APIv2
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:
• 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).
• 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.
• 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 operatorbool()).
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).
BoxedTypes. 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.
RecordTypes. 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;
• 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.
• 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 autop: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.
• 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.
• 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).
PlainTypes. 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).
SubclassesandInterfaces. 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
// 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>());
SUBCLASSIMPLEMENTATIONAPI
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:
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).
CODELAYOUTANDBUILDSETUP
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):
#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 the items are also marked deprecated in GIR data,
then these are skipped by default. However, if the string deprecated:<NAMESPACE>:<VERSION> matches (a
regexp) in specified ignore data/files, then deprecated items will be considered for the namespace in
question, after being checked as usual against the ignore list.
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.
OVERRIDINGOREXTENDING
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':
initialsetup
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.
classdefinition
these hooks allow extending the wrapped class with new or tweaked methods
globalextradefinitions
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.
CODEGENERATION
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 BUGSANDLIMITATIONS). 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.
(RATIONALEOF)v2CHANGES
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). For reasons of
consistency and to avoid collision with generated methods, some more "custom methods" may have had _
appended (e.g. CBoxed::allocate_()).
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