases have been specially marked (in overrides) to make the owning wrappers copyable as well. Likewise, a _Ref of such cases can be (implicitly) assigned/copied to an owning one (in each case triggering a g_boxed_copy which is then known to be plain and cheap). If desired, additional wrappers could be marked as copyable, in which case a wrapper copy invokes a potentially more expensive (and non-identity preserving) g_boxed_copy. Also, or alternatively, if GI_ENABLE_BOXED_COPY_ALL is defined and truthy, then all boxed wrappers are copyable in that way.

Record Types. Plain records (i.e. structs with no registered GType) are handled in a similar fashion, with g_free as "deleter" (and without any copy support). Since no lifecycle resource management (construction, destruction) is available for such types, there are (quite some) limitations to what code generation or binding can do here (see also discussion in corresponding section).

Strings. A string (e.g. char*) is also regarded and wrapped in a similar way. That is, a gi::cstring wraps (and owns and manages) a C char* and gi::cstring_v is the corresponding non-owning variant. Obviously, the former bears resemblance to std::string whereas the latter to std::string_view. In fact, as there is no real definitive "string API" (in C or glib), their API is fairly similar (though not guaranteed identical) to the std counterparts. Also, various conversions from/to std counterparts should allow for convenient type interchange. Additional integration with other string types is also possible by further specialization of gi::convert::converter (see gi/string.hpp source for details).

Collections. That is, GList, GSList, GPtrArray, GHashTable or plain arrays (zero-terminated or not). Similar to std container, each collection wrapper is a templatized gi::Collection type, with (a.o.) a type parameter for the contained type. As with some of the above types, such wrappers come in an owning and non-owning variants, as specified by another (type) parameter and obtained from annotations, i.e. transfer none, transfer container or transfer full. Note that the "ownership" specifies both ownership of the container and of the contained elements. Of course, where needed, code generation will select and specify the proper type (e.g. as function parameter). Following aspects are worth mentioning;

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Templatized constructors and conversion operators support construction from/of and assignment from/to (e.g.) std container types. Likewise so for "similar" (duck-ed) types, where "similar" refers to member types and constructor signatures.

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A (std) container-ish API is also provided, though neither identical nor fully compatible (a.o. due to limitations of the C wrappee's API). However, the none (ownership) variant is considered read-only and so it does not provide any "modification" API parts and only a const iterator. As almost no wrapper methods are const, an auto p : coll (range-for) pattern is recommended (wrappers are cheaply copied). Other variants do support modification as well as iteration that allows for a auto &p : coll pattern (if so desired). In particular, this applies to the full variant, which is the recommended one for "standalone" use (as container), as it safely manages ownership of both itself and elements.

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Wrappers of refcounted collections (GPtrArray, GHashTable) are otherwise similar to object wrappers. So they always manage a refcount (and are copyable) regardless of ownership variant (none, etc). The other wrappers are similar to boxed wrappers, e.g. copyable in none variant, but otherwise assume unique ownership and are non-copyable.

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A gi::CollectionParameter may also used by code generation for a function input parameter. In case of none ownership, this type/instance will temporarily hold ownership of a collection that may be created by conversion from another container. Temporarily here refers to the duration of the call during which the parameter instance exists. It is not (and should not be) used elsewhere.

In short, one can choose to work with std types and convert to collection wrappers upon function call/return, but for simple cases (or beyond), the collection wrapper might well serve (without conversion).

Plain Types. Various enum, (static) method, functions, typedef (for callback) fill in the rest.

Functions. Functions that involve the usual GError return pattern are wrapped in a few ways. On the one hand, in a straight way, where the error is a (wrapped error) output parameter. Alternatively, the error parameter is removed from the signature. In that case it is "returned" by either throwing the (wrapped) error (which is also a std::exception subclasss), or by returning a suitable expected type (with the wrapped error type as error type). While throwing is default behaviour, the latter can be requested using --expected option.

In case of a GError in (function) callback or virtual method signature, it is always retained as a (wrapped) error output parameter and preferably used to report an error that way. Alternatively, an exception can be thrown, preferably then a GLib::Error instance. Callback wrapping code will catch any exception and report (to C caller) using GError output along with a zero-initialized return value, which is likely but not necessarily a good choice.

Note, however, that the aforementioned catch only applies if exception support is enabled. Auto-detection of this should usually work, but if needed can be specified by defining GI_CONFIG_EXCEPTIONS expclitly (truth/falsy).

Subclasses and Interfaces. Some additional specifications on how subclasses and interfaces are mapped may also be in order. A subclass in the GObject world is directly mapped as a subclass in the C++ binding. However, if a GObject implements an interface, the generated class does not inherit from the interface's (generated) class. This is mostly of a matter of implementation choice (and to ensure its lightweight simplicity). However, knowledge of implemented interfaces is not always available at compile time, e.g. in case of dynamically loaded GStreamer elements (though it is more likely in case of Gtk hierarchy). Since there would be no inheritance in the dynamic case, a consistent choice is not to have it at any time. However, for ease of use, some helper code is generated when an implemented interface is known at generation/compile time, as illustrated in the following snippet from an example

c++ // use a cast if not known, either to a class or interface auto bin = gi::object_cast<Gst::Bin>(playbin_); // known at compile time; overloaded interface_ method auto cp = bin.interface_ (gi::interface_tag<Gst::ChildProxy>());

SUBCLASS IMPLEMENTATION API¶

There may be times when one would want to make a custom subclass of GObject, or of some Gtk widget. In the same vein, (current) implementation choices imply that one should not simply inherit from Gtk::Window. Part of the motivation here is that such subclassing depends on style and setting, i.e. it is rather rare when in a GStreamer setting, but less so in e.g. Gtk. As such, the possibly rare cases should not burden or complicate the basic wrapping usecase.

So, how to subclass then? By a slight twist by using the impl namespace variations, as in following excerpt from an example:

```c++ class TreeViewFilterWindow : public Gtk::impl::WindowImpl { // ... public: // Assume (hypothetically) that Window also implements FakeInterface // with a set_focus method, then a compilation failure will be triggered (as // it can no longer be detected whether set_focus is defined in this class). // Then the following inner struct is needed to resolve so manually; struct DefinitionData { // the last parameter specifies whether the method is defined // (which may well be false in all class/interface cases if not defined) GI_DEFINES_MEMBER(WindowClassDef, set_focus, true) GI_DEFINES_MEMBER(FakeInterfaceDef, set_focus, false) }; // NOTE for the auto-detection to work, the methods must be accessible // so either they should be defined public, or (e.g.) WindowClassDef // must be declared friend, or the above manual resolution can be used.

TreeViewFilterWindow () : Gtk::impl::WindowImpl (this) { // ... }

void set_focus_ (Gtk::Widget focus) noexcept override { } }; ``` Parent (class or interface) methods can then be overridden or implemented in the usual way by simply defining them in the subclass. It is also possible to define custom signal and properties in the subclass, as illustrated in the gobject.cpp example. As mentioned, the inner DefinitionData struct in the above fragment is usually not needed, but only in case of conflict/duplication of class/interface member(s).

Since this is considered an optional feature, the impl parts are not generated by default, but only if the --class option is specified. Since the virtual methods share some similarities with callbacks they are also subject to some limitations (see corresponding section). As such, it may happen that some virtual methods do not have a wrapper. If the --class-full option is specified, then a passthrough virtual method (with C signature as-is) is then generated instead, which can then be overridden and implemented as a fallback. So the custom type registration (that happens behind the scenes) can then still be used, albeit at the expense of dealing with a plain C signature and types (which is similar to directly calling a C function as a fallback if no wrapper function was generated for some reason).

CODE LAYOUT AND BUILD SETUP¶

The generated code is written to the top-level with the following layout. Each GIR namespace has a corresponding subdirectory, say ns (and also a C++ namespace, cppgir::repository::ns). The top-levels headers for a namespace are then:

ns.hpp

a regular header providing the namespace's declarations. It will also include the dependent namespaces' top headers. If the macro GI_INLINE is defined, then it will also include ...

ns_impl.hpp

contains the definitions corresponding to the declarations. Normally, this would be a .cpp file, but as they might be included directly in the inline case, they have been named xxx_impl.hpp instead.

ns.cpp

this merely includes ns_impl.hpp and is as such no different than the latter, except for more traditional naming. Compiling this file in the non-inline case provides all the definitions for the namespace in the resulting object file.

So, in summary, it comes down to setting up the build system to build each of the namespaces' .cpp, as is also done in this repo's CMake build setup. There is one other shortcut build setup that is illustrated by the gtk-obj.cpp example file, which includes all definitions (recursively):

c++ #define GI_INCLUDE_IMPL 1 #include <gtk/gtk.hpp>

Note, however, this is only possible if there is exactly 1 top-level namespace, as doing this for several namespaces will lead to duplicate definitions.

Some items (functions, types) may be marked as deprecated (in source code). while still present in GIR data. Wrappers will still be generated and pragma are issued to avoid warnings that might otherwise occur. Generic gi support tries to avoid using deprecated code. There is, however, one exception regarding the use of g_object_newv, which is deprecated but may have to be used if support for an older GLib is required. This can be arranged by defining GI_OBJECT_NEWV (and the deprecation warning should also be silenced when dealing with newer version).

If you have specified the --class option, then the generated code will possibly contain classes that inherit from several classes (representing interfaces). Since various interfaces may have overlapping member names, this might trigger compilation warnings. These are not suppressed by default, as you may need to be made aware of this. However, if it does no harm in your particular case, then defining GI_CLASS_IMPL_PRAGMA should arrange for proper suppression.

OVERRIDING OR EXTENDING¶

It is possible to add functions or methods or override existing names (by effect of name hiding). To this end, the generated code contains various 'optional include hooks' using the __has_include directive. This way, code in externally supplied (include) files can be inserted into the class definition chain. There are roughly 3 such 'hook points':

initial setup

this part is (conditionally) included before the namespace's C headers are included. This allows specifying define's to tweak subsequent headers or to add headers that also need to be include'd, and which may not have been specified in the GIR.

class definition

these hooks allow extending the wrapped class with new or tweaked methods

global extra definitions

these are included after all generated code, and supports adding of new global functions, typedef's, type trait helper declarations, ...

The reader is invited to examine the default overrides in this repo as well as the generated code to see how this fits together based on a simple naming scheme and use of macros. In particular, see the provided GLib overrides. Suffice it to add that the _def suffix refers to 'default' as supplied by this repo and which are installed alongside the common headers. The corresponding non-suffixed filenames should be used by project specific custom additions.

CODE GENERATION¶

It might be necessary to exclude a GIR entry from processing, either because it is a basic type handled by custom code (e.g. GObject, GValue, ...) or because of a faulty annotation. The latter can be a glitch in the annotation itself, or one that actually refers to a symbol in a non-included private header. The exclusion can be directed by so-called ignore files, and at least one such is supplied as a system default ignore containing known and essential cases to exclude (and without which code generation would not produce valid code). Such a file consists of lines of regular expressions (# commented lines are ignored). At generation time, each symbol is turned into a <NAMESPACE>:<SYMBOLKIND>:<SYMBOL> string, and excluded if it matches one of the lines' regular expression. So, for instance, GObject:record:Value prevents processing of GValue, since there is already special-case code for that in the common header code. Further expression examples are found in the default ignore file. Additional files can be specified by the --ignore option.

As each entry is processed, some notification may be given regarding a perceived inconsistency in an annotation or an unsupported case (see also BUGS AND LIMITATIONS). When the reported cases have been (manually) checked and considered harmless, the corresponding notices can be suppressed by specifying suppression files to --suppression. The format of such files is the same as ignore files, except that a match then simply serves to decrease reporting verbosity. Such a file could be hand-crafted, but it can also be auto-generated by a run when specifying --gen-suppression.

Besides excluding problematic GIR parts, one might also consider solutions to some problematic GIRs used by other projects, such as fixed GIRs maintained by gtk-rs https://gtk-rs.org/gir/book/tutorial/finding_gir_files.html#gtk-dependencies in the referenced repo https://github.com/gtk-rs/gir-files.

(RATIONALE OF) v2 CHANGES¶

Consider the following python session using gobject-introspection:

>>> import gi
>>> gi.require_version('Gst', '1.0')
>>> from gi.repository import Gst
>>> Gst.init(None)
>>> c = Gst.caps_from_string('video/x-raw')
>>> c.get_structure(0)
<Gst.Structure object at 0x7fe284096760 (GstStructure at 0x1bb4420)>
>>> c.get_structure(0)
<Gst.Structure object at 0x7fe2840b5d00 (GstStructure at 0x1bb43a0)>

What happens here? A different GstStructure* is created each time, even though the same one is returned (by C code) in each case. The python binding here has no other choice than to use g_boxed_copy() on the transfer none return value. If it would not, it would be carrying around an unguarded/unowned and hence potentially dangling pointer (in some PyObject wrapper), which is a definite no-go in a scripted setting that must always ensure valid objects.

v1 API followed a similary "scripted" style approach where all objects/pointers should always be safe and valid, with (roughly) std::shared_ptr in place of PyObject. Of course, also then with similar (copy) effects as in the above excerpt and in e.g. issue #32 https://gitlab.com/mnauw/cppgir/-/issues/32.

v2 now follows a different approach. After all, C++ is much closer to C, and it is customary to mind about (potentially dangling) references and such, and where and how (not) to use e.g. std::string_view. And so while types/objects are now no longer always "owning" (and as such always safe), the type conventions do clearly specify whether or not they do (own). As such, standard C++ practices should handle what v2 API provides, while avoiding superfluous and potentially surprising copies or any other "automagic". In particular, the v2 bindings are therefore even more "tight and direct" than before, with a typical wrapper being only a cast away from the wrappee (and matching in size and semantics).

Migration. In practice, only limited changes have been needed in the included examples. Of course, your mileage may vary, depending on usage of "boxed types" as well as use of (type deduction) auto versus explicit type specification. Some _Ref types may have to be used instead here or there, as well as possibly some std::move on "owning" variants (unless overall boxed copy is enabled).

WORKAROUNDS¶

As C++ allows direct mixing/calls with C, there are usually some fallback workarounds when confronted with one of the limitations. First of all, note that a C++ wrapper typically has e.g. a gobj_() method that provides the underlying C pointer/object. Conversely, gi::wrap can be used to obtain a wrapper from a C pointer/object obtained by some means. With that in mind, the following are some workarounds;

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function call; using/given the above, the C function can then (simply) be called directly

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custom subclass virtual method; use --class-full to generate a virtual method with plain C signature

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signal; use Object::connect_unchecked (see also gst.cpp example)

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callback; use gi::callback_wrapper (see also in same example location as above)