Function object

[Deep India]

when we have to use a function object in our code?

[vlad from moscow]

Usually with standard algorithms.

[Thumper]

Or whenever you need a function that behaves as an object as well.
EDIT: Or an object that behaves as a function.

[pata]

If your looking for examples on when you would use it, here are some:

http://www.cprogramming.com/tutorial/functors-function-objects-in-c++.html