Question

I’m well aware that in C++

int someValue = i++;
array[i++] = otherValue;

has different effect compared to

int someValue = ++i;
array[++i] = otherValue;

but every once in a while I see statements with prefix increment in for-loops or just by their own:

for( int i = 0; i < count; ++i ) {
     //do stuff
}

or

for( int i = 0; i < count; ) {
    //do some stuff;
    if( condition ) {
        ++i;
    } else {
        i += 4;
    }
}

In the latter two cases the ++i looks like an attempt to produce smarty-looking code. Am I overseeing something? Is there a reason to use ++i instead of i++ in the latter two cases?

Answer 1

If we ignore force of habit, ‘++i’ is a simpler operation conceptually: It simply adds one to the value of i, and then uses it.

i++ on the other hand, is “take the original value of i, store it as a temporary, add one to i, and then return the temporary”. It requires us to keep the old value around even after i has been updated.

And as Konrad Rudolph showed, there can be performance costs to using i++ with user-defined types.

So the question is, why not always just default to ++i?

If you have no reason to use `i++´, why do it? Why would you default to the operation which is more complicated to reason about, and may be slower to execute?