tdesktop/cmake/external/glib/cppgir/docs/cppgir.md

cppgir(1) - 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 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 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.

• --const-method: Mark (almost) all generated methods in generated wrappers as const. Alternatively, perhaps more recommended, see also the helper gi:cs_ptr type.

• --class-args MIN_OPTIONAL: If >= 0, minimum number of non-required arguments that triggers generation of a CallArgs signature variant (see below for details).

• --basic-container: Also generate a collection signature for an input collection of basic type (e.g. int, etc). See below for some details and discussion.

• --output-top: Also generate convenience .cpp and .hpp files in root output directory per namespace, as may be useful for some build tool setups.

• --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.

• For an input collection parameter of basic type (e.g. int), the original C signature is typically used. That is, 2 parameters (int* and gsize). The rationale here is that the same signature may also occur for an output collection (of specified input size). Preserving the original signature ensures that it can be used whether or not the annotation is correct. The latter may not be the case as these APIs are typically "low-level" (e.g. involve some buffers), and as such are often not considered by "scripted binding". It is also an efficient and clear API as any buffer's "location" (e.g. std::vector or otherwise) can easily be provided by means of these 2 parameters. However, if desired and specified by --basic-container option, then an additional collection-based signature is generated as well.

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 explicitly (truth/falsy).

If a function has (non-GError) output parameters, then there is a signature where these outputs are parameters (as in the plain C case) and another variant where these are incorporated in the return value, which may then become a std::tuple<> type.

If so configured, some so-called CallArgs variations may also be generated. In this case, (roughly) a custom xyz_CallArgs struct type is generated (for each function xyz) with members corresponding to the function arguments along with a function with 1 argument (of that custom struct type). In case of many (optional) arguments, this argument could be specified using designated initializer syntax, thereby allowing a sort-of "call by keyword". Again, there are variations with output parameters as return value or not. Besides some potential advantages, there are also some drawbacks, however. First and foremost, when using the member names in designated initializers, these names then become essentially part of the API, although the name's origin as plain C function parameter does not provide stability guarantee. Also, suffice it to say a great many struct types can be generated this way. This could be mitigated by employing a suitable "generation level", e.g. 2. In that case, only functions that have at least 2 (or more) non-required arguments will have such a custom type and signature generated.

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

// 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.

Before going into the details of "how", let's first consider what a "subclass" actually means in this context. It will probably involve a (C++) (sub)class of some generated class (related to a GObject class type) with potentially extra/custom properties and signals. In particular, the latter implies it also involves a new GType that defines a subtype (of the aforementioned GObject class type). An instance of such class/type then consists of a C++ instance that is 1-to-1 associated with a GObject instance of the custom defined (sub) GType.

In turn, this leads to (at least, for now) 2 possible ways to create instances;

• triggered on C++ side, in the usual way through a constructor. The bottom of the constructor chain registers a custom GType (if still needed), creates an instance (g_object_new()) and associates it (with the C++ instance).

  The advantage is that standard use of C++ constructor applies. The downside is that the defined GType can not be (safely) used in the GObject (eco)system, as any g_object_new()'d instance is incomplete (as it lacks a C++ instance).

• triggered on C-side by g_object_new(). In this case, the registered type's custom (GObject) constructor first delegates to parent constructor to create a GObject subtype instance and then new's a C++ object, and again the bottom of the constructor chain associates it with created GObject instance.

  In this case, an instance has to be created based on GType. However, this type can be safely used in any GType based factory system, e.g. when referenced (by name) in XML/UI file or (by number) in a GStreamer plugin factory.

In summary, (up to) 2 different ways to create objects, and so (up to) 2 different custom GType that can be defined by a "subclass". The latter one is more recent and is also the recommended approach as instances are always properly created. Also, if desired, some additional helper new_() member can be defined that acts as a surrogate constructor.

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.

  // this (super)constructor signature leads to a C++ side type
  TreeViewFilterWindow () : Gtk::impl::WindowImpl (this)
  {
    // ...
  }

  // this (super)constructor signature leads to a C-side type
  // NOTE InitData is not (easily) instantiated,
  //    so this constructor is only used by the internal C-side mechanics
  TreeViewFilterWindow (const InitData &id)
      : Gtk::impl::WindowImpl (this, id, "TreeViewFilterWindow")

  void set_focus_ (Gtk::Widget focus) noexcept override
  {
  }

  // the above constructor is also re-used during (C-side) type registration
  // (in that case with an "empty" id and no associated GObject setup)
  // that can avoided by providing a separate ...
  GType get_type_()
  {
    // no interfaces, properties or signals to declare
    return register_type_<TreeViewFilterWindow>("TreeViewFilterWindow", 0, {}, {}, {});
  }
};

// create an instance of (either) type
// it prefers the latter C-side type, if supported, and supports extra arguments
// (a second template parameter allows for specific selection,
// see code comments and examples for details)
gi::make_ref<TreeViewFilterWindow>()

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).

It is also a fairly advanced feature, with various aspects not immediately obvious. For example, the resulting "instances" have "2 sides"; there is C++ object instance as well as an associated (derived) GObject instance (which internally refer to each other in 1-to-1 association ). In particular, it follows that their destruction must be closely coordinated. This is potentially tricky as the GObject side is traditionally reference-counted, whereas the C++ object could be anything (stack-allocated or otherwise). The latter can be stack-allocated if it is ensured that there is no lingering reference in the GObject world. So, it depends on the use-case as to how to proceed. But in overall, it is probably recommended to manage lifetime and ownership based on GObject (side) reference count. The gi::make_ptr and gi::ref_ptr helpers can be helpful in this regard.

There are also situations where the C API (implementation) involves some "low-level tricks" which do not port over in a simple or straight way (e.g. GtkBuilder). Those are likely in need of some custom overrides or extensions (see also below) which may or may not already be provided for some particular API/situation. It is highly advised to browse provided overrides and extensions and/or consult the closest relevant related example (which usually showcases what is available, along with additional explanations).

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.
• *.cppm; module wrappers, see next subsection for details.

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.

The generated code may be quite extensive and so it may present a "heavy build". Other than that the above allows for a number of different build setups, cppgir tries not to impose any particular approach. In particular, standard tried-and-tested build optimizations can be applied, such as precompiled headers (with some good results, as reported in issue #99).

C++ Module support

At this time of writing, module support is still new-ish in compilers and (not in the least) build tools. Unfortunately, all major compilers also take a different approach in handling these wrt command-line argument, source file extension or binary output. Note that this also likely complicates any compile_commands.json based tooling (e.g. LSP server as used by IDEs) along with other issues as outlined in this post and originally raised long ago.

By comparison, precompiled headers are long since well supported, so these may be a more recommended alternative approach.

Failing that, a next natural fit might be "header units" (in module spec sense). Unfortunately, compiler setup here varies somewhat and build tool support may be limited.

So, a next step is a pure/real module. Unfortunately, "core gi" can not be provided in that form;

• in addition to code, it also "exports" some macros, which can not really be export'ed or import'ed (only from a "header unit")
• the core is somewhat intertwined with basic types from GObject, e.g. some code-generated types are forward declared (which satisfies for their purposes in "core gi"). However, forward and real declaration can not pass module boundary.

But "core gi" combined with (generated) GLib and GObject can be wrapped in a module (along with a separate "macro API header", gi_inc.hpp).

Again, cppgir does not impose a particular module layout/setup, but provides the basic parts and pieces to do so. The generated code also provides a few example module wrappers (with no stability guarantee), also see gst.cpp and gtk.cpp examples for possible usage details;

• ns.cppm (exports gi.repo.ns); module that wraps/provides ns (inline) code, so a typical compilation produces .o code of ns and a BMI/CMI that exports ns declarations
• ns_rec.cppm (exports gi.repo.ns.rec); module that wraps/provides ns and all (recursive) dependencies, so a typical compilation produces .o code of ns and dependencies, along with a BMI/CMI with corresponding declarations.

The above have been tested with gcc-15 and clang-18 with some varied measure of success (apparently, GCC fails to link the non-recursive approach). They each come with an extra "knob"; if GI_MODULE_EXTERN is defined, then the module purview is essentially extern "C++" { ... }. So all declarations are then attached to global module, as opposed to the named module, and as such do not incur any modified ABI linkage (@M) (so it may be recommended).

Other variations are likely possible, e.g. module partitions, separate module implementation, etc.

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 in the referenced repo.

(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.

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_()).

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'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).

In particular, methods are usually not marked const either, as there is similarly no (semantic) data to decide either way. As such, it is not recommended to use const wrapper types. However, they may arise due to generic templates or when captured in a lambda. In such cases, the helper (template) type gi::cs_ptr may be useful, or alternatively one might set the option to mark all code-generated methods as const.

Floating (into darkness). Gobject docs 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). Or perhaps there is an API variant using closures which may be useful in combination with some Closure::from_* helpers (see still same example).

SEE ALSO

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