• ++i will increment the value of i, and then return the incremented value.

i = 1;
j = ++i;  //i is 2, j is 2

T& T::operator++() // pre-increment, return *this by reference
{
  return *this;
}

• i++ will increment the value of i, but return the original value that i held before being incremented.

i = 1
j = i++;  //i is 2, j is 1

T T::operator++(int) // post-increment
{
  T copy(*this);
  ++(*this);// pre-increment here seems logical
  return copy;
}

For a for loop, either works, ++i seems more common. There is a couple of comments regarding the efficiency of ++i and i++. In any non-student project compiler, there will be no performance difference.
It's different for a C++ object, since operator++() is a function and the compiler can't know to optimize away the creation of a temporary object to hold the intermediate value.