nullptr read access violation while checking in loop

Question

I have this code:

while (current->next->data <= temp->data && current->next != nullptr)
        {
            current = current->next;
        }

when I run it i get the error:

Exception thrown: read access violation. current->next was nullptr.

I believe I should not be receiving this error because it is the exact thing I check for; I am new to this, is there something I am doing wrong?

What is the first expression that gets executed in the condition? Hint: It's not `current->next != nullptr` :) — Rakete1111, Jan 30 '18 at 09:21
So when you used your debugger, what did it tell you was null? — UKMonkey, Jan 30 '18 at 09:23

score 1 · Answer 1 · answered Jan 30 '18 at 09:21

while (current->next->data <= temp->data && current->next != nullptr)

This will fail for the very first iteration if current->next is null. You should swap the order of condition check.

while (current->next != nullptr && current->next->data <= temp->data)

score 0 · Answer 2 · edited Jun 20 '20 at 09:12

By the http://en.cppreference.com/w/cpp/language/operator_logical

lhs && rhs

For the built-in logical AND operator, the result is true if both operands are true. Otherwise, the result is false. This operator is short-circuiting: if the first operand is false, the second operand is not evaluated

The lhs expression will be executed first, so you should put your current->next null pointer check at the left side of && in condition.

score 0 · Answer 3 · answered Jan 30 '18 at 09:27

Standard says [expr.log.and]

The && operator groups left-to-right. The operands are both contextually converted to bool (Clause 7). 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.

So, in

current->next->data <= temp->data && current->next != nullptr

First evaluated first operand which is current->next->data <= temp->data. And here you have the error if current->next is nullptr.

nullptr read access violation while checking in loop

3 Answers3