unsigned char data[24] = {
    0x18, 0x00, 0x04, 0x01,
    0x00, 0x00, 0x00, 0x00,
    0x5E, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00
};

struct UpdateFileHeader
{
   UINT16 HdrSize; // always 0x0018
   BYTE override; // 0x01 => override with new file; 0x04 => update file (bsdiff)
   BYTE version; // always 0x04
   UINT32 unknown3;
   UINT32 OldFileSize; // file size before the update (uncompressed)
   UINT32 NewFileSize; // size of the entire file after the update is applied (uncompressed)
   UINT32 unknown6; // always 0
   UINT32 unknown7; // always 0
};

Hardcoded values work:
            const char* output = reinterpret_cast<const char*>(data);

            OutFile.write(output, 24)


Trying to use structure to write variable data:
            OutFile.write(reinterpret_cast<const char*>(test.HdrSize), sizeof(UINT16));
            OutFile.write(reinterpret_cast<const char*>(test.override), sizeof(BYTE));
            OutFile.write(reinterpret_cast<const char*>(test.version), sizeof(BYTE));
            OutFile.write(reinterpret_cast<const char*>(test.unknown3), sizeof(UINT32));
            OutFile.write(reinterpret_cast<const char*>(test.OldFileSize), sizeof(UINT32));
            OutFile.write(reinterpret_cast<const char*>(test.NewFileSize), sizeof(UINT32));
            OutFile.write(reinterpret_cast<const char*>(test.unknown6), sizeof(UINT32));
            OutFile.write(reinterpret_cast<const char*>(test.unknown7), sizeof(UINT32));