I've tried the following sample code:
#import "Foundation/Foundation.h"
#interface example
{
#public NSString* name;
}
#end
#implementation example #end
int main()
{
example* me;
me->name = #"World";
}
And it appears my code hates me at this point. I do understand how much of a bad idea it is to make a field public, but I'm not sure why I'm getting an error at that last line in main().
There is a lot wrong here
You class should subclass NSObject so it should be declared as
#interface example : NSObject
{
#public NSString* name;
}
#end
Next you actually need an instance of the class to get at it's values e.g.
example *me = [[example alloc] init];
NSLog(#"%#", me->name);
Next classes are named starting with an uppercase letter and normally have a prefix e.g. I would use
PSExample
Next please don't access instance variables like this, you should make your objects state available through accessors rather than give direct access.
You're not allocating or initializing your me variable. You probably want to inherit from NSObject and then use this:
example *me = [[example alloc] init]
At the very least you need to alloc it.
You need to initialize your variable before you can access it. Also you should derive your class from NSObject.
#import "Foundation/Foundation.h"
#interface example : NSObject
{
#public NSString* name;
}
#end
#implementation example #end
int main()
{
example* me = [[example alloc] init];
me->name = #"World";
}
Related
While I'm more than familiar with C#, I'm totally new at Objective C and iOS development. So I'm learning the language. What I don't understand is why the following code throws a compiler error (and yes, this is from the exercises at Programming with Objective C:
SNDPerson:
#interface SNDPerson : NSObject
#property NSString *first;
#property NSString *last;
+ (SNDPerson *)person;
#end
#implementation SNDPerson
+ (SNDPerson *)person
{
SNDPerson *retVal = [[self alloc] init];
retVal.first = #"Ari";
retVal.last = #"Roth";
return retVal;
}
#end
SNDShoutingPerson:
#import "SNDPerson.h"
#interface SNDShoutingPerson : SNDPerson
#end
#implementation SNDShoutingPerson
// Implementation is irrelevant here; all it does is override a method that prints a string
// in all caps. This works; I've tested it. However, if necessary I can provide more code.
// The goal here was for a concise repro.
#end
Main method:
- int main(int argc, const char * argv[])
{
SNDShoutingPerson *person = [[SNDShoutingPerson alloc] person]; // Error
...
}
The error is "No visible #interface for "SNDShoutingPerson" declares the selector "person".
Shouldn't this work? SNDShoutingPerson inherits from SNDPerson, so I would have assumed it got access to SNDPerson's class factory methods. Did I do something wrong here, or do I have to declare the method on SNDShoutingPerson's interface as well? The exercise text implies that what I did should Just Work.
Omit the +alloc when calling the class method:
SNDShoutingPerson *person = [SNDShoutingPerson person];
Briefly:
+ (id)foo denotes a class method. This takes the form:
[MONObject method];
- (id)foo denotes an instance method. This takes the form:
MONObject * object = ...; // << instance required
[object method];
Also, you can declare + (instancetype)person in this case, rather than + (SNDPerson *)person;.
change the line SNDShoutingPerson *person = [[SNDShoutingPerson alloc] person]; // Error
to
SNDShoutingPerson *person = [[SNDShoutingPerson alloc] init];
Cheers.
If you want to call class method:
SNDPerson person = [SNDPerson person];
person is a class method, but you're trying to call it with the incompletely constructed instance returned by alloc. Kill the alloc and just do [SNDShoutingPerson person].
This has nothing to do with subclasses, by the way. You would get the same error if you had written [[SNDPerson alloc] person].
Ok, I've read a lot around these days about this topic and I alwyas get confused because the answers is different every search I make.
I need to know the best way to declare instance variables in iOS. So far I know I should only declare them inside .m file and leave .h clean. But I can't do it: the compiler gives me compilation erros.
Here is some code from .m only.
#interface UIDesign ()
// .m file
{
NSString *test2 = #"test2";
}
#property (nonatomic, assign) int privateInt;
#end
#implementation UIDesign
{
NSString *test1 = #"test1";
}
Both strings are declared incorrectly and I don't know why. The compiler says: expected ';' at end of declaration list.
So the question is: how can I declare instance variables? I will only need them inside the class.
You cannot initialize instance variables. They are all initialized to nil or zeroes. So compiler expect a semicolon when you are writing an equal sign.
You can initialize them in init method.
You are attempting to add an instance variable to a class extension or category which is unsupported. [EDIT 2013-05-12 06-11-08: ivars in class extension are supported, but not in categories.] As an alternative:
#interface UIDesign : NSObject
#end
#interface UIDesign ()
#property (nonatomic, assign) int privateInt;
#end
#implementation UIDesign
#synthesize privateInt = _privateInt;
- (void)someMethod {
self.privateInt = 42;
}
#end
On the other hand, if you just want to declare an instance variable inside the implementation, just do it there:
#implementation UIDesign {
int _privateInt;
}
#end
EDIT: just noticed that you're also attempting to initialize instance variables in the declaration which is also unsupported. So:
#interface UIDesign : NSObject
#end
#implementation UIDesign {
NSString *_test;
}
- (id)init {
self = [super init];
if( !self ) return nil;
_test = #"Foo";
return self;
}
#end
I have Class A and Class B.
Class B subclasses Class A
Class A has a static constant NSString variable
In a method of Class B I need to use the static constant NSString variable of Class A. What are my options ?
I tried declaring the same thing again, but caused problem (internal inconsistency), tried without using (says, variable undeclared).
Any idea how I can solve this problem ?
Thanks.
Make a class method returning that constant, like this:
+(NSString*) constString {
return myConstString;
}
You can declare static constant strings in the .h file. That way they are public and can be used by other classes that import the header file, including your subclass.
Alternatively, you can declare a reference to the string in your Class B using extern:
extern NSString *const MyString;
That basically tells the compiler that the value of that string is defined elsewhere in the code and it doesn't have to worry about it.
By "Class A has a static constant NSString variable", I assume you're referring to having something like the following defined in your Class A .m file:
static NSString * const MyString = #"MyString";
#implementation ClassA
#end
To allow Class A and its subclasses to see the value, you can do something like this:
MDClassAPrivate.h:
#import <Foundation/Foundation.h>
static NSString * const MDBlahBlahKey = #"MDBlahBlah";
MDClassA.h:
#import <Foundation/Foundation.h>
#interface MDClassA : NSObject {
}
#end
MDClassA.m:
#import "MDClassA.h"
#import "MDClassAPrivate.h"
#implementation MDClassA
#end
MDClassB.h:
#import "MDClassA.h"
#interface MDClassB : MDClassA {
}
#end
MDClassB.m:
#import "MDClassB.h"
#import "MDClassAPrivate.h"
#implementation MDClassB
#end
By moving the static const variables to a separate file, you can allow any class to import them in the implementation file.
My personal solution to this problem would be a pseudo-protected method that returns the constant string.
// ClassA.m
#interface ClassA ()
- (NSString *)constantString;
#end
#implementation ClassA
- (NSString *)constantString {
return #"MyConstantString";
}
#end
Then following on into your subclass:
// ClassB.m
#interface ClassA ()
// This is a method redeclaration to avoid build warnings
- (NSString *)constantString;
#end
#implementation ClassB
- (void)someMethod {
NSString *theConstantString = [self constantString];
// do stuff...
}
#end
The assumption here is that your constant string really is constant, if you want it to be dynamic you would need to modify this solution slightly, but it can still handle it.
You can use 'userDefaults' to set a value that is accessible in the whole application.
[[NSUserDefaults standardUserDefaults] setObject:object forKey:#"objectKey"];
The below coding is working and I can see the values in my second screen. But I am using the same in other classes with different variables in this format. But it dosent show me the variable if after i type the classname with a dot. I cant figure this out. Is there any way to pass values to other class.
InstallProfiler_2 *installProfiler2 = [[InstallProfiler_2 alloc] initWithNibName:#"InstallProfiler_2" bundle:nil];
installProfiler2.profilerType2 = profilerType;
[self.navigationController pushViewController:installProfiler2 animated:NO];
[installProfiler2 release];
Make sure that:
You have imported the class header.
The #property declarations are in this header and not a class extension.
#property refers to ivars so when you say
if after i type the classname with a dot
this terminology is incorrect, you probably mean after you start typing the name of the variable which has points to an instance of a class.
ClassA.h
#interface ClassA : NSObject
#property (nonatomic, weak) NSInteger myInt;
#end
ClassA.m
#implementation ClassA
#synthesize myInt = _myInt;
#end
ClassB.m
#import "ClassA.h" // <- Import the header of the class
# implementation ClassB
// .. other methods and stuff
- (void)myMethod;
{
ClassA *instanceOfClassA = [[ClassA alloc] init]; // <- Working with an instance not a class
instanceOfClassA.myInt = 1;
}
#end
UPDATE
Make sure your #property () does not have readonly between the round brackets.
Also make sure you have either #synthesize'd the ivar in the implementation or have provided both a getter and a setter for the ivar.
Failing that show some relevant code so we can actually see what your doing - we are answering pretty blindly here.
The dot syntax is only available with property/synthesize
Create a custom setter/getter:
+ (BOOL)awesomeClassVar {
return _classVar;
}
+ (void)setAwesomeClassVar:(BOOL)newVar {
_classVar = newVar;
}
then call as a method from the other class:
BOOL theOtherClassVar = [AwesomeClass awesomeClassVar];
[AwesomeClass setAwesomeClassVar:!theOtherClassVar];
Having some difficulty with what I thought would be straight forward. I am trying to make an array of class objects but running into different problems. The goal was to create one class that held an array of another class of objects.
Any help appreciated:
// Basic class unit
#interface MobRec : NSObject {
NSString *MName;
int Speed;
}
#end
// Master Class holding an array of units
#interface MobDefs : NSObject {
MobRec *MobInfo;
}
#property(retain) MobRec *MobInfo;
#end
#synthesize MobInfo;
1) From reading it seems I should create and NSMutableArray but how do you declare an NSMutableArray of custom class objects? All iterations I try cause errors. Previously I had predefined the size in the class as MobInfo[20]; but that didnt seem to be good for anything.
2) How do you properly #Synthesize an array of class objects?
I think you misunderstand what #synthesize does. It creates accessor methods to get and set the property (i.e., it would create a getter method to return that NSMutableArray and a setter method to allow you to replace it with another NSMutableArray). To create an NSMutableArray, you would just create one like any other object in that class's initializer.
NSMutableArray doesn't have any type checking as you add (or read) from it, so you can add any objects you want to it.
In this case I'd have something like:
// MobRec Class
#interface MobRec : NSObject {
NSString *mName;
int speed;
}
#property(retain)NSString *name;
#property(assign)int speed;
#end
#implementation MobRec
#synthesize mName, speed;
#end
// MobDefs Class
#import "MobRec.h"
#interface MobDefs : NSObject {
NSMutableArray *mobInfo;
}
#property(retain) NSMutableArray *mobInfo;
#end
#implementation MobDefs
#synthesize mobInfo;
- (id)init {
mobInfo = [[NSMutableArray alloc] init];
MobRec *aNewMobRec = [[MobRec alloc] init];
[mobInfo addObject:aNewMobRec];
[aNewMobRec release];
}
I've included the basics for adding to the array as well so you can see how its used. Oh and don't forget you have to release the MobInfo in the dealloc method.
But mostly look at NSMutableArray
Your MobDefs class is not necessary, you can use an NSMutableArray directly. NSArray can hold objects of any type, and is declared in just the same way as any other object (such as your MName property).