bitiwise exclusive or ^ with stdbool.h bool types in C and assignment

问题

I have a toy program in which I use the <stdbool.h> library to use the type bool. I have a few arrays of bool's, and the following commands work in a loop.

I have read online that I should not use bit-wise logic with bools. They appear to work here. Is this just luck of the draw with my compiler, or did I just misunderstand warnings about bitwise operators?

#include <stdio.h> 
#include <stdbool.h>

#define N 5
int main(void) {
  bool a[N] = { true, true, false, false, false };
  bool b[N] = { true, false, false, true, false };

  /* to count how many pairs are opposite */
  int count = 0;
  for (int i = 0; i < N; ++i){ count += (a[i] ^ b[i]); } 
  printf(" # pairs opposite: %i\n", count);

  /* flip the values of a */
  for (int i = 0; i < N; ++i){ 
        a[i] = (a[i] ^ true);
        printf(" %i", a[i]);
  } 
  printf("\n");

  /* flip only the value of a that are true in b */
  for (int i = 0; i < N; ++i){ 
        a[i] = (a[i] ^ b[i]);
        printf(" %i", a[i]);
  } 
  printf("\n");
}

回答1:

In C the values for true and false are 1 and 0 respectively. So that will work fine.

However you have to remember that everything non-zero is "true". So unless you have true or 1, any other "true" value is not going to do what you expect using bitwise operations.

Example: Both 2 and 1 are "true", but 2 & 1 will not be "true".

回答2:

I have read online that I should not use bit-wise logic with bools. They appear to work here. Is this just luck of the draw with my compiler, or did I just misunderstand warnings about bitwise operators?

stdbool's bool is an alias for the standard _Bool type, which the standard declares to be among the standard unsigned integer types. As a result, _Bool values are valid operands for the bitwise operators, and bitwise operations involving them have well-defined results. Moreover, when any scalar value is converted to type _Bool, the result is either 0 or 1, depending on whether the original value compares equal to 0.

Thus, if you thought the warnings were telling you that a compiler might reject your particular example code, or that the resulting program might behave differently than if you used, say, unsigned int in place of _Bool, then yes, you did misunderstand the warnings.

As others have already remarked, this is more about the boolean interpretation of non-_Bools than about bitwise operations on _Bools. Specifically, the boolean interpretation of the result of a bitwise operation in which at least one of the operands is a non-_Bool is not necessarily the same as the result of the corresponding logical operation.

来源：https://stackoverflow.com/questions/48588556/bitiwise-exclusive-or-with-stdbool-h-bool-types-in-c-and-assignment

标签

boolean

bitwise-operators