Is there a performance difference between i++ and ++i in C?

Question

Is there a performance difference between i++ and ++i if the resulting value is not used?

Answer 1

A better answer is that ++i will sometimes be faster but never slower.

Everyone seems to be assuming that i is a regular built-in type such as int. In this case there will be no measurable difference.

However if i is complex type then you may well find a measurable difference. For i++ you must make a copy of your class before incrementing it. Depending on what's involved in a copy it could indeed be slower since with ++it you can just return the final value.

Foo Foo::operator++()
{
  Foo oldFoo = *this; // copy existing value - could be slow
  // yadda yadda, do increment
  return oldFoo;
}

Another difference is that with ++i you have the option of returning a reference instead of a value. Again, depending on what's involved in making a copy of your object this could be slower.

A real-world example of where this can occur would be the use of iterators. Copying an iterator is unlikely to be a bottle-neck in your application, but it's still good practice to get into the habit of using ++i instead of i++ where the outcome is not affected.