Question

I have been working on a legacy C++ application and am definitely outside of my comfort-zone (a good thing). I was wondering if anyone out there would be so kind as to give me a few pointers (pun intended).

I need to cast 2 bytes in an unsigned char array to an unsigned short. The bytes are consecutive.

For an example of what I am trying to do:

I receive a string from a socket and place it in an unsigned char array. I can ignore the first byte and then the next 2 bytes should be converted to an unsigned char. This will be on windows only so there are no Big/Little Endian issues (that I am aware of).

Here is what I have now (not working obviously):

//packetBuffer is an unsigned char array containing the string '123456789' for testing //I need to convert bytes 2 and 3 into the short, 2 being the most significant byte //so I would expect to get 515 (2*256 + 3) instead all the code I have tried gives me //either errors or 2 (only converting one byte unsigned short myShort; myShort = static_cast<unsigned_short>(packetBuffer[1]) 

Answer 1

Well, you are widening the char into a short value. What you want is to interpret two bytes as an short. static_cast cannot cast from unsigned char* to unsigned short*. You have to cast to void*, then to unsigned short*:

unsigned short *p = static_cast<unsigned short*>(static_cast<void*>(&packetBuffer[1])); 

Now, you can dereference p and get the short value. But the problem with this approach is that you cast from unsigned char*, to void* and then to some different type. The Standard doesn’t guarantee the address remains the same (and in addition, dereferencing that pointer would be undefined behavior). A better approach is to use bit-shifting, which will always work:

unsigned short p = (packetBuffer[1] << 8) | packetBuffer[2];