I'm beginner ObectiveC user so please keep potential answers simple.
I have quite a few years experience with C and C++.
Now, with ObjectiveC I want to create two objects, not using properties. My question is "what is wrong here" not "how to do it differently".
So there is my code:
#implementation News
NSString *_title;
NSString *_excerpt;
NSString *_content;
NSString *_thumbnailURL;
NSString *_date;
-(id)initWithTitle:(NSString *)title excerpt:(NSString *)excerpt content:(NSString*)content thumbnail:(NSString *)thumbnailURL date:(NSString *)date {
self = [super init];
if (self) {
_title = [[NSString alloc] initWithString:title];
_excerpt = [[NSString alloc] initWithString:excerpt];
_content = [[NSString alloc] initWithString:content];
_thumbnailURL = [[NSString alloc] initWithString:thumbnailURL];
_date = [[NSString alloc] initWithString:date];
}
return self;
}
-(void)showData {
NSLog(#" title:%#", _title);
NSLog(#" excerpt:%#", _excerpt);
NSLog(#" thumbnailURL:%#", _thumbnailURL);
NSLog(#" date:%#", _date);
NSLog(#" getContent:%#", _content);
}
#end
Now I want to create two objects:
News *nws = [[News alloc] initWithTitle:#"title1" excerpt:#"excerpt1" content:#"content1" thumbnail:#"thumbnail1" date:#"date1"];
News *nws2 = [[News alloc] initWithTitle:#"title3" excerpt:#"excerpt3" content:#"content3" thumbnail:#"thumbnail3" date:#"date3"];
After that want to show whats is inside this objects:
[nws showData];
[nws2 showData];
Result is that both objects have the same values inside. All ended with "3". I thought that nws object will containt values ending with "1" and nws2 will contain values with "3". But it isnt working like that. Why? Where is an error?
Please help and thanks!
I asked in twitter and got the following comment from #Bavarious:
https://gist.github.com/11c22c0edea5391a3799 (bold added)
Any variable declared outside of #interface … {} or #implementation … {} is treated as a regular C variable.
In your example, _excerpt
is a global (file scope) variable with static storage duration and
could equivalently be placed at the top of the file before
#interface, or between #interface and #implementation, or
between the implementation of two methods — it’s the same mechanism
where file scope variables in C are defined outside of a function
block.
Variables with static storage duration can be used to realise class
variables, a concept that doesn’t exist in Objective-C.
Your variables are defined as global variables (and not instance variables), this why they have the same same value referenced from your instances.
Embedding them in {} is a possible solution in your case.
Related
Here is my code:
I want to be able to create a global NSMutableArray that can store Budget* objects that can then be written to a .pList file... I'm only learning what pLists are, and I am a bit hazy about how to implement them...
Where am I going wrong here?
- (IBAction)btnCreateBudget:(id)sender
{
Budget *budget = [[Budget alloc] init];
budget.name = self.txtFldBudgetName.text;
budget.amount = [self.txtFldBudgetAmount.text intValue];
// Write the data to the pList
NSMutableArray *anArray = [[NSMutableArray alloc] init]; // I want this to be a global variable for the entire app. Where do I put this?
[anArray addObject:budget];
[anArray writeToFile:[self dataFilePath] atomically:YES];
/* As you can see, below is where I test the code. Unfortunately,
every time I run this, I get only 1 element in the array. I'm assuming
that this is because everytime the button is pressed, I create a brand new
NSMutableArray *anArray. I want that to be global for the entire app. */
int i = 0;
for (Budget * b in anArray)
{
i++;
}
NSLog(#"There are %d items in anArray",i);
}
-(NSString *) dataFilePath
{
NSArray *path = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentDirectory = [path objectAtIndex:0];
return [documentDirectory stringByAppendingPathComponent:#"BudgetData.plist"];
}
edit: I'd like to add that I am creating the anArray array so that it can be accessible by other views. I understand that this can be done with NSNotification? or Should I do this the appDelegate classes? The end goal is to have the anArray object populate a UITableView that is in a separate View.
Just put the declaration outside the method instead of inside it.
NSMutableArray *anArray = nil;
- (IBAction)btnCreateBudget:(id)sender
{
...
if ( anArray == nil )
anArray = [[NSMutableArray alloc] init];
...
}
If it's only used inside the one file, make it "static" instead to prevent name collisions with other files:
static NSMutableArray *anArray = nil;
If it's only used inside the one method, make it "static" and put it inside that method:
- (IBAction)btnCreateBudget:(id)sender
{
static NSMutableArray *anArray = nil;
...
if ( anArray == nil )
anArray = [[NSMutableArray alloc] init];
...
}
Note that people usually use some kind of naming convention for global variables, like "gArray", to easily differentiate them from local variables, instance variables, or method parameters.
Global variable is not necessary in this case. You can do something like this:
Read old data to mutable array (initWithContentsOfFile:).
Add new record to the array.
Save the array to same file.
But the second problem in your code is that if your Budget class is not a property list type (NSString, NSData, NSArray, or NSDictionary objects) writeToFile: will not save it sucessfully.
You need to make sure that your Budget class invokes NSCoder and then the NSCoder initWithCoder: and NSCoder decodeWithCoder: methods. Otherwise, writeToFile: will not work for you NSObject class.
But I digress. The answer to the original question should be the following.
In your .h file you need to do the following.
#interface WhateverClassName : UIViewController
{
NSMutableArray *anArray;
}
#property(nonatomic, retain) NSMutableArray *anArray;
#end
Then, you need to make sure you #synthesize the NSMutableArray so that you don't get any freaky warnings. This is done just after the #implementation line in your .m file.
Then, within the function that you want it to be allocated into memory, simply do the following.
anArray = [[NSMutableArray alloc] initWithObjects:nil];
This is now a global variable. It is global in the sense that it can be used from any function and is not limited to use in one function.
If you would like to have data accessible to the entire application or context ("global"), you can use a singleton. However, do this with care and make sure it is actually necessary and appropriate. I would suggest doing plenty of reading up on it prior to any implementation of a singleton. Carter Allen has a good basic implementation here.
According to "The end goal is to have the anArray object populate a UITableView that is in a separate View" you wouldn't need to write anything to a file, database or singleton. Just set the object. Like stated by Sebastien Peek.
If you wish for offline data storage, look into sqlite, json, plist , etc
I have three NSString properties declared like this:
#property(nonatomic,retain) NSString *currentPassword;
#property(nonatomic,retain) NSString *newPassword;
#property(nonatomic,retain) NSString *confirmPassword;
I initialize them in a viewDidLoad method:
currentPassword = [[NSString alloc]init];
newPassword = [[NSString alloc]init];
confirmPassword = [[NSString alloc]init];
The funny thing is that they are the same object after initialize them as different objects!
Is this some kind of compiler optimization?
Thank you
Is this some kind of compiler optimization?
Not quite. It's a special case value for a constant, and an optimization of a common concrete immutable type/value which has been implemented by the NSString class.
NSString is immutable. There's no reason multiple instances of the same empty string are needed. In such simple cases, -[NSString init] can take the form:
static NSString* const EmptyNSString = #"";
- (id)init
{
self = [super init];
[self release];
return EmptyNSString;
}
similarly, + [NSString string]:
+ (id)string
{
return EmptyNSString;
}
So there are a few static immutable objects which are used this way where it makes sense. Other obvious examples include + [NSArray array] and + [NSNumber numberWithBool:].
Each one of these constants can represent what would have been many many many thousands of unique allocations produced during your program's execution.
This happens to work because NSString as a class cluster: You are returned an object of one of many opaque types which implements the interface declared by NSString. Therefore, a NSMutableString type could then implement init appropriately:
- (id)init
{
self = [super init];
if (nil != self) { ... }
return self;
}
Finally, you should in almost all cases declare your NSString properties as copy.
As NSString objects are immutable (i.e. cannot be changed after they are created) and there's no sense in creating several different instances of the same immutable strings, system tries to reuse existing objects whenever possible.
Using constructor with no parameters may be one of examples. You can also check that +stringWithString: (and -initWithString:) also return the (retained) parameter string, and copy method in NSString is equivalent to retain.
Remember that optimization is only possible because we know NSString instance is not going to change and the same tests with NSMutableString most likely will to create new string instances.
P.S. About NSAssert usage:
NSAssert Generates an assertion if a given condition is false.
So your assert condition should be reversed:
NSAssert(currentPassword && newPassword && confirmPassword,#"nil field");
When you have NSString as a property you should specify the attribute 'copy' instead.
NSString is defined as an immutable type, so whenever the compiler can optimize things by combining identical strings, it should. if you use #"myString" in two separate places in your code, they will be referencing the very same object. #"" strings are of class NSConstantString
I've been reading about NSArrays and NSDictionaires and I think I need the later. I'm trying to populate an object from a small database table. So I can access the string values via a record id. I have to do this several times so putting it into an object makes sense.
I have the basics...
- (void)viewDidLoad {
// WORKING START
NSMutableDictionary *dictCategories = [[NSMutableDictionary alloc] init];
[dictCategories setValue:#"Utility" forKey:#"3"];
[dictCategories setValue:#"Cash" forKey:#"5"];
NSString *result;
result = [dictCategories objectForKey:#"3"];
NSLog(#"Result=%#", result);
// WORKING END
// Can't get this bit right, current error Request for member
// 'getCategories' in something not a structure or union
NSMutableDictionary *dictCategories2 = self.getCategories;
NSLog(#"Result2=%#", [dictCategories2 objectForKey:#"5"]);
[super viewDidLoad];
}
-(NSMutableDictionary*)getCategories {
NSMutableDictionary *dictCategories = [[NSMutableDictionary alloc] init];
[dictCategories setValue:#"Utility" forKey:#"3"];
[dictCategories setValue:#"Cash" forKey:#"5"];
return dictCategories;
}
you are calling the method wrong,try [self getCategories]
You're not being clear on what isn't working, but a few things that are obviously wrong (JonLOo might be spot on though) ...
Firstly. You're using the wrong methods, or at least there's a better one -- setValue:forKey: should/could be setObject:forKey: instead. This might be one of the reasons for your issue.
Secondly. You're over-allocating and not releasing properly. dictCategories2 in your viewDidLoad will vanish into the void and bring with it the allocated memory for dictCategories defined in the getCategories method. An easy standard fix for this is to change
NSMutableDictionary *dictCategories = [[NSMutableDictionary alloc] init];
in getCategories into
NSMutableDictionary *dictCategories = [NSMutableDictionary dictionary];
It will be autoreleased using the latter method by the system.
Thirdly. You want to read up on #property. Instead of getFoo, setBar, the Ob-C standard is to use #properties to (pre)define setters and getter methods. You can then override these to populate default data into your methods when appropriate. You also (probably) want to store the dictionary in your interface as an instance variable, rather than letting it be deallocated all the time. Example of a #property implementation that does this:
#interface foo {
NSMutableDictionary *ingredients;
}
#property (nonatomic, retain) NSMutableDictionary *ingredients;
#end
// ....
#implementation foo
#synthesize ingredients;
// ...
// the #synthesize command above will create getter and setter methods for us but
// we can override them, which we need to do here
- (NSMutableDictionary *)ingredients
{
if (ingredients != nil) {
// we've already got an ingredients variable so we just return it
return ingredients;
}
// we need to create ingredients
ingredients = [[NSMutableDictionary alloc] init];
[ingredients setObject:#"foo" forKey:#"bar"]
return ingredients;
}
In the viewDidLoad method (or anywhere else where you think ingredients might not have been initialized yet), you would do e.g.
NSMutableDictionary *dict = self.ingredients;
Anywhere else you can opt to use just ingredients without self, but if it's nil, your method will never be called, and you will get nil thrown at you.
This is useful in many cases, and is necessary if we want to ever read or write the ingredients variable from outside of our class. It's outside of what you're asking about, but I brought it up because you're trying to do something similar with self.getCategories.
Hope that helps.
I want to use NSMutableDictionary to cache some data i will use later. My custom object is following:
#interface MyData : NSObject {
NSRange range;
NSMutableArray *values;
}
#property (nonatomic, retain) NSMutableArray *values;
and implement:
- (id)init {
if (self = [super init]) {
values = [[NSMutableArray alloc] init];
}
return self;
}
and when i wanna cache it, i use it like this:
NSMutableDictionary *cache = [[NSMutableDictionary alloc] init];
NSString *key = #"KEY";
MyData *data = [[MyData alloc] init];
// save some data into data
[data.values addObject:"DATA1"];
[data.values addObject:"DATA2"];
//... ...
[cache setObject:data forKey:key];
My questions is the count of cache.values is zero when i retrieve this object later as follow:
[cache objectForKey:#"KEY"];
i can retrieve "data" and the object's memory address is the same as the address when i put it into cache.
what's wrong? i need some kind guys help, any info is helpful. thanks
As Carl Norum pointed out, you're passing C strings to addObject:. addObject:, as its name suggests, requires a pointer to a Cocoa object; a C string is a pointer to characters. You need to pass NSString objects there; for literal strings, this simply requires prefixing them with #: "Fred" is a constant C string, whereas #"Fred" is a constant NSString object.
Is cache an instance variable? It looks like it's not; it appears to be a local variable, which means you're creating a new dictionary object every time. That's why there's nothing you've added previously (to previous dictionaries) in the new one. It also means you're leaking those previous dictionaries, since you're not releasing them (not in the code you showed, anyway).
Make cache an instance variable and only create the dictionary when you don't already have one (i.e., when cache == nil). Creating the dictionary in your init method is one good way. And make sure you manage its lifetime appropriately, so you don't leak and/or crash.
First of all your objects your adding don't look right it should have an # before the string. Like #"DATA1"
Second when you add an object to a dictionary or an array it does not make an actual copy of it. It just creates a pointer to it so if those objects are destroyed or moved somewhere also they are also gone out of your dictionary. A better way to make a cache of your values would be to copy the objects like so:
MyData* cache = [[MyData alloc] init];
for (int i = 0; i < [data.values count]; i ++){{
[cache.values addObject:[NSString stringWithString:[data.values objectAtIndex:i]]];
}
Don't use a dictionary in this situation.
I am curious how I might override the description method that is used when you do the following (see below) for an object. I basically want to better format the output, but am unsure about how I might go about setting this up.
NSLog(#"ARRAY: %#", myArray);
many thanks
EDIT_001
Although subclassing NSArray would have worked I instead decided that I would add a category to NSArray (having not used one before) Here is what I added ...
// ------------------------------------------------------------------- **
// CATAGORY: NSArray
// ------------------------------------------------------------------- **
#interface NSArray (displayNSArray)
-(NSString*)display;
#end
#implementation NSArray (displayNSArray)
-(NSString*)display {
id eachIndex;
NSMutableString *outString = [[[NSMutableString alloc] init] autorelease];
[outString appendString:#"("];
for(eachIndex in self) {
[outString appendString:[eachIndex description]];
[outString appendString:#" "];
}
[outString insertString:#")" atIndex:[outString length]-1];
return(outString);
}
#end
gary
If you're doing this a lot, the easiest way to reformat the display of your array would be to add a new prettyPrint category to the NSArray class.
#interface NSArray ( PrettyPrintNSArray )
- (NSSTring *)prettyPrint;
#end
#implementation NSArray ( PrettyPrintNSArray )
- (NSString *)prettyPrint {
NSMutableString *outputString = [[NSMutableString alloc] init];
for( id item in self ) {
[outputString appendString:[item description]];
}
return outputString;
}
#end
Obviously you'd need to alter the for loop to get the formatting the way you want it.
I'm assuming that you myArray variable is an instance of the NSArray/NSMutableArray class.
When NSLog() encounters the # character in its format string, it calls the -description: method on the object. This is a method on the root class, NSObject from which all other Cocoa classes inherit. -description: returns an NSString allowing any object that implements this method to be passed into NSLog(#"#",anyObject) and have a nicely formatted output. The string returned can be anything you care to construct.
For your specific problem, you could subclass NSMutableArray and override the -description: method with your own implementation. Then utilise your subclass instead of NSMutableArray.
For more information on NSObject and -description: see Apple's docs.
From Formatting string objects:
NSString supports the format characters defined for the ANSI C functionprintf(), plus ‘#’ for any object. If the object responds to the descriptionWithLocale: message, NSString sends that message to retrieve the text representation, otherwise, it sends a description message.
So to customize array conversion to string you should change NSArray descriptionWithLocale: implementation. Here's an example of how you can replace object method in run-time.