Maybe I do not understand C++ properly or is it a compiler's bug?
uint8_t a = 0x00;
uint8_t b = 0xFF;
if( a - b == 1 )
{
doNothing();
}
doNothing is not called (as expected), because result of (a-b) was implicitly casted to the type of second operand in compare operation. And for numbers it's signed int. Okay.
if( a - b == (uint8_t)1 )
{
doNothing();
}
doNothing STILL isn't called, but now I do not understand the reason for it! I have explicitly casted the number to uint8!
if( (uint8_t)(a - b) == 1 )
{
doNothing();
}
Now doNothing is finally called, but again why? How can subtraction of two uint8 return an int?
Compiler is uVision ARMCC for ARM Cortex M3.
Answer
In a - b, the operands are promoted to int before the subtraction, so the result is -255, not 1.
That's why both the first and second examples fail; it's nothing to do with the other operand of ==. The third converts -255 back to uint8_t, reducing it modulo 256, so the result is 1 as expected.