Class method (NSString type) called from viewcontroller returns nil?

I'm a c++ programmer new to objective-c.

I created a calculator app that is working fine using a single view. I开发者_Python百科 have a Calculations class and a ViewController. Every time a button is pressed, an IBAction method in the ViewController calls methods defined in the Calculations class to handle the input and returns the output as an NSString which I then set as the value of the label.text field.

Now I am working on a tab bar app using the same Calculations class. This app has two tabs, each with a unique set of input buttons for the calculator (both views sharing the same input/output data). The first tab is identical to my first app with the single view, so I am trying to do this in a similar fashion.

Here is the problem:

When a button is pressed, the IBAction method that handles the input runs through the calls to the Calculations class methods (shown below) without error:

-(IBAction)readInput:(id)sender {    
    [_calculations input:[sender titleForState:UIControlStateNormal]];
    inputField.text = [_calculations inputDisplay];
    outputField.text = [_calculations outputDisplay];
}

however, both the inputDisplay and outputDisplay methods return nil. Using the debugger I noticed that I am unable to "step into" the calls to _calculations methods, instead the line is skipped and the value returned by both is nil. I added the following method:

-(IBAction)setNumber:(id)sender {
    NSString *button =(NSString *)[sender titleForState:UIControlStateNormal];
    inputField.text = button;
}

and if I attach this to the input buttons I can see the display updated. This seems to be an issue with calling the _calculations member functions and tab bar views (because this issue is not present using a single view).

I realize that I left out a lot of information, but I did it to avoid providing irrelevant information. I will provide all details that are necessary if asked.

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Check to make sure _calculations is not nil.

You can send any message (call any method) on nil and it will just return nil, not cause an exception.

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Without seeing more code it is going to be a bit difficult to diagnose.

If I was trying to debug this issue I would first make sure _calculations points to the object you want it to point to. If its loaded from a NIB then it might not be getting initialised, and still be nil. You can send messages to nil objects without any issues. If an object receives a message that it cant handle (the method doesn't exist, or the target object is nil) then the return for that call will be nil.

I have in the past put initilization code into the init: method, and spent a few hours why it wasn't being called, until it dawned on me that I needed to put my init code into the viewDidLoad:, or the initWithNibName:bundle: or even the initWithCoder: selector.

HTH, Matt