C++ Tip - Packing Data

Here's a quick tip that is useful if you are sending data over a network. If you ever messed with any sockets in programming then you know you can only send bytes of data. Why is this problem? Well, you can't exactly send over a 4 byte integer in a single 1 byte char. To get around this, we do something called packing.

char Buffer[4];
int Data = 1024;

#if SDL_BYTEORDER == SDL_BIG_ENDIAN
Buffer[0] = (Data >> 24) & 0xFF;
Buffer[1] = (Data >> 16) & 0xFF;
Buffer[2] = (Data >>  8) & 0xFF;
Buffer[3] = (Data)       & 0xFF;
#elseif
Buffer[3] = (Data >> 24) & 0xFF;
Buffer[2] = (Data >> 16) & 0xFF;
Buffer[1] = (Data >>  8) & 0xFF;
Buffer[0] = (Data)       & 0xFF;
#endif

Now, we can easily send over the Buffer char array. Two things to note here. First, depending on the system you are working on, the endianness can change (to read up on what endianness is, look here: http://en.wikipedia.org/wiki/Endianness) - that is what the #if SDL_BYTEORDER == SDL_BIG_ENDIAN statement is for. Secondly, we did nothing here to specify if the data being packed is signed or unsigned. You will need to know on the other end what type of data you are receiving.

Here's how you can unpack this data:

char Buffer[4];
int Data = 0;

#if SDL_BYTEORDER == SDL_BIG_ENDIAN
Data = ((((Uint8 *)Buffer)[0] << 24) | (((Uint8 *)Buffer)[1] << 16) | (((Uint8 *)Buffer)[2] <<  8) |  ((Uint8 *)Buffer)[3] <<  0);
#elseif
Data = ((((Uint8 *)Buffer)[3] << 24) | (((Uint8 *)Buffer)[2] << 16) | (((Uint8 *)Buffer)[1] <<  8) |  ((Uint8 *)Buffer)[0] <<  0);
#endif