QByteArray to qlonglong

DRoscoe

What am I doing wrong? I am parsing a binary file, reading 4 bytes at the current position in the file:

  QByteArray seconds(playback_file.read(4));

This appears to work correctly in that the bytes in the array match the file:

seconds	"\022$uX"	QByteArray
	[0]	    	18    	0x12	char
	[1]	'$' 	36    	0x24	char
	[2]	'u' 	117    	0x75	char
	[3]	'X' 	88    	0x58	char

This is from my hex dump of the file at the same spot (pos 8-11):

61 6f 70 20 01 01 20 20 12 24 75 58 40 b7 03 20

However, when I try to convert the array to a qlonglong, it fails:

  bool ok;
  qlonglong ll_seconds = seconds.toLongLong(&ok, 16);

		
Locals		
	ll_seconds	0	int64
	ok	false	bool
	seconds	"\022$uX"	QByteArray

Chris Kawa

toLongLong is not for byte mangling. It interprets the bytes as characters of a string i.e. it would convert a string "123" into an integer 123.
If you want to just use the bytes directly use the bytes directly:

qlonglong foo = *reinterpret_cast<const qlonglong*>(seconds.constData());

Note that you might have to reverse the array first, depending on your data format and executable endianness. For platform independent format you might want to take a look at QDataStream.

DRoscoe

@Chris-Kawa said in QByteArray to qlonglong:

toLongLong is not for byte mangling. It interprets the bytes as characters of a string i.e. it would convert a string "123" into an integer 123.
If you want to just use the bytes directly use the bytes directly:
qlonglong foo = *reinterpret_cast<const qlonglong*>(seconds.constData());
Note that you might have to reverse the array first, depending on your data format and executable endianness. For platform independent format you might want to take a look at QDataStream.

Thanks! I might have missed something but it wasn't obvious to me from the documentation that there was an assumption of string format in those QByteArray functions. It seems a bit counter-intuitive.

I switched it to use the QDataStream and its working fine now. I don't have an endianness concern right now, but who knows where this tool will be used in the future.

Chris Kawa

Yeah, for better or worse QByteArray has grown to be kind of a "safer const char*" in that it's not only for storing bytes, as the name would suggest, but also provides a variety of functions that you would expect from a string handling class. It's in the docs

QByteArray can be used to store both raw bytes (including '\0's) and traditional 8-bit '\0'-terminated strings. Using QByteArray is much more convenient than using const char *. Behind the scenes, it always ensures that the data is followed by a '\0' terminator, and uses implicit sharing (copy-on-write) to reduce memory usage and avoid needless copying of data.

but I do think it has grabbed more functionality than it should. Oh well...