Question

I want to convert a four character string (i.e. four characters) into a long (i.e. convert them to ASCII codes and then put them into the long).

As I understand it, this is done by writing the first character to the first byte of the long, the second to the adjacent memory location, and so on. But I don’t know how to do this in C++.

Can someone please point me in the right direction?

Thanks in advance.

Answer 1

Here’s your set of four characters:

const unsigned char buf[4] = { 'a', '0', '%', 'Q' };

Now we assemble a 32-bit unsigned integer:

const uint32_t n = (buf[0]) | (buf[1] << 8) | (buf[2] << 16) | (buf[3] << 24);

Here I assume that buf[0] is the least significant one; if you want to go the other way round, just swap the indices around.

Let’s confirm:

printf("n = 0x%08X\n", n); // we get n = 0x51253061
                           //               Q % 0 a

Important: Make sure your original byte buffer is unsigned, or otherwise add explicit casts like (unsigned int)(unsigned char)(buf[i]); otherwise the shift operations are not well defined.

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Word of warning: I would strongly prefer this algebraic solution over the possibly tempting const uint32_t n = *(uint32_t*)(buf), which is machine-endianness dependent and will make your compiler angry if you’re using strict aliasing assumptions!

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As was helpfully pointed out below, you can try and be even more portable by not making assumptions on the bit size of a byte:

const unsigned very long int n = buf[0] |
              (buf[1] << (CHAR_BIT)     |
              (buf[2] << (CHAR_BIT * 2) |
              (buf[3] << (CHAR_BIT * 3)   ;

Feel free to write your own generalizations as needed! (Good luck figuring out the appropriate printf format string 😉 .)