In Java, the following code is fine, no error.
class ConstantA{
public static String MY_TEST = "My Test";
}
import ConstantA;
Class TestClass{
public void test(){
System.out.println(ConstantA.MY_TEST); // it's work fine.
}
}
According to above java concept, it doesn't work on Objective-C,
in ConstantA.h file
extern NSString * const MY_TEST;
#interface ConstantA : NSObject
#end
in ConstantA.m file
NSString * const MY_TEST = #"My Test";
#implementation ConstantA
#end
in main.m file ( error occurs here)
#import "ConstantA.h"
int main(int argc, char *argv[])
{
#autoreleasepool {
NSLog(#"%#",ConstantA.MY_TEST); // error: Property 'MY_TEST' not found on object of type 'ConstantA'
}
}
Can help me to solve this problem?
thanks so much
#import "ConstantA.h"
int main(int argc, char *argv[])
{
#autoreleasepool {
NSLog(#"%#", MY_TEST); // error: Property 'MY_TEST' not found on object of type 'ConstantA'
}
}
Use directly the global variable, without class name:
MY_TEST and NOT ConstantA.MY_TEST.
Use NSLog(#"%#",MY_TEST);.
MY_TEST is not scoped by ConstantA in any way; it's simply a global variable. Objective-C doesn't support class-level "static variables" like you'd find in Java or C++. Classes can only have instance variables
You can sorta imitate a class variable with a global variable, but it's still just a global; it's not namespaced to the class in any way.
Put your Global string in the .h file
property (nonatomic, retain) NSString *gMyString;
In the .m file
synthetize gMyString;
It should work... I had the same problem last week !
Related
I have declared a functions in a header file like this (pseudo code):
// funcs.h
#import <Foundation/Foundation.h>
NSString* func1();
void func2();
Then I have the implementations in
//funcs.mm
#import "funcs.h"
NSString* func1()
{
// do something and use C++ functions
}
void func2()
{
// do more
}
When I include the header in a third file
// AppDelegate.mm
#import "funcs.h"
....
string1 = func1();
func2();
....
Whenever I try to use these functions I get "Use of undeclared identifier" errors.
What am I doing wrong? Isn't function declaration in files in Objective-C++ the same as in normal C?
A .mm file is Objective-C++. Plain functions in a .mm file are not C functions, but C++ functions. Objective-C is the same as C, but you are not using Objective-C, you are using Objective-C++.
I would strongly recommend that you only use .mm files for interfacing with C++. For example, your AppDelegate should be a .m file and not .mm.
You need to write C functions like this:
extern "C" {
NSString* func1()
{
// do something and use C++ functions
}
}
Also, in the .h file you should declare them like this:
#if __cplusplus
extern "C" {
#endif
NSString* func1(void);
#if __cplusplus
}
#endif
The macro __cplusplus might be misspelled, google for it.
extern "C" tells the C++ or Objective-C++ compiler that a function should have C linkage.
This is probably pilot error on my part, but I am a little confused why this does not return an int (as thats the type of the property identified by the key path). Does valueForKeyPath: return an object instead, can anyone explain.
// Simple Object
#interface Hopper : NSObject
#property(nonatomic, assign) int mass;
#end
// Test
Hopper *hopper = [[Hopper alloc] init];
[hopper setMass:67];
NSLog(#"HOPPER: %d", [hopper valueForKeyPath:#"mass"]);
.
WARNING: Conversion specifies type 'int' but the argument has type 'id'
Yes, it returns an objc object:
- (id)valueForKeyPath:(NSString *)keyPath;
Details for automatic conversions from non-objc objects to objc objects (e.g. NSNumber and NSValue) is covered in Accessor Search Patterns for Simple Attributes.
Therefore, you would use the objc object format specifier %#:
NSLog(#"HOPPER: %#", [hopper valueForKeyPath:#"mass"]);
valueForKeyPath returns an object. int and char types are not objects. Access the property via the . operator or similar.
NSLog(#"HOPPER: %d", [hopper mass]);
NSLog(#"HOPPER: %d", hopper.mass);
Edit: Didn't fully read example code, updated answer
I declare a variable and some methods in the global nsobject class like
#interface classGlobal : NSObject {
NSString *myGuid;
}
#property(nonatomic,assign)NSString *myGuid;
and i synthesize in the .m class. but when i try to access the myGuid variable in the same class (classGlobal.m) then it shows the error "instance variable 'myGuid' accessed in class method".
So please suggest how i solve this issue.
It means that instance variables cannot be accessed from class methods. A class method is declared using a + instead of a -. If you need to use global variables I suggest you take a look at this question which answers it pretty well. And here is another one.
The compiler complains, that you are using myGuid in a scope, where it is not accessible/defined. The declaration of myGuid in the interface part does not define a global variable, but an instance member variable. If you need a global variable (say, becaue you have to access it from a class method declared with + instead of -), declare as usual in your .m file:
MyClass.m:
static NSString* myGuid = nil;
+ (void) someClassMethod {
if( myGuid == nil ) ...
}
The answer to this quesion explains that opaque pointers are a good way to include C++ member variables in an Objective C++ header. I'm getting compile errors when trying to follow the example. Here's the relevant code from my header, with the corresponding compiler errors shown as comments:
struct ADSR_opaque; // error: forward declaration of 'struct ADSR_opaque'
#interface LoopyPulser : NSObject{
float _pulseRate;
UInt32 tickInterval;
UInt32 step;
InMemoryAudioFile * audioFilePlayer;
ADSR_opaque* env; // error: expected specifier-qualifier-list before 'ADSR_opaque'
Pattern * pattern;
float loopLengthRatio;
float volume;
}
Is there something simple I'm doing wrong here?
I don't have any problem with the following minimal sample:
struct ADSR_opaque;
#interface LoopyPulser : NSObject {
struct ADSR_opaque* env;
}
#end
If you include the header in plain Objective-C files (not Objective-C++), you have to add struct.
Alternatively use typedefs:
struct ADSR_opaque_;
typedef struct ADSR_opaque_ ADSR_opaque;
#interface LoopyPulser : NSObject {
ADSR_opaque* env;
// ...
How to create & access static string in iPhone (objective c)?
I declare static NSString *str = #"OldValue" in class A.
If i assign some value to this in class B as str = #"NewValue".
This value persists for all methods in class B. But if I access it in class C (after assignment in B) I am getting it as OldValue.
Am I missing something? Should i use extern in other classes?
Thanks & Regards,
Yogini
Update: As of Xcode 8, Objective-C does have class properties. Note, it's mostly syntactic sugar; these properties are not auto-synthesized, so the implementation is basically unchanged from before.
// MyClass.h
#interface MyClass : NSObject
#property( class, copy ) NSString* str;
#end
// MyClass.m
#import "MyClass.h"
#implementation MyClass
static NSString* str;
+ (NSString*) str
{
return str;
}
+ (void) setStr:(NSString*)newStr
{
if( str != newStr ) {
str = [newStr copy];
}
}
#end
// Client code
MyClass.str = #"Some String";
NSLog( #"%#", MyClass.str ); // "Some String"
See WWDC 2016 What's New in LLVM. The class property part starts at around the 5 minute mark.
Original Answer:
Objective-C doesn't have class variables, which is what I think you're looking for. You can kinda fake it with static variables, as you're doing.
I would recommend putting the static NSString in the implementation file of your class, and provide class methods to access/mutate it. Something like this:
// MyClass.h
#interface MyClass : NSObject {
}
+ (NSString*)str;
+ (void)setStr:(NSString*)newStr;
#end
// MyClass.m
#import "MyClass.h"
static NSString* str;
#implementation MyClass
+ (NSString*)str {
return str;
}
+ (void)setStr:(NSString*)newStr {
if (str != newStr) {
[str release];
str = [newStr copy];
}
}
#end
Unlike Java, where a static variable is scoped for all instances of a class, static in C means that a variable is accessible only from within the file where it is declared. It allows you to do things like declare a static variable inside a function, which sets the value only the first time through, like this.
One thing you haven't mentioned is the relationship between classes A, B, and C. If they are in an inheritance hierarchy, and you're expecting the static variable to be inherited as in Java, the method described by zpasternack will work.
If the three classes are unrelated, and you just want to access the value declared in A, then extern is a more appropriate way to go. In this case, you want to declare the variable as extern in ClassA.h, then define it in Class.m. As long as ClassB and ClassC import ClassA.h, they will be able to link against the same extern definition.
One fine point is that, instead of using extern by itself, it's more robust to use OBJC_EXPORT, which is defined in objc-api.h and handles compiling under C++ as well. Here's a code sample:
// ClassA.h
OBJC_EXPORT NSString* commonString;
...
// ClassA.m
NSString* commonString = #"OldValue";
// ClassB.m
#import "ClassA.h"
...
commonString = #"NewValue"; // Can be inside a function or method
Of course, using externed variables in this way creates an infamous, much-maligned global variable, which is fragile in that anyone can read or write it, and access is uncontrolled. This is the simple approach, and answers your question about using static vs. extern. However, as a design principle, the encapsulation provided by wrapping the variable with class methods is much safer, albeit more complex. In object-oriented languages, when the effect you're trying to achieve is that of a class-static method, encapsulation is probably the right way to go.