Convert int to little-endian formated bytes in C++ for blobId in Azure

Question

Working with a base64 encoding for Azure (http://msdn.microsoft.com/en-us/library/dd135726.aspx) and I dont seem to work out how to get the required string back. I'm able to do this in C# where I do the following.

int blockId = 5000;
var blockIdBytes = BitConverter.GetBytes(blockId);
Console.WriteLine(blockIdBytes);
string blockIdBase64 = Convert.ToBase64String(blockIdBytes);
Console.WriteLine(blockIdBase64);

Which prints out (in LINQPad):

Byte[] (4 items)
| 136         |
| 19          |
| 0           |
| 0           |

iBMAAA==

In Qt/C++ I tried a few aporaches, all of them returning the wrong value.

const int a = 5000;
QByteArray b;

for(int i = 0; i != sizeof(a); ++i) {
  b.append((char)(a&(0xFF << i) >>i));
}

qDebug() << b.toBase64(); // "iIiIiA==" 
qDebug() << QByteArray::number(a).toBase64(); // "NTAwMA=="
qDebug() << QString::number(a).toUtf8().toBase64(); // "NTAwMA=="

How can I get the same result as the C# version?

Answer 1

I ended up doing the following:

QByteArray temp;
int blockId = 5000;

for(int i = 0; i != sizeof(blockId); i++) {
  temp.append((char)(blockId >> (i * 8)));
}

qDebug() << temp.toBase64(); // "iBMAAA==" which is correct

Answer 2

See my comment for the problem with your for loop. It's shifting by one bit more each pass, but actually it should be 8 bits. Personally, I prefer this to a loop:

    b.append(static_cast<char>(a >> 24)); 
    b.append(static_cast<char>((a >> 16) & 0xff)); 
    b.append(static_cast<char>((a >> 8) & 0xff)); 
    b.append(static_cast<char>(a & 0xff)); 

The code above is for network standard byte order (big endian). Flip the order of the four operations from last to first for little endian byte order.

Answer 3

I think this would be clearer, though may be claimed to be ill styled...

int i = 0x01020304;
char (&bytes)[4] = (char (&)[4])i;

and you can access each byte directly with bytes[0], bytes[1], ... and do what ever you want to do with them.