在TAppDecTop.cpp  ,最重要的是decode 函数，下面将对其进行分析，是解码上层的一个重要函数。

代码如下，代码后将进行分析。

Void TAppDecTop::decode()
{Int                 poc;TComList<TComPic*>* pcListPic = NULL;ifstream bitstreamFile(m_pchBitstreamFile, ifstream::in | ifstream::binary);if (!bitstreamFile){fprintf(stderr, "\nfailed to open bitstream file `%s' for reading\n", m_pchBitstreamFile);exit(EXIT_FAILURE);}InputByteStream bytestream(bitstreamFile);// create & initialize internal classesxCreateDecLib();xInitDecLib  ();m_iPOCLastDisplay += m_iSkipFrame;      // set the last displayed POC correctly for skip forward.// main decoder loopBool recon_opened = false; // reconstruction file not yet opened. (must be performed after SPS is seen)while (!!bitstreamFile){/* location serves to work around a design fault in the decoder, whereby* the process of reading a new slice that is the first slice of a new frame* requires the TDecTop::decode() method to be called again with the same* nal unit. */streampos location = bitstreamFile.tellg();AnnexBStats stats = AnnexBStats();Bool bPreviousPictureDecoded = false;vector<uint8_t> nalUnit;InputNALUnit nalu;byteStreamNALUnit(bytestream, nalUnit, stats);// call actual decoding functionBool bNewPicture = false;if (nalUnit.empty()){/* this can happen if the following occur:*  - empty input file*  - two back-to-back start_code_prefixes*  - start_code_prefix immediately followed by EOF*/fprintf(stderr, "Warning: Attempt to decode an empty NAL unit\n");}else{read(nalu, nalUnit);if( (m_iMaxTemporalLayer >= 0 && nalu.m_temporalId > m_iMaxTemporalLayer) || !isNaluWithinTargetDecLayerIdSet(&nalu)  ){if(bPreviousPictureDecoded){bNewPicture = true;bPreviousPictureDecoded = false;}else{bNewPicture = false;}}else{bNewPicture = m_cTDecTop.decode(nalu, m_iSkipFrame, m_iPOCLastDisplay);if (bNewPicture){bitstreamFile.clear();/* location points to the current nalunit payload[1] due to the* need for the annexB parser to read three extra bytes.* [1] except for the first NAL unit in the file*     (but bNewPicture doesn't happen then) */bitstreamFile.seekg(location-streamoff(3));bytestream.reset();}bPreviousPictureDecoded = true; }}if (bNewPicture || !bitstreamFile){m_cTDecTop.executeLoopFilters(poc, pcListPic);printf("\npoc =%d\n",poc);}if( pcListPic ){printf("\nnaluType =%d\n",nalu.m_nalUnitType);if ( m_pchReconFile && !recon_opened ){if (!m_outputBitDepthY) { m_outputBitDepthY = g_bitDepthY; }if (!m_outputBitDepthC) { m_outputBitDepthC = g_bitDepthC; }m_cTVideoIOYuvReconFile.open( m_pchReconFile, true, m_outputBitDepthY, m_outputBitDepthC, g_bitDepthY, g_bitDepthC ); // write moderecon_opened = true;}if ( bNewPicture && (   nalu.m_nalUnitType == NAL_UNIT_CODED_SLICE_IDR|| nalu.m_nalUnitType == NAL_UNIT_CODED_SLICE_IDR_N_LP|| nalu.m_nalUnitType == NAL_UNIT_CODED_SLICE_BLA_N_LP|| nalu.m_nalUnitType == NAL_UNIT_CODED_SLICE_BLANT|| nalu.m_nalUnitType == NAL_UNIT_CODED_SLICE_BLA ) ){xFlushOutput( pcListPic );}// write reconstruction to fileif(bNewPicture){xWriteOutput( pcListPic, nalu.m_temporalId );}}}xFlushOutput( pcListPic );// delete buffersm_cTDecTop.deletePicBuffer();// destroy internal classesxDestroyDecLib();
}

代码xCreateDecLib 和 xInitDecLib 重要是初始化四叉树和解码需要的全局变量和申请内存。

当提供一个码流文件的文件名后，进行ifstream bitstreamFile(m_pchBitstreamFile, ifstream::in | ifstream::binary); 以二进制方式打开文件名，码流以字节方式InputByteStream bytestream(bitstreamFile);进行读操作。

我们都知道，HEVC/H265 是以NAL方式组织数据的，解析VPS,SPS,PPS,SEI,SEI_SUFFIX 后，其他的是一个个slice的NAL数据，而Deblocking & SAO Filters 等滤波是对整个picuture进行滤波操作，出现从第二帧开始，每帧的第一个slice两次进行解析，这是参考软件的一个bug或不好的地方，其实完全可以知道是否是最后一个slice，不必进行两次打开。

所以出现：

bitstreamFile.clear();

bitstreamFile.seekg(location-streamoff(3));
bytestream.reset();

大家有兴趣，可以先增加一个变量，判断是否是最后一个slice，就不需要执行上面代码了。

如下代码是从来不会执行，因为HEVC/H265没有cavlc，也就不会有slice part A ，slice part B，slice part C ，是现实的编解码告诉了设计者，slice part A ，slice part B，slice part C 没有人使用，就被抛弃了，实际的编解码从来没有实现slice part A ，slice part B，slice part C 等编解码的。

 if( (m_iMaxTemporalLayer >= 0 && nalu.m_temporalId > m_iMaxTemporalLayer) || !isNaluWithinTargetDecLayerIdSet(&nalu)  )
      {
        if(bPreviousPictureDecoded)
        {
          bNewPicture = true;
          bPreviousPictureDecoded = false;
        }
        else
        {
          bNewPicture = false;
        }
      }

解码和滤波后当然就是输出重构数据，xWriteOutput( pcListPic, nalu.m_temporalId )， 如果slice是NAL_UNIT_CODED_SLICE_IDR，NAL_UNIT_CODED_SLICE_IDR_N_LP，NAL_UNIT_CODED_SLICE_BLA_N_LP，NAL_UNIT_CODED_SLICE_BLANT，NAL_UNIT_CODED_SLICE_BLA中的一种，将解码产生的picture全部清空，没有任何参考帧，相当于一个新的sequence进行解码了。

其他的代码很简单，请自己分析。