rtpax rtpax - 1 year ago 66
C++ Question

Changing one member of char array changes another

I wrote some code as an exercise that essentially performs mathematical operations on any size piece of data using only (except for looping) bit-wise operations. It's mostly functional but there is one problem which I cannot understand and I can't recreate it out of context and there is a bit of code that goes along with it, but here's the snippet that's giving me trouble

const __rint<size> _leftshiftto(__rint<size>arg)
if (arg.val[0] & 0x80)//negative arg
return _rightshiftto(arg._negto());
uint32_t byte = (arg._int32() / 8);
uint32_t carbyte = size - byte;
byte = byte;
uint8_t bit = (arg._int8() % 8);
uint8_t carbit = 8 - bit;

for (uint32_t n = 0; n<carbyte; ++n)
printf("%02hhx <- %02hhx\n",val[n],val[n+byte]);
val[n] = val[n + byte];
for (uint32_t n = carbyte; n < size; ++n)
val[n] = 0x00;

for (uint32_t n = 0; n<size-1; ++n)
val[n] <<= bit;
val[n] &= val[n + 1] >> carbit;
val[size - 1] <<= bit;

return *this;

The full code is at https://github.com/rtpax/rmath/blob/master/rint.h

What's happening is that it works for small enough values, but evaluates to zero for input 8 or above (so if it shifts a whole byte).

__rint<4> a(5);
//evaluates to 0x00000005
//evaluates to 0x00000050
__rint<4> b(5);
//evaluates to 0x00000005
//evaluates to 0x00000000

What's stranger is that if you change the line

val[n] = 0x00;


val[n] = 0xff;//or anything 0x80 and above

It does work except that the rightmost bytes will be filled with whatever number you put there (so
from above would evaluate to
I'm at a loss on this one, if anyone could offer an explanation I appreciate it.

some other notes:

I'm compiling using g++ on Windows 10

Something that seems related is that when I print out my hexes they display as 4 digits rather than two if they are at least 0x80 (which would print as 0xff80)

Bitshifts from 1 to 7 also don't work for larger numbers, but I assume that those failures are for the same reason as the whole byte shift failures.

Answer Source

& is the logical and operator: it requires a 1 in both sources to yield a 1 in the destination. 0101 & 0011 yields 0001.

val[n] &= val[n + 1] >> carbit;

This will set val[n] to zero, since we know the low bits of val[n] are zero, and we shift the val[n + 1] bits so the corresponding high bits will be zero:

11111000 &
00000111 =

You want the | operator:

11011000 |
00000010 =

Also, note that your code modifies the this object, but you return by value, making a copy.

Your return type probably should be

const __rint<size>&

Return by const value makes very little sense here.

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