How to initialize a subscriber only when a 'if' condition meets?

edit

Hello. I want to initialize a subscriber only when

if(i>0){              // 'i' is received as node argument
  ros::Subscriber sub = nh.subscribe("/image_rect_color/compressed", 1, imageCallback);
  }

Problem I am facing is: main thread comes into if-scope but couldn't initialize the subscriber. I guess problem is introduction of new scope.

BTW:This code is in main().

How can this problem be solved?

Thanks