boolean - C++, set bool to false, return old

Question

This code:

bool b = isTrue;
isTrue = false;

saves old isTrue value to b, then sets isTrue to false. All good, but is there a way to perform this in one line? In the end I would like to achieve this:

bool b = isTrue ? /* assign true and flip `isTrue` value to false */ : false;

question from:https://stackoverflow.com/questions/65946859/c-set-bool-to-false-return-old

Answer 1

std::exchange can be used to do exactly this:

bool b = std::exchange(isTrue, !isTrue);

It's equivalent to:

bool b = (isTrue := !isTrue)

Where := is a magical assignment operator that returns the old value rather than the freshly-assigned value.

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If you instead would like b to have the old value of isTrue and set isTrue to false, as the discussion in the comments suggests, you can do this with:

bool b = std::exchange(isTrue, false);