> From: ffmpeg-devel <ffmpeg-devel-boun...@ffmpeg.org> On Behalf Of > Andreas Rheinhardt > Sent: Wednesday, March 18, 2020 21:29 > To: ffmpeg-devel@ffmpeg.org > Subject: Re: [FFmpeg-devel] [PATCH 1/3] lavformat: Prepare to make > avio_enum_protocols const correct > > Fu, Linjie: > Yes, updating the callers is necessary (as I already said in the commit > message), but easy. The typical old call is (the ... usually includes a > loop): > void *opaque = NULL; > char *name; > > ... > name = avio_enum_protocols(&opaque, flag); > ... > > Changing the type of opaque to const void* will make this code > const-correct. (The opaque now points to one of the constant pointers in > the url_protocols array, so the user having a pointer to nonconst to > them is wrong.) > > - Andreas > > PS: The reason that the callers need to update their code is that there > is no automatic conversion from type ** to const type **, so that an > explicit cast is required. Such an automatic conversion would be > dangerous as it could be used to remove const from any pointer without a > cast. See http://c-faq.com/ansi/constmismatch.html > PPS: There would be another way to get rid of this warning: Instead of > making the void **opaque store a pointer to a pointer to a const entry > of the url_protocols array one make the opaque point to a value of type > uintptr_t that stores an index in the url_protocols array. This would > still require a cast, but it would not cast const away. But I don't > really like this approach, as forcing the user to use pointers to const > void makes it clearer that the user is not to meddle with the opaque.
No more concerns, this fix looks reasonable. Thanks for the kind elaborations. - Linjie _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
