It is probably related to this technical note https://developer.apple.com/library/content/qa/qa1490/_index.html

&gt; IMPORTANT: For 64-bit and iPhone OS applications, there is a linker bug that prevents -ObjC from loading objects files from static libraries that contain only categories and no classes. The workaround is to use the -all_load or -force_load  flags. -all_load forces the linker to load all object files from every archive it sees, even those without Objective-C code. -force_load is available in Xcode 3.2 and later. It allows finer grain control of archive loading. Each -force_load option must be followed by a path to an archive, and every object file in that archive will be loaded.


Need to store values from foreach loop into an array, need help doing that. 

The code below does not work, only stores the last value, tried `$items .= ...,` but that is not doing the trick either, any help will be appreciated.

```php
foreach($group_membership as $i =&gt; $username) {
    $items = array($username);
}
    
print_r($items);
```

How to store values from foreach loop into an array?

It seems to be common knowledge that hash tables can achieve O(1), but that has never made sense to me.  Can someone please explain it? Here are two situations that come to mind:

A.  **The value is an int smaller than the size of the hash table.** Therefore, the value is its own hash, so there is no hash table. But if there was, it would be O(1) and still be inefficient.

B.  **You have to calculate a hash of the value.** In this situation, the order is O(n) for the size of the data being looked up.  The lookup might be O(1) after you do O(n) work, but that still comes out to O(n) in my eyes.

And unless you have a perfect hash or a large hash table, there are probably several items per bucket. So, it devolves into a small linear search at some point anyway.

I think hash tables are awesome, but I do not get the O(1) designation unless it is just supposed to be theoretical.

Wikipedia&#39;s [article for hash tables](http://en.wikipedia.org/wiki/Hash_table) consistently references constant lookup time and totally ignores the cost of the hash function.  Is that really a fair measure?

---

**Edit:** To summarize what I learned:

   * It is technically true because the hash function is not required to use all the information in the key and so could be constant time, and because a large enough table can bring collisions down to near constant time.

   * It is true in practice because over time it just works out as long as the hash function and table size are chosen to minimize collisions, even though that often means not using a constant time hash function.


Can hash tables really be O(1)?

I can&#39;t find anywhere what the -all_load flag do when compiling Objective-C code.

I have some issues uploading binaries to Apple, the they say it&#39;s because I didn&#39;t use this flag, but my code compiles even without it.

Can some one help me with that?

Thanks

What does the -all_load linker flag do?

I can't find anywhere what the -all_load flag do when compiling Objective-C code.
I have some issues uploading binaries to Apple, the they say it's because I didn't use this flag, but my code compiles even without it.
Can some one help me with that?
Thanks

In an attempt to follow best practices, we&#39;re trying to use the proper JavaScript/jQuery events according to what device you are using. For example, we&#39;re building a mobile site that has an &lt;a&gt; tag that will have an onclick or touch event. In the case of an iPhone, we&#39;d like to use the &quot;touchstart&quot; event. We&#39;d like to test if their device supports &quot;touchstart&quot; before we bind that handler to the object. If it doesn&#39;t, then we will bind &quot;onclick&quot; instead.

What is the best way to do this?

How to check browser for touchstart support using JS/jQuery?

My website renders well on the iPhone/Safari browser, with one exception: My text input fields have a weird rounded style which doesn&#39;t look good at all with the rest of my website.

Is there a way to instruct Safari (via CSS or metadata) not to round the input fields and render them rectangular as intended?

Turn off iPhone/Safari input element rounding

I have an app with a navigation bar consisting of 2 bar buttons. I would like to hide and show this navigation bar when a user double taps the screen.


Initially, the navigation bar should be hidden. When a user double taps the screen, the navigation bar should come up with an animation, like what can be seen in the iPhone&#39;s photo gallery.

How can i do something this? Suggestions are always appreciated.

Navigation bar show/hide 

I&#39;m working on a iPhone app which has a pretty large UITableView with data taken from the web, so I&#39;m trying to optimize its creation and usage.


I found out that `dequeueReusableCellWithIdentifier` is pretty useful, but after seeing many source codes using this, I&#39;m wondering if the usage I make of this function is the good one.

Here is what people usually do:

    UITableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:@&quot;Cell&quot;];
   
    if (cell == nil) {
      cell = [[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:@&quot;Cell&quot;];
    
    // Add elements to the cell
    return cell;

And here is the way I did it:

    // The cell row
    NSString identifier = [NSString stringWithFormat:@&quot;Cell %d&quot;, indexPath.row]; 
    
    UITableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:identifier];

    if (cell != nil)
      return cell;

    cell = [[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:identifier];
    // Add elements to the cell
    return cell;

The difference is that people use the same identifier for every cell, so dequeuing one only avoids to alloc a new one.

For me, the point of queuing was to give each cell a unique identifier, so when the app asks for a cell it already displayed, neither allocation nor element adding have to be done.

&lt;i&gt;In fine&lt;/i&gt; I don&#39;t know which is best, the &quot;common&quot; method ceils the table&#39;s memory usage to the exact number of cells it display, whilst the method I use seems to favour speed as it keeps all calculated cells, but can cause large memory consumption (unless there&#39;s an inner limit to the queue).

Am I wrong to use it this way? Or is it just up to the developer, depending on his needs?

iPhone - dequeueReusableCellWithIdentifier usage

How would I store CGRect objects in a NSMutableArray, and then later retrieve them?

How to store CGRect values in NSMutableArray?

What is the default value of a `BOOL` variable in Objective-C?

Default value of BOOL

In my application I need to get the hour and minute separately:

    NSString *currentHour=[string1 substringWithRange: NSMakeRange(0,2)];
    		int currentHourInNumber=[currentHour intValue];

Consider `string1` contains `11:59:13 AM` which is coming from datepicker.

Here if I use above code, it&#39;s okay to get hour if it&#39;s greater than 9. Else I need to change `NSMakeRange(0,1)` to get hour between 1 to 9.

Are there any methods to get the hour, minutes, etc?

Thanks in advance, please provide me sample code.


How do I get hour and minutes from NSDate?

Is there a way to make a `UIScrollView` auto-adjust to the height (or width) of the content it&#39;s scrolling?

Something like:

    [scrollView setContentSize:(CGSizeMake(320, content.height))];

How do I auto size a UIScrollView to fit its content

I searched, but surprisingly couldn&#39;t find an answer.

I have a long `NSString` that I want to shorten. I want the maximum length to be around 20 characters. I read somewhere that the best solution is to use `substringWithRange`. Is this the best way to truncate a string? 

	NSRange stringRange = {0,20};
	NSString *myString = @&quot;This is a string, it&#39;s a very long string, it&#39;s a very long string indeed&quot;;
	NSString *shortString = [myString substringWithRange:stringRange];

It seems a little delicate (crashes if the string is shorter than the maximum length). I&#39;m also not sure if it&#39;s Unicode-safe. Is there a better way to do it? Does anyone have a nice category for this?

How can I truncate an NSString to a set length?

I have a C project that produces ten executables, all of which I would like to be linked in statically. The problem I am facing is that one of these executables uses a 3rd-party library of which only the shared-object version is available.

  If I pass the `-static` flag to gcc, ld will error saying it can&#39;t find the library in question (I presume it&#39;s looking for the .a version) and the executable will not be built.  Ideally, I would like to be able to tell &#39;ld&#39; to statically link as much as it can and fail over to the shared object library if a static library cannot be found.

In the interium I tried something like `gcc -static -lib1 -lib2 -shared -lib3rdparty foo.c -o foo.exe` in hopes that &#39;ld&#39; would statically link in lib1 and lib2 but only have a run-time dependence on lib3rdparty. Unfortunatly, this did not work as I intended; instead the `-shared` flag overwrote the `-static` flag and everything was compiled as shared-objects.

Is statically linking an all-or-nothing deal, or is there some way I can mix and match?

Can I mix static and shared-object libraries when linking?

In [gcc-strict-aliasing-and-casting-through-a-union](https://stackoverflow.com/questions/2906365/gcc-strict-aliasing-and-casting-through-a-union) I asked whether anyone had encountered problems with union punning through pointers.  So far, the answer seems to be _No_.

This question is broader: Do you have _any_ horror stories about gcc and strict-aliasing?

Background: Quoting from [AndreyT&#39;s answer in c99-strict-aliasing-rules-in-c-gcc](https://stackoverflow.com/questions/2771023/c99-strict-aliasing-rules-in-c-gcc/2771041#2771041):

&gt;&quot;Strict aliasing rules are rooted in parts of the standard that were present in C and C++ since the beginning of [standardized] times. The clause that prohibits accessing object of one type through a lvalue of another type is present in C89/90 (6.3) as well as in C++98 (3.10/15). ... It is just that not all compilers wanted (or dared) to enforce it or rely on it.&quot;

Well, **gcc** is now daring to do so, with its `-fstrict-aliasing` switch.  And this has caused some problems.  See, for example, the excellent article  http://davmac.wordpress.com/2009/10/ about a Mysql bug, and the equally excellent discussion in http://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html.

Some other less-relevant links:

 * [performance-impact-of-fno-strict-aliasing](https://stackoverflow.com/questions/1225741/performance-impact-of-fno-strict-aliasing)  
 * [strict-aliasing](https://stackoverflow.com/questions/754929/strict-aliasing)  
 * [when-is-char-safe-for-strict-pointer-aliasing](https://stackoverflow.com/questions/262379/when-is-char-safe-for-strict-pointer-aliasing)  
 * [how-to-detect-strict-aliasing-at-compile-time](https://stackoverflow.com/questions/725138/how-to-detect-strict-aliasing-at-compile-time)  

So to repeat, do you have a horror story of your own?  Problems ***not*** indicated by `-Wstrict-aliasing` would, of course, be preferred.  And other C compilers are also welcome.

***Added June 2nd***:  The first link in [Michael Burr&#39;s answer](https://stackoverflow.com/a/2959468/4095830), which does ***indeed*** qualify as a horror story, is perhaps a bit dated (from 2003).  I did a quick test, but the problem has apparently gone away.

Source:

    #include &lt;string.h&gt;
    struct iw_event {               /* dummy! */
        int len;
    };
    char *iwe_stream_add_event(
        char *stream,               /* Stream of events */
        char *ends,                 /* End of stream */
        struct iw_event *iwe,       /* Payload */
        int event_len)              /* Real size of payload */
    {
        /* Check if it&#39;s possible */
        if ((stream + event_len) &lt; ends) {
                iwe-&gt;len = event_len;
                memcpy(stream, (char *) iwe, event_len);
                stream += event_len;
        }
        return stream;
    }

The specific complaint is:

&gt;Some users have complained that when the [above] code is compiled without the -fno-strict-aliasing, the order of the write and memcpy is inverted (which means a bogus len is mem-copied into the stream).

Compiled code, using **gcc** 4.3.4 on CYGWIN wih -O3 (please correct me if I am wrong--my assembler is a bit rusty!):

    _iwe_stream_add_event:
            pushl       %ebp
            movl        %esp, %ebp
            pushl       %ebx
            subl        $20, %esp
            movl        8(%ebp), %eax       # stream    --&gt; %eax
            movl        20(%ebp), %edx      # event_len --&gt; %edx
            leal        (%eax,%edx), %ebx   # sum       --&gt; %ebx
            cmpl        12(%ebp), %ebx      # compare sum with ends
            jae L2
            movl        16(%ebp), %ecx      # iwe       --&gt; %ecx
            movl        %edx, (%ecx)        # event_len --&gt; iwe-&gt;len (!!)
            movl        %edx, 8(%esp)       # event_len --&gt; stack
            movl        %ecx, 4(%esp)       # iwe       --&gt; stack
            movl        %eax, (%esp)        # stream    --&gt; stack
            call        _memcpy
            movl        %ebx, %eax          # sum       --&gt; retval
    L2:
            addl        $20, %esp
            popl        %ebx
            leave
            ret

And for the second link in Michael&#39;s answer,

    *(unsigned short *)&amp;a = 4;

**gcc** will usually (always?) give a warning.  But I _believe_ a valid solution to this (for **gcc**)  is to use:

    #define CAST(type, x) (((union {typeof(x) src; type dst;}*)&amp;(x))-&gt;dst)
    // ...
    CAST(unsigned short, a) = 4;

I&#39;ve asked SO whether this is OK in [gcc-strict-aliasing-and-casting-through-a-union](https://stackoverflow.com/questions/2906365/gcc-strict-aliasing-and-casting-through-a-union), but so far nobody disagrees.



gcc, strict-aliasing, and horror stories

How does one combine two GCC compiled .o object files into a third .o file?

    $ gcc -c  a.c -o a.o
    $ gcc -c  b.c -o b.o
    $ ??? a.o b.o -o c.o
    $ gcc c.o other.o -o executable

If you have access to the source files the `-combine` GCC flag will merge the source files before compilation:

    $ gcc -c -combine a.c b.c -o c.o

However this only works for source files, and GCC does not accept `.o` files as input for this command.

Normally, linking `.o` files does not work properly, as you cannot use the output of the linker as input for it. The result is a shared library and is not linked statically into the resulting executable.

    $ gcc -shared a.o b.o -o c.o
    $ gcc c.o other.o -o executable
    $ ./executable
    ./executable: error while loading shared libraries: c.o: cannot open shared object file: No such file or directory
    $ file c.o
    c.o: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, not stripped
    $ file a.o
    a.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped


combine two GCC compiled .o object files into a third .o file

I am implementing a divide and conquer polynomial algorithm so I can benchmark it against an OpenCL implementation, but I can&#39;t get `malloc` to work. When I run the program, it allocates a bunch of stuff, checks some things, then sends the `size/2` to the algorithm. Then when I hit the `malloc` line again it spits out this:

```none
malloc.c:3096: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &amp;((av)-&gt;bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) &amp;&amp; old_size == 0) || ((unsigned long) (old_size) &gt;= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) &amp; ~((2 * (sizeof(size_t))) - 1))) &amp;&amp; ((old_top)-&gt;size &amp; 0x1) &amp;&amp; ((unsigned long)old_end &amp; pagemask) == 0)&#39; failed.
Aborted
```

The line in question is:

    int *mult(int size, int *a, int *b) {
        int *out,i, j, *tmp1, *tmp2, *tmp3, *tmpa1, *tmpa2, *tmpb1, *tmpb2,d, *res1, *res2;
        fprintf(stdout, &quot;size: %d\n&quot;, size);

        out = (int *)malloc(sizeof(int) * size * 2);
    }

I checked size with a `fprintf`, and it is a positive integer (usually 50 at that point).  I tried calling `malloc` with a plain number as well and I still get the error.  I&#39;m just stumped at what&#39;s going on, and nothing from Google I have found so far is helpful.

Any ideas what&#39;s going on?  I&#39;m trying to figure out how to compile a newer GCC in case it&#39;s a compiler error, but I really doubt it.




Why do I get a C malloc assertion failure?

    template&lt;typename T&gt;
    class CConstraint
    {
    public:
        CConstraint()
        {
        }
	
        virtual ~CConstraint()
        {
        }
		
        template &lt;typename TL&gt;
        void Verify(int position, int constraints[])
        {		
        }
	
        template &lt;&gt;
        void Verify&lt;int&gt;(int, int[])
        {	
        }
    };

Compiling this under g++ gives the following error:

&gt; Explicit specialization in non-namespace scope &#39;class CConstraint&lt;T&gt;&#39;


In VC, it compiles fine. Can anyone please let me know the workaround?

Explicit specialization in non-namespace scope

This warning:

    LINK : warning LNK4098: defaultlib &#39;MSVCRT&#39; conflicts
      with use of other libs; use /NODEFAULTLIB:library

is a fairly common warning in Visual Studio. I&#39;d like to understand the exact reason for it and the right way (if at all) to handle it.

This comes up in a debug build, compiled with `/MDd`. The project is linked to things like windows `Version.dll` and `pdh.dll` which themselves link with `MSVCRT.dll`. Obviously, I don&#39;t have the debug versions of these and can&#39;t compile them.  

So I added `/NODEFAULTLIB:MSVCRT` to the linker command line and it actually did remove the warning. But what does this actually do? And why is it necessary?

Resolving LNK4098: defaultlib &#39;MSVCRT&#39; conflicts with

How can I create global variables that are shared in C? If I put it in a header file, then the linker complains that the variables are already defined. Is the only way to declare the variable in one of my C files and to manually put in `extern`s at the top of all the other C files that want to use it? That sounds not ideal.

shared global variables in C

During a refactor of an iOS project, I ran into this bear of a bug. During the linking phase, I get this message:

&gt;ld: duplicate symbol _OBJC_IVAR_$_TinCanViewController.currentViewController in /path/to/TinCanViewController-E98A666B7AF2673A.o and /path/to/TinCanViewController-E98A666B7AF2673A.o

As far as I can tell, it looks like it claims `TinCanViewController-E98A666B7AF2673A.o` is declaring the specified symbol twice; both paths are pointing to the exact same .o file. I ran `nm` on that specific file, and it only included that symbol once: 

&gt; 00008150 S _OBJC_IVAR_$_TinCanViewController.currentViewController

I ran `nm` on all the other .o files in the directory to see if they were somehow declaring this symbol, too, but they&#39;re not. This happens to any member I add to the TinCanViewController class - it&#39;s not specific to currentViewController.

I feel like I must be somehow linking against the class twice somehow, but I&#39;ve pretty assiduously gone through and checked all references to this class. In the refactored version, there are basically none. The AppDelegate includes it, but right now it&#39;s basically just a passthrough class that loads another ViewController at the start. No other classes in the project include it.

Any suggestions on what might be causing this or how I might debug it better?


Content Type	Original Author	Original Content on Stackoverflow
Question	Guy Ephraim	View Question on Stackoverflow
Solution 1 - Iphone	Sharjeel Aziz	View Answer on Stackoverflow

What does the -all_load linker flag do?

Iphone Problem Overview

Iphone Solutions

Solution 1 - Iphone

Can hash tables really be O(1)?

How to store values from foreach loop into an array?

Attributions