Question

I accidentally typed =! instead of != that caused a huge bug in a system that went undetected for a while; I have fixed it since but I’m curious as to what =! does.

I had something like this

void foo(int param)
{
    int a = 0;

    ... 

    if (a =! param)
    {
        // never got here even when `a` was not equal to `param`  
    }
    ...
}

Can someone explain what the above if statement is evaluating ?

Answer 1

This expression:

a =! param

assigns the value !param to a. !param is negated version of param evaluated in boolean context.

Assignment operators return the value of the right hand side, so, if (a = !param) also executes the if body, if !param is true.