In FFmpeg, the three concepts of inheritance, encapsulation and polymorphism are realized through structures and function pointers. In the C language, structure embedding and pointers are required to implement class inheritance; structure and function pointers are required to implement encapsulation; and techniques such as function pointers and parameterization are required to implement polymorphism.
Next, I will explain in detail how to implement class inheritance, encapsulation and polymorphism through structure and function pointers in FFmpeg, as well as some key concepts and technologies involved.
1. Inheritance
In the C language, implementing class inheritance requires the use of structure embedding and pointers. In FFmpeg, many structures have a parent-child relationship, and child structures are nested in parent structures. This nesting relationship is similar to the inheritance relationship in C++. For example, the AVStream structure contains a pointer codecpar of AVCodecContext type, indicating the codec parameters of the stream. This relationship is equivalent to the child structure AVCodecContext inheriting the parent structure AVStream.
There is also a structure called AVClass in FFmpeg, which is used to realize the function similar to the virtual base class in C++. The AVClass structure has a pointer to the parent class of the AVClass type, and its subclasses can also have a pointer to the parent class of the AVClass type, so nesting can achieve a multi-inheritance effect.
2. Packaging
In the C language, encapsulation requires the use of structures and function pointers. Many data structures in FFmpeg are encapsulated in a single structure, and the corresponding operations are implemented through function pointers. For example, the functions av_packet_alloc() and av_packet_unref() in AVPacket are all encapsulated in the AVPacket structure to realize the encapsulation and operation of AVPacket.
An important concept of encapsulation is private data (priv_data). Private data refers to member variables in a structure that are only included in the structure and cannot be directly accessed from the outside. In FFmpeg, many data structures have private data, such as AVCodecContext, AVCodec, AVFilter and so on. If you need to add custom members to a structure, you can put these members in private data, so as to ensure the encapsulation of the outside world.
Three, polymorphism
In C language, polymorphism needs to be combined with technology such as function pointer and parameterization. In FFmpeg, many operations (such as decoding, encoding, filtering, etc.) need to process different data structures, so different function pointers need to be implemented to point to corresponding processing functions. For example, the decode function in AVCodecContext is a function pointer, which is used to point to the decoding functions of different codecs, thereby achieving polymorphism.
FFmpeg also defines some structures for storing function pointers related to specific types or operations. For example, the AVCodec structure contains function pointers decode and encode pointing to decoding and encoding functions, and these function pointers are used to represent the decoding and encoding functions supported by the codec.
At the same time, FFmpeg also uses parameterization technology to make functions more polymorphic. For example, the filter chain (filter graph) in FFmpeg can be dynamically adjusted at runtime according to the input and output data types. This is to achieve dynamic polymorphism by encapsulating information such as input and output data types in an AVFilterLink structure, and then passing the structure as a parameter to the corresponding processing function.
Four. Summary
In FFmpeg, since the C language itself does not support the concepts of classes and objects, it cannot directly implement inheritance, encapsulation and polymorphism like C++. However, FFmpeg still implements the corresponding mechanism through structures, function pointers, etc. These mechanisms not only effectively encapsulate data and operations, but also enable advanced features such as polymorphism and inheritance. Among them, private data and parametric technology are very important key technologies.
Overall, the design of FFmpeg is very ingenious, it is perfectly compatible with the features of C language, and realizes many advanced features similar to C++. This also laid a solid foundation for FFmpeg to become the industry's leading multimedia processing library.