Short-Circuit Evaluation of C++ Logical AND OR

C++’s logical AND operator && and logical OR operator || are short-circuit evaluations.

• &&: If the first operand is not satisfies the condition (== false), the second one will not evaluated.
• ||: If the first operand is satisfies the condition (== true), the second one will not evaluated.

const auto x = 1;

if ((x > 1) && (x < 10)) { // (x > 1) is false, then (x < 10) is ignored.
    ...
}

if ((x > 0) || (x < 10)) { // (x > 0) is true, then (x < 10) is ignored.
    ...
}

Using this system, we can write a code like below:

assert ((!(myclass = getInstance(key))) || !myclass->isValid());

// Equals to below:
myclass = getInstance(key);
if (myclass != NULL) assert(!myclass->isValid())

Personally, I feel the second writing style is easier to read, though.

References:

• https://en.wikipedia.org/wiki/Short-circuit_evaluation
• https://en.cppreference.com/w/cpp/language/operator_logical