the “|=” operator in c++

Question

I have a question about "|=" in c++, how this operator works, for example:

bool result;

result |= callFunctionOne(sig);
result |= callFunctionTwo(sig);
result |= callFunctionThree(sig);
result |= callFunctionFour(sig);

and the function called above, will reutrn "true" if the paramater sig is processed in the function, otherwish, return "false";

the sig can be processed only in one function each time, how the "|=" works?

Answer 1

| is bitwise OR.

|= says take what is returned in one of your function and bitwise OR it with the result, then store it into result. It is the equivalent of doing something like:

result = result | callFunctionOne(sig);

Taking your code example:

bool result;

result |= callFunctionOne(sig);
result |= callFunctionTwo(sig);
result |= callFunctionThree(sig);
result |= callFunctionFour(sig);

and your logic of

will reutrn "true" if the paramater sig is processed in the function, otherwish, return "false";

So that means that if you don't define result, it will be by default FALSE.

result = false;

callFunctionOne returns TRUE

result = result | callFunctionOne;

result equals TRUE.

result = false;

callFunctionOne returns FALSE

result = result | callFunctionOne

result equals FALSE.

While it may seem that this is a boolean OR, it still is using the bitwise OR which is essentially OR'ing the number 1 or 0.

So given that 1 is equal to TRUE and 0 is equal to FALSE, remember your truth tables:

p   q   p ∨ q
T   T   T
T   F   T
F   T   T
F   F   F

Now, since you call each function after another, that means the result of a previous function will ultimately determine the final result from callFunctionFour. In that, three-quarters of the time, it will be TRUE and one-quarter of the time, it will be FALSE.

Answer 2

a |= b is equivalent to a = a | b.

Answer 3

To put it simple, it will assign true to result if any of those functions return true, and false otherwise. But there is one problem - result must be initialized, otherwise this bit operation will be performed on random initial value and could return true even if all of the functions returns false.

The operator itself is called "bitwise inclusive or".

Answer 4

The | operator is bitwise OR (if you know the boolean logic, on a bool variable it acts as a boolean OR). If one or many calls return true, result will be true. If all calls are returning false, the result is false.

Each time, it's the equivalent of result = result | callFunctionXXX(sig);

Remark: in your sample code, the variable result is not initialized. It should be "bool result = false;"