objective c - NSLogging UILabel's text outputs null -

i have custom view controller called timerviewcontroller , subclasses of called firstviewcontroller , fourthviewcontroller.

i declared instance of firstviewcontroller in firstviewcontroller's .h named controller.

in viewdidload method of fourthviewcontroller's .m, have:

controller = [self.storyboard instantiateviewcontrollerwithidentifier:@"maincontroller"]; 

in storyboard, i've declared view controller id maincontroller , declared custom class of fourthviewcontroller. then, in fourthviewcontroller's .m have:

controller.mainlab.text = [nsmutablestring stringwithformat:@"this string"]; nslog(@"%@", controller.mainlab.text); 

this, however, outputs (null).

why happen?

mainlab must nil. outlet isn't connected xib.

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as aside, using stringwithformat: string isn't format wasteful.