gstreamer/sdk-qt-gstreamer-vs-c-gstreamer.md

QtGStreamer vs C GStreamer

QtGStreamer is designed to mirror the C GStreamer API as closely as possible. There are, of course, minor differences. They are documented here.

Common Functions

C GStreamer	QtGStreamer
gst_element_factory_make()	QGst::ElementFactory::make(const QString &factoryName, const char *elementName=NULL)
gst_parse_bin_from_description()	QGst::Bin::fromDescription(const QString &description, BinFromDescriptionOption ghostUnlinkedPads=Ghost)
gst_caps_from_string()	QGst::Caps::fromString(const QString &string)
g_signal_connect()	QGlib::connect(GObject* instance, const char *detailedSignal, T *receiver, R(T::*)(Args...) slot, ConnectFlags flags)

Naming Convention

QtGStreamer follows a strict naming policy to help make cross referencing easier:

Namespaces

The "G" namespace (GObject, GValue, etc...) is referred to as "QGlib".

The "Gst" namespace (GstObject, GstElement, etc...) is referred to as "QGst".

Class Names

Class names should be the same as their G* equivalents, with the namespace prefix removed. For example, "GstObject" becomes "QGst::Object", "GParamSpec" becomes "QGlib::ParamSpec", etc...

Method Names

In general the method names should be the same as the GStreamer ones, with the g[st]_<class> prefix removed and converted to camel case.

For example,

gboolean gst_caps_is_emtpy(const GstCaps *caps);

becomes:

namespace QGst {
    class Caps {
        bool isEmpty() const;
    }
}

There are cases where this may not be followed:

1. Properties. Most property getters have a "get" prefix, for example, gst_object_get_name(). In QtGStreamer the "get" prefix is omitted, so this becomes just name().
2. Overloaded members. In C there is no possibility to have two methods with the same name, so overloaded members usually have some extra suffix, like "_full". For example, g_object_set_data() and g_object_set_data_full(). In C++ we just add a method with the same name, or put optional parameters in the existing method.
3. Other cases where the glib/gstreamer method name doesn't make much sense. For example, gst_element_is_locked_state(). That doesn't make sense in english, as "sate" is the subject and should go before the verb "is". So, it becomes stateIsLocked().

Reference Counting

Reference counting is handled the same way as Qt does. There is no need to call g_object_ref()`` and g_object_unref().

Access to GStreamer Elements

QtGStreamer provides access to the underlying C objects, in case you need them. This is accessible with a simple cast:

ElementPtr qgstElement = QGst::ElementFactory::make("playbin");
GstElement* gstElement = GST_ELEMENT(qgstElement);