Prefix vs Postfix increment in C++

It doesn’t matter much, but you should use ++i instead of i++ in C++.

First let’s look at how ++i and i++ differ:

// How pre-increment works. Increment, _then_ return i.
#include <iostream>

int main() {
  int i = 0;
  std::cout << ++i;  // prints 1.
}

// How post-increment works. Return i, _then_ increment.
#include <iostream>

int main() {
  int i = 0;
  std::cout << i++;  // prints 0.
}

Let’s try using it in a for loop:

#include <iostream>

int main() {
  auto it = GetMyIterator();
  // Pre-increment increments the iterator and returns a _reference_ to it.
  for (; !it->end(); ++it) {
    // ...
  }
}

But what happens when we use post-increment in a for loop?

#include <iostream>

int main() {
  auto it = GetMyIterator();
  // it++ is supposed to return `it`, but it no longer exists because you incremented it.
  // So that means it's actually returning a copy. You'll find that when you
  // try to implement the post-increment iterator, you have to do a copy!
  for (; !it->end(); it++) {
    // ...
  }
}

As illustrated above: if i is not an int but an iterator, post-increment ends up returning an unneeded copy of the iterator’s first value. Compilers are generally smart enough to optimize this away, but in the rare case that they don’t / can’t, it’s wise to use pre-increment.