Provided by: cppgir_2.0-2_amd64 
NAME
cppgir - GObject-Introspection C++ binding wrapper generator
SYNOPSIS
cppgir [OPTION...] --output DIRECTORY GIR...
DESCRIPTION
cppgir reads each of the specified GIR and converts these (and any dependencies) into
C++14 wrapper code that collectively then make up a 'binding' (in GObject-Introspection
https://wiki.gnome.org/Projects/GObjectIntrospection terminology). Each GIR can be
specified as a full pathname to the .gir file or simply by the basename (i.e. no path or
.gir suffix), with or without version. Of course, in the latter case, the .gir must be in
a standard location, or other options must specify additional whereabouts.
OPTIONS
See BACKGROUND later on for further details on some of the concepts used in the following
descriptions.
--output DIRECTORY
Specifies the top-level directory in which to generate code. It will be created if
it does not yet exist.
--gir-path PATHS
Adds a colon-separated list of additional directories within which to (recursively)
search for a .gir file (if not specified by full pathname).
--debug LEVEL
Debug level or level of verbosity, higher numbers are more verbose.
--ignore FILES
Adds a colon-separated list of so-called ignore files.
--suppression FILES
Adds a colon-separated list of so-called suppression files.
--gen-suppression FILE
Specifies a suppression file to generate during this run.
--class
Requests generation of implementation class code needed for subclassing.
--class-full
Requests generation of a plain as-is C signature fall-back method for an otherwise
unsupported unwrapped method. Only applicable if --class is also specified. It also
requires use of the latest custom subclass (signature) approach (see below for
details on that), as these plain methods are not "activated" in case of legacy
approach (for backwards compatibility).
--expected
Use an error return type based on std::expected http://wg21.link/p0323 proposal (as
opposed to throwing exception).
--dl Use dlopen/dlsym to generate (most) calls rather than usual "direct" calls. As
such, a great many calls might then fail at runtime. So, if combined with
--expected all those calls will use the above error return type.
--dump-ignore
(only if compiled with embedded ignore) Dumps embedded ignore data.
ENVIRONMENT
In stead of command-line options, environment variables can also be used. Note, however,
that options are still taken into account even when variables have been set. The following
environment variables are considered, and have the same meaning as the corresponding
command-line option:
`GI_DEBUG`, `GI_IGNORE`, `GI_SUPPRESSION`, `GI_GEN_SUPPRESSION`, `GI_OUTPUT`,
`GI_CLASS`, `GI_CLASS_FULL`, `GI_EXPECTED`, `GI_DL`, `GI_GIR_PATH`
In addition to the above, GI_GIR can specify a colon-separated lists of GIRs (specified as
on command-line). XDG_DATA_DIRS is also used as additional source of directories to search
for GIRs (within a gir-1.0 subdirectory).
BACKGROUND
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:
○ 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 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;
○ 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 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.
○ 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).
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).
BUGS AND LIMITATIONS
The generated code's coverage is pretty good and comfortably serves most cases that arise
in practice as also illustrated by the examples. Nevertheless, the following should be
mentioned:
Callback types. Only callback types that have an explicit user_data parameter are
supported. That includes (fortunately) cases such as connecting to a signal, or a
GstPadProbeCallback, though a GstPadChainFunction is excluded. The reason is a technical
one; the user_data parameter is used to pass data used by callback wrapper code. A typical
(script) runtime binding handles this using libffi https://github.com/libffi/libffi's
closure API. In effect, a little bit of executable code is then generated at runtime, and
the address of that code then essentially serves as surrogate user_data that can carry
extra meta-data for use by the runtime. This could also be employed here to lift the
user_data limitation, it would take a bit extra work, but would more importantly then also
incur an additional dependency.
Callback handling. Even if user_data is present, other aspects of a callback signature may
not be supported (at this time), e.g. certain (sized) array parameters. However, few (if
any) of such actual cases are known at this time. Note that both signals and virtual
methods are somewhat similar to a callback and as such share similar limitations.
Whereas the above items could (in theory) be resolved, the following are more inherent
limitations (by the very context and nature of e.g. annotations). Fortunately, though, the
practical impact is fairly limited (if any).
const handling. In C++, this is a Bigger Thing. For instance, a simple 'getter' should
preferably be marked const. However, on the original C-side of things, only very limited
consideration is given to this. Even if there is some const, it is not treated with all
that much respect, e.g. g_value_take_boxed starts const but it is merrily cast away along
the way. As such, there is not much to find on const-ness in annotation data, and so no
point in inventing any. Rather, the focus is simply on getting the proper function calls
done along with automagic refcount and resource management (much as any runtime binding
would do, with no regard for const whatsoever in that case).
Floating (into darkness). Gobject docs https://docs.gtk.org/gobject/floating-refs.html
mention the following about floating references (i.e. transfer floating);
Floating references are a C convenience API and should not be used in modern GObject
code. Language bindings in particular find the concept highly problematic, as floating
references are not identifiable through annotations, ...
Indeed, by the time floating makes it into the parsed annotation, it has become none. And
in case of a "factory" some_widget_new(), floating behaves more like full as the caller
must "take ownership" to avoid a leak. So a "floating" none is quite different from a
"real" none (e.g. "getter" method). But no way to know from annotation data. So, in case
of none, an object wrapper always ref_sink()s. If it was floating, it has taken suitable
ownership. If it was really none, then it is now managing an extra refcount. And in either
case, it will release/decrement upon destruction. Essentially, this follows the
recommendation given in referenced docs. In practice, it actually Just Works.
It gets really tricky when this is combined with e.g. lists. So what does none mean in
this case (in annotation)? In the worst case, the contained elements might actually be
floating, so one would have to go through the list and ref_sink them all
(un)conditionally? Suffice it to say, no such "automagic" is handled/injected by any
wrapper code. Fortunately, at this time there does not seem to be such a "multiple
factory" API. Even if there were, then in practice the calling code is likely to loop over
the list and access the elements. The ensuing C++ wrappers (even if existing only briefly)
would then effectively ref_sink(), so again we are ok. And last but not least, by the
above quoted recommendation, there should be no such new tricky API coming along. So,
again, it Just Works. If needed, any such old or new API can and should be handled by
custom overrides.
Boxed (by darkness). This refers to so-called "plain records" which are "C structs" with
no registered GType (referred to as "C boxed" types in cppgir code), e.g. GOptionEntry or
GstMapInfo. While their fields may be described in annotations, there is no information
regarding the "ownership" of any data (which may even vary upon context). In particular,
also no way to create/free. This corresponds with their frequent stack-allocated use in C
code in typically "low-level" API which is usually not considered "binding friendly".
Based on the mild assumption that 0-initialized data makes a valid instance, they are
treated somewhat similar to (GType) boxed types and as such can be used in some limited
(function call) situations. Any improvement beyond that is likely to remain in the purview
of overrides.
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;
○ function call; using/given the above, the C function can then (simply) be called
directly
○ custom subclass virtual method; use --class-full to generate a virtual method with
plain C signature
○ signal; use Object::connect_unchecked (see also gst.cpp example)
○ callback; use gi::callback_wrapper (see also in same example location as above)
SEE ALSO
g-ir-scanner(1)
January 2024 CPPGIR(1)