[GStreamer] gst_element_link 及 pad 在继承体系中如何存在

前言：

pad 是每个 element实例 都有的，是 elemenet 之间沟通的代理人，没有 pad 的 element 没法于其他element交流。

考虑到gstreamer存在继承体系，那么如果继承类element不在init函数里创建pad，那么在gst_element_link的时候是否可以使用父类element实例的pad呢? 下面从源码来找答案。

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gst_element_link

在 gstutils.c 找到 gst_element_link(...) 的实现。

/**
 * gst_element_link:
 * @src: (transfer none): a #GstElement containing the source pad.
 * @dest: (transfer none): the #GstElement containing the destination pad.
 *
 * Links @src to @dest. The link must be from source to
 * destination; the other direction will not be tried. The function looks for
 * existing pads that aren't linked yet. It will request new pads if necessary.
 * Such pads need to be released manually when unlinking.
 * If multiple links are possible, only one is established.
 *
 * Make sure you have added your elements to a bin or pipeline with
 * gst_bin_add() before trying to link them.
 *
 * Returns: %TRUE if the elements could be linked, %FALSE otherwise.
 */
gboolean
gst_element_link (GstElement * src, GstElement * dest)
{
  return gst_element_link_pads (src, NULL, dest, NULL);
}

直接调用 gst_element_link_pa