Hi . I want a program that receive a number and print its subcollections . like this :
n=3 :
cout : {} , {A} , {B} , {C} , {A,B} , {A,C} , {B,C} , {A,B,C}
(with one of the A,B,C , ... characters .)
Please help me . :(

Read up on loops as they are likely to figure prominently in your program. Provide a better description of your program, it will help you help yourself as well as helping others help you. Show some effort besides just posting your version of the problem description. In particular provide code that you're having trouble with, error messages and pertinent code, etc. Here's the most common place to start a project that most of use can help with.

#include <iostream>
using namespace std;
int main()
{
  return 0;
}

Before you even compile that however, you need a better description of the problem.