Question

Objective C: How do I convert from an NSString to a char[]?

Asked by: ehensens

Hi all,

I have an iPhone app with a view where the user enters a text value into a UITextField and I need to get the values as character arrays. Please see the attached code. Also, please let me know if there's any other questions I can answer to make this question more clear! Thank you for all your help!

#define MAX_SIZE 500
 
...
 
NSString *directoryAsNS = directoryField.text;
 
char m_directory[MAX_SIZE] = .................

                                  
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Asked On
2009-03-23 at 06:34:41ID24255398
Tags

Objective C

,

iPhone

,

XCode

,

Cocoa

Topics

Apple Programming

,

Objective-C Programming Language

Participating Experts
2
Points
500
Comments
10

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Answers

 

by: AJViennaPosted on 2009-03-23 at 10:58:31ID: 23960465

You need getCString
Note that char is not sufficient to store all international characters. So the encoding is important.
See also the documentation of NSString's getCString method and the values for NSStringEncoding. To e.g. convert to ASCII you can write:  

[directoryAsNS getCString:m_directory maxLength:MAX_SIZE, encoding:NSASCIIStringEncoding];
                                              
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by: ehensensPosted on 2009-03-23 at 11:18:58ID: 23960652

Hi AJVienna, thanks for your response. I am having some problems though:

Your solution gives me three errors:

error: 'encoding' was not declared in this scope
error: expected ']' before ':' token
error: expected ';' before ':' token

Please see my exact code below:

#define MAX_HOST_SIZE 500
 
NSString *directoryAsNS = directoryField.text;
 
char m_directory[MAX_HOST_SIZE];
 
[directoryAsNS getCString:m_directory maxLength:MAX_HOST_SIZE, encoding:NSASCIIStringEncoding];

                                              
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by: ehensensPosted on 2009-03-24 at 10:03:00ID: 23970620

I found the problem, just take out the comma after "MAX_SIZE"

 

by: AJViennaPosted on 2009-03-24 at 13:49:06ID: 23973193

I explained which function to use and how. Just having a little typo in it does IMHO not disqualify the answer.

 

by: ehensensPosted on 2009-03-24 at 14:18:28ID: 23973536

I am not saying your solution is incorrect to punish you.

My concern is not with how many points you can rack up.

My concern is with other people finding your solution with the little typo, having their program not compile, and getting frustrated for an hour of wasted time.

If you wish to repost your solution with the correct syntax, I will gladly accept it as an answer.

 

by: xhmr_xPosted on 2009-03-25 at 02:34:42ID: 23977316

I hope this could help

NSString *directoryAsNS = directoryField.text;
 
unsigned int length = [directoryAsNS length];
 
char m_directory[length+1] ;
directory[length] = '\0';
strncpy(m_directory, directoryAsNS, length);

                                              
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by: ehensensPosted on 2009-03-25 at 06:06:13ID: 23978917

Hi xhmr x,

Thanks for your comment. Since AJVienna's solution already works fine with a shorter syntax, I am going to go ahead and use that, although your solution might very well work. Although it is nice since you don't have to name the encoding if you're not sure. Thanks for the tip!

 

by: xhmr_xPosted on 2009-03-25 at 06:27:48ID: 23979124

your welcome ehensens. I wanted to remind that ^we can use C and its libraries^

 

by: AJViennaPosted on 2009-03-25 at 09:46:07ID: 23981744

Ok, I understand your concern and I did not read from your problem soon enough to spare you the search for the problem. @xhmr_x although I did not try your solution I strongly believe that it would not work correctly. The NSString pointer is not the same as a native char based C-string. The result will copy the content of the object, but internally it uses an unicode representation. If it is UTF-8 it will work fine as long as the code uses only ASCII codes, but it will break as soon as the code uses non-ASCII characters. If it is UTF-16 the result will make no sense at all. In both cases the code may crash may clip the string, because it might require more bytes than the NSString representation.

The correct solution is thus:

char m_directory[MAX_HOST_SIZE];
[directoryAsNS getCString:m_directory maxLength:MAX_HOST_SIZE encoding:NSASCIIStringEncoding];

                                              
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by: ehensensPosted on 2009-03-26 at 10:27:15ID: 31561361

Sorry, would have accepted this earlier, I overlooked it.

20120131-EE-VQP-002

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