Guarantee function call in logical AND expression

Question

I am in the process of refactoring some code using C++ atomics. The code looks like this:

std::atomic<bool> someFlag{}; // This can be set to true using a public method

// ...

const bool cond1 { someFunction() };
const bool cond2 { otherFunction() };

if (someFlag.load())
{
    someFlage.store(false);

    if (cond1 && cond2)
    {
        performSomeAction();
    }
}

I'm currently planning to rewrite the if statement like this:

if (std::atomic_exchange(&someFlag, false) &&
    cond1 && cond2)
{
    performSomeAction();
}

What is extremely important, is that after this if statement, the someFlag variable is set to false. I therefore want to make sure that the call to atomic_exchange always occurs, regardless of the value of cond1 and cond2. Can I be guaranteed that this will be the case since boolean expressions are evaluated left to right, regardless of optimisation settings?

score 16 · Accepted Answer · edited Jan 14 '20 at 11:50

16

Yes, the order is guaranteed. From cppreference.com:

Every value computation and side effect of the first (left) argument of the built-in logical AND operator && and the built-in logical OR operator || is sequenced before every value computation and side effect of the second (right) argument.

edited Jan 14 '20 at 11:50

Toby Speight

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answered Jan 14 '20 at 10:26

Renat

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score 7 · Answer 2 · answered Jan 14 '20 at 10:26

In if (std::atomic_exchange(&someFlag, false) && cond1 && cond2)

std::atomic_exchange(&someFlag, false) will be called first.
If evaluate to true, evaluates cond1
If cond1 is true, evaluates cond2.
and finally performSomeAction() if cond2 is also true.

Guarantee function call in logical AND expression

2 Answers2