Question on using volatile in a return type

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Hello all,

actually I am asking as well about semantics and best practices...

To start with, I am waiting on a condition variable (in a loop that is).
The actual condition is a bool flag located elsewhere, which is
to be passed in as a function parameter. So the correct type would be
"volatile bool&"

void func1 (volatile bool& flag) {
    // init...

    while (!flag && !err)
        err = pthread_cond_wait (&cond, &mutex);

    // ...
}

The "volatile" should give the compiler a hint not to employ optimisations
but fetch the value from the original location, where it may have been changed
by another thread meanwhile -- is this correct?

And: does the flag referred to have to be declared as volatile at the original
location? (usually somewhere in a class?). Or is it sufficient to define the
reference as volatile bool& ?

Now, assumed I want to use a functor instead of the bool flag.
What would be the correct and the "best" way to define it?

class Check1 {
    bool operator() () { ... }
}

class Check2 {
    volatile bool operator() () { ... }
}

class Check3 {
    volatile bool& operator() () { ... }
}

My understanding is that for Check3 the "volatile" is necessary, is this
correct? But Check1 should be ok, because it's returning a value and it will
actually be re-invoked in each loop iteration?

And besides, would you consider the definition within Check2 good practice,
bad practice, expressive, superfluous, ....?

Thanks
Hermann Vosseler

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