if conditions execute order in C++

Question

I have a if block as below in C++:

if( node != NULL && node->next !=NULL ){
    //do some stuff
}

Can anybody tell me do I need to split node and node->next in two if block? or is it guaranteed that node!=NULL will executed before node->next!=NULL ?

You don't need to split it. short-circuit eval will prevail and avoid the secondary expression if the first is already deemed false. — WhozCraig, Sep 04 '14 at 18:50
As long as `operator&&` has not been overloaded for your type (which in this case is the result of `operator!=` which is a `bool` so you can trust that `operator&&` hasn't been overloaded) it will short circuit and work as expected. If someone has overloaded `operator&&` it loses the ability to short circuit. — YoungJohn, Sep 04 '14 at 19:05

score 15 · Accepted Answer · edited Jun 20 '20 at 09:12

This is a short circuit evaluation and the operator && guarantees that the left-hand side expression will be fully evaluated before the right-hand side is evaluated

From the standards:

5.14 Logical AND operator [expr.log.and]
logical-and-expression: 
      inclusive-or-expression
      logical-and-expression && inclusive-or-expression 
The && operator groups left-to-right. The operands are both contextually converted to bool (Clause 4). The result is true if both operands are true and false otherwise. Unlike &, && guarantees left-to-right evaluation: the second operand is not evaluated if the first operand is false.

The result is a bool. If the second expression is evaluated, every value computation and side effect associated with the first expression is sequenced before every value computation and side effect associated with the second expression.

score 2 · Answer 2 · answered Sep 04 '14 at 18:52

No, you do not. The && operator short-circuits; if the left operand evaluates to false then the right operand is not evaluated at all, because the result is already known. (Similarly, the || operator will not evaluate the right operand when the left operand is true.)

score 0 · Answer 3 · answered Sep 04 '14 at 18:51

0

You do not need to. node!=NULL will execute first and if it is false it will not evaluate the rest of the conditions.

answered Sep 04 '14 at 18:51

RockOnRockOut

751
7
18

score 0 · Answer 4 · answered Sep 04 '14 at 18:54

If the first condition is false then the second condition will not be evaluated because in any case the full expression will be equal to false independing on what is the result of evaluatuin of the second condition.

According to the C++ Standard

1 The && operator groups left-to-right. The operands are both contextually converted to bool (Clause 4). The result is true if both operands are true and false otherwise. Unlike &, && guarantees left-to-right evaluation: the second operand is not evaluated if the first operand is false.

if conditions execute order in C++

4 Answers4

5.14 Logical AND operator [expr.log.and]