C++ smart pointers to FFmpeg objects

Question

Can I create and use C++ smart pointers for different pointer types from FFmpeg?

1. "AVCodecContext *" which is used only as a pointer in all functions except deallocation.

Alloc:

AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);

Free:

void avcodec_free_context(AVCodecContext **avctx);

Use:

int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);

Then the smart pointer:

std::shared_ptr<AVCodecContext> av_codec_context(avcodec_alloc_context3(av_codec),
[](AVCodecContext* _context)
{
  if (_context) avcodec_free_context(&_context);
});
avcodec_open2(av_codec_context.get(), av_codec, NULL)

Is this correct?

2. "AVDictionary **" which is used in all functions only as a pointer to a pointer.

Alloc and use:

int av_dict_set(AVDictionary **pm, const char *key, const char *value, int flags);

where pm is a Pointer to a pointer to a dictionary struct. If *pm is NULL a dictionary struct is allocated and put in *pm.

Free:

void av_dict_free(AVDictionary **m);

Then a smart pointer:

std::shared_ptr<AVDictionary*> av_dict(new (AVDictionary*),
[](AVDictionary** _dict)
{
  if (_dict)
  {
    if(*_dict)
      av_dict_free(_dict);
    delete _dict;
  }
});
av_dict_set(av_dict.get(), "key", "value", 0);

Is this correct?

3. "AVFormatContext *" which is used both as a pointer and as a pointer to a pointer.

Alloc:

AVFormatContext *avformat_alloc_context(void);

Free:

void avformat_free_context(AVFormatContext *s);

Use:

int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);

or

int avformat_open_input(AVFormatContext **ps, const char *url, const AVInputFormat *fmt, AVDictionary **options);

where ps is a Pointer to user-supplied AVFormatContext (allocated by avformat_alloc_context). May be a pointer to NULL, in which case an AVFormatContext is allocated by this function and written into ps.

Then a smart pointer:

std::shared_ptr<AVFormatContext> av_format_context(avformat_alloc_context(),
[](AVFormatContext* _context)
{
  if(_context)
    avformat_free_context(_context);
});
avformat_find_stream_info(av_format_context.get(), NULL);

Is this correct? But how can I use it with the avformat_open_input() function, which needs a pointer to a pointer and may want to create an object by this pointer?

Answer 1

1. is correct.

pointer on pointer is unneeded, you just need single pointer, i.e:

std::shared_ptr<AVDictionary> make_AVDictionaryPtr(const char *key,
                                                   const char *value,
                                                   int flags)
{
    AVDictionary* p = nullptr;
    av_dict_set(&p, key, value, flags); // "equivalent to" p = some_alloc(..)

    return std::shared_ptr<AVDictionary>{
        p,
        [](AVDictionary* dict) {
            if (dict) {
                av_dict_free(&dict);
            }
        }
    };
}
// ...
auto av_dict = make_AVDictionaryPtr("key", "value", 0);
av_dict_set(av_dict.get(), "key2", "value2", 0);

3. Is this correct?

Yes

But how can I use it with the avformat_open_input() function, which needs a pointer to a pointer and may want to create an object by this pointer?

As for 2. if you want to use it directly.