Question

Possible Duplicates:
Why use ++i instead of i++ in cases where the value is not used anywhere else in the statement?
Incrementing in C++ – When to use x++ or ++x?

i++ vs. ++i  

When is this used in real scenarios?

Answer 1

The obvious is when you want the old value returned, you use post-increment.

The more subtle things are that pre-increment should really never be slower and could be faster due to the lack of creating a temporary and returning the old value when using post-increment.

A real scenario for using post-increment in C++ is when erasing from standard containers. For example:

set<int> ctr;
ctr.insert(1);
ctr.insert(10);
ctr.insert(12);
ctr.insert(15);

set<int>::iterator it = set.begin();

// Post-increment so the value returned to erase is that of the previous
// iteration (i.e. begin()), yet the iterator stays valid due to the local
// iterator being incremented prior to the erase call
ctr.erase(it++);

// Still valid iterator can be used.
cout << "Value: " << *it << "\n";  

In response to the compiler optimization, it is true yet I think it’s always important to convey as precisely as possible what you’re trying to accomplish. If you don’t need the returned value from x++, then don’t ask for it. Also, I’m not sure you would always get the same optimization if the type your incrementing is not a simple type. Think iterators that are not just plain pointers. In cases such as this, your mileage may vary with regard to optimization. In short, always pre-increment unless you need the returned (i.e. old value) of the post-increment operator.