Difference between (++i) and (i++)

Question

In C++ I understand that (++i) should return a reference to i because the need of concatenation of operators, but what I can't figure out is:

Why (i++) should return i by value?

Can anyone please clarify.

Answer 1

i++ returns a value because it is returns the old value of i , while i is increased by 1 .

A basic implementation of this would be:

int i++() {
  int old = i;
  i = i + 1;
  return old;
}

So, if it returned a reference, it would be the wrong value ... since i 's value has been incremented!

Answer 2

++i can be written as

prefix_inc (this) {
   increase this by 1
   return this
}

Since the real i is returned, we can take reference of it. However, i++ looks like

postfix_inc (this) {
   set old_this = copy of this
   increase this by 1
   return old_this
}

as old_this is just a local variable, the reference of it is pointless after i++ is completed. So logically it should return an rvalue.

Answer 3

Let foo be some function. foo(i++) calls foo(i) with the old value of i and increments i , hence the need to build a temporary copy. foo(++i) increments i and then calls foo with the incremented value, so for better performance we can reuse the same variable, no need to have a temporary copy.

Answer 4

i++ This returns the value of the i before it is incremented. So the idea is that if you want to use i in a function, and then increment the value after using it, you can do that in one step. As an example, here is how I would overload that operator for integers.

Integer Integer::operator++()
{
    Integer returnValue = *this;
    this->increment();
    return returnValue;
}

So it increments the value and then returns what it used to be. It also doesn't return a reference, because returning a reference would be different from what was originally passed, which would break cascading.

++i This increments the value of i, and then returns the new value. So you could use this in a situation where you want to increment i and then use the new value in your function.

Integer Integer::operator++(Integer i)
{
    i.increment();
    return i;
}

So the value it returns is the incremented value of i.

Answer 5

int i = 0;
Console.Writeline(i++); // Output 0, after that, i will be 1


int x = 0;
Console.Writeline(++x); // Output 1

Note: code is in C#

Answer 6

While prefix ++i returns the incremented value and the suffix i++ returns the old value and increments it afterwards the operator selection is significant if you care for the CPU cycles. Prefix increment is faster ;-)

Answer 7

5 cents:

As a consequence of i++ making a copy, it is slower for non-POD variables (ie iterators). You should use ++i anywhere when possible.

I personaly always use for(...;...;++i) instead of for(...;...;i++) , although compiller should optimize that.

Answer 8

如果你曾经处于重要的场景中，那你就错了。