从二进制文件读取时获取错误的数据

Question

I'm having an issue reading in some bytes from a yuv file (it's 1280x720 if that matters) and was hoping someone could point out what I'm doing wrong. I'm getting different results using the read command and using an istream iterator . Here's some example code of what I'm trying to do:

void readBlock(std::ifstream& yuvFile, YUVBlock& destBlock, YUVConfig& config, const unsigned int x, const unsigned int y, const bool useAligned = false)
{
    //Calculate luma offset
    unsigned int YOffset = (useAligned ? config.m_alignedYFileOffset : config.m_YFileOffset) +
    (destBlock.yY * (useAligned ? config.m_alignedYUVWidth : config.m_YUVWidth) + destBlock.yX);// *config.m_bitDepth;

//Copy Luma data
//yuvFile.seekg(YOffset, std::istream::beg);
    for (unsigned int lumaY = 0; lumaY < destBlock.m_YHeight && ((lumaY + destBlock.yY) < config.m_YUVHeight); ++lumaY)
    {
        yuvFile.seekg(YOffset + ((useAligned ? config.m_alignedYUVWidth : config.m_YUVWidth)/* * config.m_bitDepth*/) * (lumaY), std::istream::beg);
        int copySize = destBlock.m_YWidth;
        if (destBlock.yX + copySize > config.m_YUVWidth)
        {
            copySize = config.m_YUVWidth - destBlock.yX;
        }
        if (destBlock.yX >= 1088 && destBlock.yY >= 704)
        {
            char* test = new char[9];
            yuvFile.read(test, 9);

            delete[] test;
            yuvFile.seekg(YOffset + ((useAligned ? config.m_alignedYUVWidth : config.m_YUVWidth)/* * config.m_bitDepth*/) * (lumaY));
        }

        std::istream_iterator<uint8_t> start = std::istream_iterator<uint8_t>(yuvFile);
        std::copy_n(start, copySize, std::back_inserter(destBlock.m_yData));

    }
}
struct YUVBlock
{
std::vector<uint8_t> m_yData;
std::vector<uint8_t> m_uData;
std::vector<uint8_t> m_vData;
unsigned int m_YWidth;
unsigned int m_YHeight;
unsigned int m_UWidth;
unsigned int m_UHeight;
unsigned int m_VWidth;
unsigned int m_VHeight;

unsigned int yX;
unsigned int yY;
unsigned int uX;
unsigned int uY;
unsigned int vX;
unsigned int vY;
};

This error only seems to be happening at X =1088 and Y = 704 in the image. I'm expecting to see a byte value of 10 as the first byte I read back. When I use

yuvFile.read(test, 9);

I get 10 as my first byte. When I use the istream iterator:

std::istream_iterator<uint8_t> start = std::istream_iterator<uint8_t>(yuvFile);
    std::copy_n(start, copySize, std::back_inserter(destBlock.m_yData));

The first byte I read is 17. 17 is the byte after 10 so it seems the istream iterator skips the first byte.

Any help would be appreciated

Answer 1

There is a major difference between istream::read and std::istream_iterator .

std::istream::read performs unformatted read.
std::istream_iterator performs formatted read.

From http://en.cppreference.com/w/cpp/iterator/istream_iterator

std::istream_iterator is a single-pass input iterator that reads successive objects of type T from the std::basic_istream object for which it was constructed, by calling the appropriate operator>> .

If your file was created using std::ostream::write or fwrite , you must use std::istream::read or fread to read the data.

If your file was created using any of the methods that create formatted output, such as std::ostream::operato<<() , fprintf , you have a chance to read the data using std::istream_iterator .