`nil` should only be used in place of an `id`, what we Java and C++ programmers would think of as a pointer to an object. Use `NULL` for non-object pointers.

Look at the declaration of that method:

    - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
        change:(NSDictionary *)change context:(void *)context

Context is a `void *` (ie a C-style pointer), so you&#39;d definitely use `NULL` (which is sometimes declared as `(void *)0`) rather than `nil` (which is of type `id`).

They&#39;re technically the same thing (0), but nil is usually used for an Objective-C object type, while NULL is used for c-style pointers (void *).

They&#39;re technically the same thing and differ only in style:

 - Objective-C style says `nil` is what to use for the `id` type (and pointers to objects).
 - C style says that `NULL` is what you use for `void *`.
 - C++ style typically says that you should just use `0`.

I typically use the variant that matches the language where the type is *declared*.

`NULL` is the **`C equivalent`** of `nil`, a pointer to nothing;

 where **`nil is zero typed as id`**, 

**`NULL is zero typed as void*`**. 

**One important point** you can’t send a message to NULL. So it is preferred to use nil in objective-C at many places.

They almost are the same thing except,

`nil` is used in an Objective-C style.
where `NULL` is for C type pointers and is typdef&#39;ed to `(void *)`.

I built a web application containing a WCF service contract and a Silverlight control which makes calls to that WCF service. On my development and test servers it works great. 

When I deploy to our live server and run the application I get an exception of type `System.ServiceModel.ServiceActivationException` which states that the service can not be activated due to an exception during compilation.  The exception is:

&gt; This collection already contains an address with scheme http. There can be at most one address per scheme in this collection.

I read that this exception may be thrown if the web site has more than one host header, which is true on our live server.  Apparently WCF services hosted in IIS can have only one base address.  How can I get around this issue?

WCF service startup error &quot;This collection already contains an address with scheme http&quot;

How would I generate a random date that has to be between two other given dates?

The function&#39;s signature should be something like this:

    random_date(&quot;1/1/2008 1:30 PM&quot;, &quot;1/1/2009 4:50 AM&quot;, 0.34)
                       ^                       ^          ^

                date generated has  date generated has  a random number
                to be after this    to be before this

and would return a date such as: `2/4/2008 7:20 PM`

Generate a random date between two other dates

In `observeValueForKeyPath:ofObject:change:context:` - why do the docs use `NULL` instead of `nil` when not specifying a context pointer?

NULL vs nil in Objective-C

<p>In <code>observeValueForKeyPath:ofObject:change:context:</code> - why do the docs use <code>NULL</code> instead of <code>nil</code> when not specifying a context pointer?</p>


I&#39;m used to programming and having log messages be viewable. I know you used to be able to use `NSLog()` to trace out messages when debugging Cocoa applications. What is the best way to &quot;trace&quot; messages when coding in an iPhone Xcode development environment?



How do I debug with NSLog(@&quot;Inside of the iPhone Simulator&quot;)?

I have the value `25.00` in a `float`, but when I print it on screen it is `25.0000000`.  
How can I display the value with only two decimal places?

Make a float only show two decimal places

In Objective-C 2.0, is it possible to make a method where the argument is optional? That is, you can have a method call like this:

    [aFraction print];

as well as this:

    [aFraction print: someParameter];

in the same program.

Apple&#39;s **Objective-C 2.0 Programming Language** guide contrasts Obj-C with Python and seems to say this is not allowed. I&#39;m still learning and I want to be sure. If it *is* possible, then what is the syntax, because my second code example does not work.

**Update:**
OK, I just made two methods, both named &quot;print&quot;. 

*header*

    -(void) print;
    -(void) print: (BOOL) someSetting; 



*implementation*

    -(void) print {
    	[self print:0];
    }
    
    -(void) print: (BOOL) someSetting {
    	BOOL sv;
    	sv = someSetting;
    	
    	if ( sv ) {
    		NSLog(@&quot;cool stuff turned on&quot;);
    	}
    	else {
    		NSLog(@&quot;cool stuff turned off&quot;);
    	}
    }


*the relevant program lines*

        ...
    	printParamFlag = TRUE;
    	
    // no parameter
    	[aCodeHolder print];

    // single parameter
    	[aCodeHolder print:printParamFlag];
        ...


I can&#39;t believe that worked. Is there any reason I shouldn&#39;t do this?

Optional arguments in Objective-C 2.0?

Being new to objective-c, cocoa, and iPhone dev in general, I have a strong desire to get the most out of the language and the frameworks.

  One of the resources I&#39;m using is Stanford&#39;s CS193P class notes that they have left on the web.  It includes lecture notes, assignments and sample code, and since the course was given by Apple dev&#39;s, I definitely consider it to be &quot;from the horse&#39;s mouth&quot;.

Class Website:  
&lt;http://www.stanford.edu/class/cs193p/cgi-bin/index.php&gt;

  Lecture 08 is related to an assignment to build a UINavigationController based app that has multiple UIViewControllers pushed onto the UINavigationController stack.  That&#39;s how the UINavigationController works.  That&#39;s logical.  However, there are some stern warnings in the slide about communicating between your UIViewControllers.

I&#39;m going to quote from this serious of slides:  
&lt;http://cs193p.stanford.edu/downloads/08-NavigationTabBarControllers.pdf&gt;

Page 16/51:
&gt; How Not To Share Data
&gt; =====================
&gt; - Global Variables or singletons
&gt;   - This includes your **application delegate**
&gt; - Direct dependencies make your code less reusable
&gt;   - And more difficult to debug &amp; test

  Ok.  I&#39;m down with that.  Don&#39;t blindly toss all your methods that will be used for communicating between the viewcontroller into your app delegate and reference the viewcontroller instances in the app delegate methods.  Fair &#39;nuff.

  A bit further on, we get this slide telling us what we *should* do.

Page 18/51:
&gt; Best Practices for Data Flow
&gt; ============================
&gt; - Figure out **exactly** what needs to be communicated
&gt; - **Define input parameters** for your view controller
&gt; - For communicating back up the hierarchy, **use loose coupling**
&gt;   - Define a generic interface for observers (like delegation)

  This slide is then followed by what appears to be a place holder slide where the lecturer then apparently demonstrates the best practices using an example with the UIImagePickerController.  I wish the videos were available! :(

  Ok, so... I&#39;m afraid my objc-fu is not so strong.  I&#39;m also a bit confused by the final line in the above quote.  I&#39;ve been doing my fair share of googling about this and I found what appears to be a decent article talking about the various methods of Observing/Notification techniques:  
&lt;http://cocoawithlove.com/2008/06/five-approaches-to-listening-observing.html&gt;

  Method #5 even indicates delegates as an method!  Except.... objects can only set one delegate at a time.  So when I have multiple viewcontroller communication, what am I to do?

  Ok, that&#39;s the set up gang.  I know I can easily do my communication methods in the app delegate by reference&#39;s the multiple viewcontroller instances in my appdelegate but I want to do this sort of thing the *right* way.

  Please help me &quot;do the right thing&quot; by answering the following questions:

1. When I am trying to push a new viewcontroller on the UINavigationController stack, **who** should be doing this push.  **Which** class/file in my code is the correct place?
2. When I want to affect some piece of data (value of an iVar) in one of my UIViewControllers when I am in a **different** UIViewController, what is the &quot;right&quot; way to do this?
3. Give that we can only have one delegate set at a time in an object, what would the implementation look like for when the lecturer says *&quot;Define a generic interface for observers (like delegation)&quot;*.  A pseudocode example would be awfully helpful here if possible.

What&#39;s the best way to communicate between view controllers?

When working with views and view controllers in an iPhone app, can anyone explain the difference between loadView and viewDidLoad?  

My personal context, is that I build all my views from code, I do not and will not use Interface Builder, should that make any difference.

I&#39;ve found that often when I add init code to loadView, I end up with an infinite stack trace, so I typically do all my child-view building in viewDidLoad...but it&#39;s really unclear to me when each gets executed, and what is the more appropriate place to put init code.  What would be perfect, is a simple diagram of the initialization calls.

Thanks!

iPhone SDK: what is the difference between loadView and viewDidLoad?

So, I saw this:

    error:(NSError **)error

in the apple doc&#39;s. Why two stars? What is the significance?

What is double star (eg. NSError **)?

I need my application to open a window when a user double clicks on a row in an `NSTableView`. I&#39;m having a bit of a difficult time finding information or examples on how to accomplish this. Can anybody point me in the right direction?

Double click an NSTableView row in Cocoa?

I want to make a preference for hiding the Dock icon and showing an `NSStatusItem`.
I can create the StatusItem but I don&#39;t know how to remove the icon from Dock. :-/

Any ideas?

How to hide the Dock icon

I know how delegates work, and I know how I can use them.

But how do I create them?

How do I create delegates in Objective-C?

I often see legacy code checking for `NULL` before deleting a pointer, similar to,

    if (NULL != pSomeObject) 
    {
        delete pSomeObject;
        pSomeObject = NULL;
    }

Is there any reason to checking for a `NULL` pointer before deleting it? What is the reason for setting the pointer to `NULL` afterwards?

Is there any reason to check for a NULL pointer before deleting?

What&#39;s the equivalent to the following:

    std::vector&lt;Foo*&gt; vec;
    vec.push_back(NULL);

when dealing with `boost::shared_ptr`? Is it the following code?

    std::vector&lt; boost::shared_ptr&lt;Foo&gt; &gt; vec;
    vec.push_back(boost::shared_ptr&lt;Foo&gt;());

Note: I may push back a lot of such objects. Should I declare a global static `nullPtr` object somewhere? That way only one of them would have to be constructed:

    boost::shared_ptr&lt;Foo&gt; nullPtr;

NULL pointer with boost::shared_ptr?

I&#39;m working on a project where i find i&#39;m checking for the following in many, many places:

    if(item.Rate == 0 || item.Rate == null) { }

more as a curiousity than anything, what&#39;s the best way to check for both cases?

I&#39;ve added a helper method which is:

    public static bool nz(object obj)
    {
        var parsedInt = 0;
        var parsed = int.TryParse(obj.ToString(), out parsedInt);
        return IsNull(obj) || (parsed &amp;&amp; parsedInt == 0);
    }


Is there a better way?

Nullable types: better way to check for null or zero in c#

I was experimenting with C++ and found the below code as very strange. 

    class Foo{
    public:
    	virtual void say_virtual_hi(){
    		std::cout &lt;&lt; &quot;Virtual Hi&quot;;
    	}
    
    	void say_hi()
    	{
    		std::cout &lt;&lt; &quot;Hi&quot;;
    	}
    };
    
    int main(int argc, char** argv)
    {
    	Foo* foo = 0;
    	foo-&gt;say_hi(); // works well
    	foo-&gt;say_virtual_hi(); // will crash the app
    	return 0;
    }

I know that the virtual method call crashes because it requires a vtable lookup and can only work with valid objects. 

I have the following questions 

 1. How does the non virtual method `say_hi` work on a NULL pointer?
 2. Where does the object `foo` get allocated?

Any thoughts?

Accessing class members on a NULL pointer

I always used `Nullable&lt;&gt;.HasValue` because I liked the semantics. However, recently I was working on someone else&#39;s existing codebase where they used `Nullable&lt;&gt; != null` exclusively instead.

Is there a reason to use one over the other, or is it purely preference?

1. 

        int? a;
        if (a.HasValue)
            // ...

vs.

2. 

        int? b;
        if (b != null)
            // ...

Content Type	Original Author	Original Content on Stackoverflow
Question	erikprice	View Question on Stackoverflow
Solution 1 - Objective C	Paul Tomblin	View Answer on Stackoverflow
Solution 2 - Objective C	Marc Charbonneau	View Answer on Stackoverflow
Solution 3 - Objective C	Andrew	View Answer on Stackoverflow
Solution 4 - Objective C	user2560622	View Answer on Stackoverflow
Solution 5 - Objective C	dan14941	View Answer on Stackoverflow

NULL vs nil in Objective-C

Objective C Problem Overview

Objective C Solutions

Solution 1 - Objective C

Solution 2 - Objective C

Solution 3 - Objective C

Solution 4 - Objective C

Solution 5 - Objective C

Generate a random date between two other dates

WCF service startup error "This collection already contains an address with scheme http"

Attributions