What is the difference between bitwise and logical OR operations when it comes to boolean variables in C ? In the integer domain, it is clear but when it comes to boolean is there any performance advantage or behavioral change between both of them?

bool VAR = TRUE , BOOL VAR2= TRUE

Is there a difference in VAR | VAR2 & VAR || VAR2

CodePudding user response：

For performance differences, logical OR short-circuits (e.g., in op1 || op2, if op1 evaluates to true, op2 is not evaluated), but bit-wise doesn't. This also creates a behavior difference:

if (op1 == nullptr || fn(op1)) { /* ... */ }

This code will never call fn with nullptr because of the short-circuit behavior. On the other hand, if the logical OR was replaced with bit-wise, fn would always be called. If fn has side effects or undesirable behavior when called with nullptr, this creates a behavior difference.

CodePudding user response：

Bitwise and logical operators are quite different:

• Logical operators work on the logical values of the variables and give back true or false which are logical values
• Bitwise operators work on the single bits of the variables

Briefly the bitwise operator consider the single bits of 2 varibales: & operation between 2 bitmasks gives back a bitmask where the compared bit is 1 if both the relative bits in the original mask are 1; | operation between 2 bitmasks gives back a bitmask where the compared bit is 1 if at least one of the relative bits in the original mask is 1.

The result of a bitwise peration depends on the size of a variable and its representation in bits. I made a little exercise trying to make bitwise operation on bool variables and you can run it on your machine. I think the result depends on the compiler and the architecture; anyway in my test the size of bool is 1 byte (8 bits) and in the case it's true just the first bit is 1:

#include <iostream>

int main()
{
    bool b1 = true;
    bool b2 = true;

    std::cout << "size of bool:" << sizeof(bool) << std::endl;

    for (int i = 0; i < 8 * sizeof(bool); i  )
    {
        std::cout << "bitwise operation | on bit:" << i << " result:" << (b1 | (1 << i)) << std::endl;
        std::cout << "bitwise operation & on bit:" << i << " result:" << (b1 & (1 << i)) << std::endl;
    }
}