How does the unary minus operator work on booleans in C++?

Question

I am currently converting some OpenCV code from C++ to Java. I can\'t use JavaCV, as we need the conversion in native Java, not a JNA. At one point in the code, I get the fo

Answer 1

What it basically does is following:

kHit >= kForeground

is an expression of type bool

-(kHit >= kForeground)

converts this bool into an int (based on true==1 and false==0) and negates it, which results in true==-1 and false==0.

This is then converted into a uchar, which results in -1==255 and 0==0.

It has to be noted that though seeming as using the underlying implementation details of the numbers, all those conversions are guaranteed by the C++ and C standards, as negative unsigned numbers are specified to behave according to twos-complement.

But if Java doesn't support this, you can always replace it by a conditional assignment:

dst[x] = (kHit>=kForeground) ? 255 : 0;