Question

I have a strange problem in C++, I’m sure I’m missing something obvious but cannot figure out what.

The purpose of the function is to set some bits of a variable. From my tests, it all seems to work however it looks like the result of the operation is not assigned back to the variable.

uint16_t setBits(uint16_t target, unsigned int source, int offset, int bitCount) {
    unsigned int mask = ~0;
    mask = mask >> ((sizeof(mask) * 8) - bitCount);
    int rightShift = (sizeof(mask) * 8 - bitCount - offset);
    mask = mask << rightShift;
    source = source << rightShift;
    printHex(target);
    printHex((target & ~mask) | (source & mask));
    target = (target & ~mask) | (source & mask);
    printHex(target);
    return target;
}

uint16_t group;
group = 0xabcd;
unsigned int source = 0x12;
group = setBits(group, source, 4, 8);

This would print:

0x0000abcd
0x0120abcd
0x0000abcd

Which for me means that (target & ~mask) | (source & mask) gives the correct result (0x0120abcd), however this result is not assigned back to target (which is still set to 0x0000abcd). Any idea what I’m doing wrong?

Answer 1

Look at the type for target:

uint16_t target

It’s only 16 bits, but you try to mask in bits higher than that.