What's this =! operator? [duplicate]

Answer 1

C++ does not have an =! operator, which immediately means that this is an = operator followed by a ! operator. So, you simply have a = !b in the if condition.

Answer 2

Operators in C++

According to C/C++ operators list there is no operator such as =!. However, there is an operator != (Not equal to, Comparison operators/relational operator)

There are two possibilities.

1. It could be typo mistake as I've noticed that =! operators is in if statement and someone is trying to type != instead of =! because != is the comparison operator which returns true or false.
2. Possibly, the developer was trying to assign the boolean negation of b to a and he/she has done a typo mistake and forgot to put a space after equal sign. This is how the compiler interprets it, anyways. According to Operator precedence in c++:
   • Operator Logical NOT (!) precedence is 3 and Associativity is Right-to-left
   • Operator Direct assignment (=) precedence is 16 and Associativity is Right-to-left

Answer 3

That is not a single operator, it is however, a great way to obfuscate code.

If it were written a=!b instead, the white space might not lead you to believe that it was a single operator.

Compilers have warnings for assignment in a conditional expression unless you wrap the entire statement in a set of parenthesis, and this is a perfect example of when this warning would be useful.

Both of these statements are functionally identical, but one generates a warning and the other does not:

if (a =! b)   // Generates a warning with `-Wparentheses` (gcc)

if ((a =! b)) // No such warning

-Wparentheses

Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.

This, of course, assumes that you are a responsible programmer and actually read the warnings your compiler spits out.

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Using white space in a more reasonable way, the statement is actually:

if (a = !b) // Assign A the value of (logical) NOT B and then test the truth

The compiler warning mentioned above is actually useful in the case where the person who wrote this code accidentally transposed ! and =. However, from the cryptic inline comments in your original question, a = !b is probably the author's intention.

Answer 4

This can be taken as a is assigned to !b.

Answer 5

I am trying to answer your question and, in agreement with Mike's answer, I am just adding up.

You can also understand it by a=!!b.
The code a=!b returns 0 to a and a=!!b returns 1 to a.

The =! is not a single operator but the mix of these two. These types of question are asked to misguide students to check their coding skills.

Answer 6

a is assigned the boolean negation of b in that line. It is just a misformatted

if( a = !b ) {

... and an evil hidden assignment inside a condition.