C++，设置布尔为假，返回旧

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;

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.

---

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);

Answer 2

it'll be

#include <iostream>

int main()
{

    bool t = !false;
    bool b = false;
//  b = t, t = !t; 
    t = !(b = t); 
    std::cout << std::boolalpha << "b : " << b << " , " << "t : " << t << " . " << "give me a bottle of rum!" << std::endl;
    return 0;
}

line that's comented works too