Re: [FFmpeg-devel] [PATCH] avutil/channel_layout: make pre-defined channel layouts C++ friendly

[James Almer <jamrial@gmail.com>]

On 8/16/2023 1:52 PM, Paul B Mahol wrote:
> On Wed, Aug 16, 2023 at 6:21 PM James Almer <jamrial@gmail.com> wrote:
> 
>> On 8/16/2023 12:44 PM, Zhao Zhili wrote:
>>> From: Zhao Zhili <zhilizhao@tencent.com>
>>>
>>> C++ doesn't support designated initializers until C++20. We have
>>> a bunch of pre-defined channel layouts, the gains to make them
>>> usable in C++ exceed the losses.
>>>
>>> Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
>>> ---
>>>    libavutil/channel_layout.h | 4 ++--
>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/libavutil/channel_layout.h b/libavutil/channel_layout.h
>>> index f345415c55..817a5ad370 100644
>>> --- a/libavutil/channel_layout.h
>>> +++ b/libavutil/channel_layout.h
>>> @@ -359,7 +359,7 @@ typedef struct AVChannelLayout {
>>>    } AVChannelLayout;
>>>
>>>    #define AV_CHANNEL_LAYOUT_MASK(nb, m) \
>>> -    { .order = AV_CHANNEL_ORDER_NATIVE, .nb_channels = (nb), .u = {
>> .mask = (m) }}
>>> +    { AV_CHANNEL_ORDER_NATIVE, (nb), { m }, NULL }
>>
>> Comment out the field names instead of removing them altogether, and add
>> a comment about how this is done for the sake of c++ projects including
>> this header.
>>
>>>
>>>    /**
>>>     * @name Common pre-defined channel layouts
>>> @@ -397,7 +397,7 @@ typedef struct AVChannelLayout {
>>>    #define AV_CHANNEL_LAYOUT_STEREO_DOWNMIX    AV_CHANNEL_LAYOUT_MASK(2,
>> AV_CH_LAYOUT_STEREO_DOWNMIX)
>>>    #define AV_CHANNEL_LAYOUT_22POINT2          AV_CHANNEL_LAYOUT_MASK(24,
>> AV_CH_LAYOUT_22POINT2)
>>>    #define AV_CHANNEL_LAYOUT_AMBISONIC_FIRST_ORDER \
>>> -    { .order = AV_CHANNEL_ORDER_AMBISONIC, .nb_channels = 4, .u = {
>> .mask = 0 }}
>>> +    { AV_CHANNEL_ORDER_AMBISONIC, 4, { 0 }, NULL }
>>>    /** @} */
>>>
>>>    struct AVBPrint;
>>
>> This needs a minor bump IMO. Something said c++ projects can look for
>> before using these defines.
>>
> 
> OK,  I had enough, I'm leaving FFmpeg once and forever.

We use designated initializers in source files and internal headers, but 
not in public installed headers, so c++ projects can include them.

It's the same reason we avoid bitfields in those too.
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