C++: Empty if statement

Question

This is more of an entry level question, but I'm wondering if it's good practice to have an empty if statement.

Consider this code:

void RabbitList::purge()
{
    if(head == NULL)
    {
        //cout << "Can't purge an empty colony!" << endl;
    }
    else
    {
        //Kill half the colony
        for(int amountToKill = (getColonySize()) / 2; amountToKill != 0;)
        {
            RabbitNode * curr = head;
            RabbitNode * trail = NULL;

            bool fiftyFiftyChance = randomGeneration(2);

            //If the random check succeeded but we're still on the head node
            if(fiftyFiftyChance == 1 && curr == head)
            {
                head = curr->next;
                delete curr;
                --size;
                --amountToKill;
            }
            //If the random check succeeded and we're beyond the head, but not on last node
            else if(fiftyFiftyChance == 1 && curr->next != NULL)
            {
                trail->next = curr->next;
                delete curr;
                --size;
                --amountToKill;
            }
            //If the random check succeeded, but we're on the last node
            else if(fiftyFiftyChance == 1)
            {
                trail->next = NULL;
                delete curr;
                --size;
                --amountToKill;
            }
            //If the random check failed
            else
            {
                trail = curr;
                curr = curr->next;
            }
        }
        cout << "Food shortage! Colony has been purged by half." << endl;
    }
}

As you can see, the if statement on line 5 is currently commented out; this was more of a debug text and I don't want to send any feedback to the console anymore. I'm pretty sure it would be considered bad practice to have the if statement do nothing. I know I could have return;

but since my return type is void it gives me an error. What if my return type is not void for example?

Answer 1

Even if your return type is void it is legal to return there, and certainly since the if has braces, atleast this isnt a bug waiting to happen. However it is not pretty and involves a little more work to read/understand.

You could reword it to

if(head == NULL) // or if(!head)
    return;

....

This should remove the need for the else, and the rest of the code is now inside the function and not a nested scope, a happy perk.

Answer 2

For a single branch, just write it directly:

if (head != 0) {
    // whatever
}

For multiple branches, sometimes having an empty first branch can simplify the conditions that follow:

if (head == 0) {
    // nothing to do
} else if (head->next == 0) {
    // whatever
} else {
    // whatever else
}

Yes, you could write that last one with an additional layer:

if (head != 0) {
    if (head->next == 0) {
        // whatever
    } else {
        // whatever else
    }
}

but the first form is clearer, especially when the second form would end up with three or four levels of if.

Oh, and

if (head == 0)
    return;

can sometimes be difficult, since it introduces an extra point of exit to the function. In the past I was a fan of this form, but in the past few years I've found that I end up removing it pretty consistently.