Is there ordering with c++ && (AND) operator

Question

a - checks whether input is defined for function b

if (a) {
    if (b) {
        /* ... */
    }
}

clearly works. Does the &&-operator always check the first input?

if (a && b) {
    /* ... */
}

Would this possibly cause undefined behaviour from b function?

Read the [docs](https://en.cppreference.com/w/cpp/language/operator_logical) — EdChum, Dec 20 '18 at 11:48
For future reference the term is called "short-circuit evaluation". — StoryTeller - Unslander Monica, Dec 20 '18 at 11:48

score 2 · Answer 1 · answered Dec 20 '18 at 11:48

2

Yes, the built-in && short-circuits.

So, if a is false, b will not be evaluated. This is guaranteed, unless there is an overload of operator&&(type of a, type of b).

answered Dec 20 '18 at 11:48

Lightness Races in Orbit

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Very very good point on the overload! – Bathsheba Dec 20 '18 at 11:50

Bathsheba · Accepted Answer · 2018-12-20T11:50:35.530

0

In the expression a && b, assuming && is the built-in operator and not an overloaded one, b is only evaluated if a evaluates to true.

That's why you can write things like if (a != nullptr && *a).

We call this short circuiting. || also has the property.

edited Dec 20 '18 at 11:50

answered Dec 20 '18 at 11:47

Bathsheba

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does same hold for || operator? Also, source? – BorticFan Dec 20 '18 at 11:49
@BorticFan: Ha, just put that in for personal pleasure. Source is the C++ standard innit. – Bathsheba Dec 20 '18 at 11:49
@BorticFan Your C++ book explains it, and the dupe contains a verbatim quote from the standard – Lightness Races in Orbit Dec 20 '18 at 11:50

score 0 · Answer 3 · answered Dec 20 '18 at 11:51

0

As Bathsheba said, if your code is

if (a && b) {
    //...
}

Then b will only be checked if a results to true. This is because the && operator is a so-called shortcut operator. The same goes for the logical OR (||).

if (a || b)//...

In this case, however, b would only be evaluated if a results to false.

These are not to be confused with the bitwise operators (& and |), which check both values regardless.

answered Dec 20 '18 at 11:51

SimonC

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Not sure I've ever heard it called a "shortcut operator". If anything, some people call `x += 42` a "shortcut operator" (versus `x = x + 42`) though even that doesn't seem to be widespread. – Lightness Races in Orbit Dec 20 '18 at 11:54
Sorry, my bad. Shortcircuit operator. I typed the answer on my phone, I guess autocorrect got me there. – SimonC Dec 20 '18 at 11:59

Is there ordering with c++ && (AND) operator

3 Answers3