I googled a lot of similar questions, but none of them had a really good answer about what i needed.
Im wondering about the best way to write constants in some .h file.
my constants file is just a clear file with no imports, just clear .h file. I import it in prefix file.
when i use
static NSInteger someInteger = 1;
or
static BOOl someBool = YES;
The compiler compiles okay but gives me a warning that this variable is unused even though im using it multiple times in different classes.
But if i use
static NSString* someString = #"";
there are not any warnings.
also if i use
const NSInteger someInteger = 1;
Compiler compiles okay for a real device, but when running on a simulator it does not compile with an error duplicate symbols for architecture i386
also what is the difference between
const NSString* someString = #"";
const NSInteger someInteger = 1;
and
NSString* const someString = #"";
NSInteger const someInteger = 1;
I ended up using static const NSInteger someInteger =1;, but i wonder if this is a right option.
So really my question is: what words should i use to successfully create a constants.h file?
For all types (both primitive or otherwise) then you need to provide a single implementation of it somewhere:
constants.h:
extern const int someValue;
extern NSString * const someString;
constants.m:
const NSInteger someValue = 1;
NSString * const someString = #"Some string";
You never want to use static variables in header files as you will end up with multiple copies of the "constant" in every implementation file that includes that header (they may not upset the linker, and cause a link error, but they are there).
Related
This question already has answers here:
Objective C Macro append to string
(3 answers)
Closed 6 years ago.
In my project, I define my urls like such:
#define TERMSURL #"http://127.0.0.1:8000/terms/"
#define PRIVACYURL #"http://127.0.0.1:8000/privacy/"
...
Since the root url (http://127.0.0.1:8000/) is always the same, is there a way to set it as a constant, and then use string substitution for the remaining pieces?
For example, in the other files, I could do something like this:
NSString *devBaseUrl = #"http://127.0.0.1:8000/";
NSString *url1 = [NSString stringWithFormat:#"%#terms/", devBaseUrl];
Is there a way to do that for my current approach?
shared.h
#define TERMSURL #"http://127.0.0.1:8000/terms/"
#define PRIVACYURL #"http://127.0.0.1:8000/privacy/"
#define URL_BASE #"http://127.0.0.1:8000/"
yourClass.m
NSString * stringUrlBase = URL_BASE;
NSString *url1 = [NSString stringWithFormat:#"%#terms/", stringUrlBase];
Sure, you can do that. I have however seen both a #define and an NSString const * const being used before. Defines are easier, and you're probably not going to save that much memory by having constants instead of individual immutable instances of NSString all over the place.
Some advice is to think about how you export the NSString constants. You'll probably want EXTERN_PRIVATE instead of EXTERN, but my sample code will allow all clients of your header to read the string constants you've declared therein.
What you can do:
Create a new .m/.c file with a header in Xcode
In the .m/.c file, declare and initialise your constants
Export the constant as necessary so other compilation units can access it
constants.h
#ifndef constants_h
#define constants_h
// Export the symbol to clients of the static object (library)
#define EXTERN extern __attribute__((visibility("default")))
// Export the symbol, but make it available only within the static object
#define EXTERN_PRIVATE extern __attribute__((visibility("hidden")))
// Make the class symbol available to clients
#define EXTERN_CLASS __attribute__((visibility("default")))
// Hide the class symbol from clients
#define EXTERN_CLASS_PRIVATE __attribute__((visibility("hidden")))
#define INLINE static inline
#import <Foundation/Foundation.h>
EXTERN NSString const * _Nonnull const devBaseUrl;
#endif /* constants_h */
constants.m
#include "constants.h"
NSString const * _Nonnull const devBaseUrl = #"http://127.0.0.1:8000/";
main.m
#import <Foundation/Foundation.h>
#import "constants.h"
int main(int argc, const char * argv[]) {
#autoreleasepool {
NSLog(#"Constant value: %#", devBaseUrl);
// Prints: Constant value: http://127.0.0.1:8000/
}
return 0;
}
I have been interested in using something along the the lines of the following code to
automate the building of my objects (since there are many of them with quite a few properties):
MyObject *myObject = [[myObject alloc] init];
unsigned int numberOfProperties = 0;
objc_property_t *propertyArray = class_copyPropertyList([MyObject class], &numberOfProperties);
for (NSUInteger i = 0; i < numberOfProperties; i++)
{
objc_property_t property = propertyArray[i];
NSString *propertyName = [NSString stringWithUTF8String:property_getName(property)];
if (propertyName)
{
id valueForProperty = [myObject valueForKey:propertyName];
[myObject setValue:valueForProperty forKey:propertyName];
}
}
free(propertyArray);
However, what I've noticed is that this code will try to run not just on the properties in my header file, but also all of my implementation properties as well, which I do not want.
Since Objective-C doesn't actually distinguish public vs private properties, I am not sure how to do this. Any thoughts on how to indicate that I'm only interested in the properties in the header file to simulate the same thing effectively?
In short, you don't. This information is not available in the compiled program. You'd need to write a custom preprocessor to do this if you really wanted to.
Usually I use the first one to define const, but I don't know the difference of the following clearly.
static NSString* kFetcherCallbackThreadKey = #"_callbackThread";
static NSString* const kFetcherCallbackRunLoopModesKey = #"_runLoopModes";
NSString* const kFetcherRetryInvocationKey = #"_retryInvocation";
static const NSUInteger kMaxNumberOfNextLinksFollowed = 25;
In C, the static keyword, used outside a function, is used to declare a symbol that will be accessible only from the file in which it's declared. Kind of «private» global variables.
The const keyword means «constant». Read, the value can't be modified.
Note the two statements are different:
const int * x;
int * const x;
The first one defines a pointer to a constant integer (its value can't be modified, but it can point to something else).
The second one defines a constant pointer to an integer (the pointer value can't be modified, but the value of the int may be).
So you can perfectly have:
const int * const x;
So in your case:
static NSString* kFetcherCallbackThreadKey = #"_callbackThread";
A pointer to a NSString instance that will be accessible only from the file in which it's declared.
static NSString* const kFetcherCallbackRunLoopModesKey = #"_runLoopModes";
A constant pointer to a NSString instance that will be accessible only from the file in which it's declared.
NSString* const kFetcherRetryInvocationKey = #"_retryInvocation";
A constant pointer to a NSString instance that may be accessed from other files of your project.
static const NSUInteger kMaxNumberOfNextLinksFollowed = 25;
A constant integer that will be accessible only from the file in which it's declared.
static means that the variable is accessible only within the compilation unit it's declared in - essentially this source file. const means its value can never change. You can use one, both, or none depending on what you're looking for.
This is a static string which will be the same reference for all instances of the class (static). If you change it in one instance, it will change in all other instances.
static NSString* kFetcherCallbackThreadKey = #"_callbackThread";
This is an NSString pointer to a constant object that is also shared between all instances (static). The const directive makes the variable immutable.
static NSString* const kFetcherCallbackRunLoopModesKey = #"_runLoopModes";
This is a pointer to a constant NSString object. It could have a different instance for each class (if NSStrings are not interned by the compiler, I'm not sure if they are), but cannot be changed (const).
NSString* const kFetcherRetryInvocationKey = #"_retryInvocation";
This is a constant static integer. It will be shared between all instances of the class (static) and cannot be changed (const).
static const NSUInteger kMaxNumberOfNextLinksFollowed = 25;
I'm making the jump from Java to Objective-C. I'm wondering if there is a concept analogous to Java's enums, that supports implementations of methods. I realize Objective-C has plain old C enumerations, but those are really just ints.
I'm looking to prevent switch-yards -- Java enums would be perfect.
Objective-C is just C with some additional markup for objects, no new types have been added.
That means, no.
For mutual exclusive flags, Apple uses strings.
header.h
extern NSString * const kNSSomeFlag;
extern NSString * const kNSOtherFlag;
extern NSString * const kNSThirdFlag ;
code.m
NSString * const kNSSomeFlag = #"kNSSomeFlag";
NSString * const kNSOtherFlag = #"kNSOtherFlag";
NSString * const kNSThirdFlag = #"kNSThirdFlag";
…
void myFunction(NSString *flag)
{
if (flag == kNSSomeFlag) {
// the code
}
}
An example of this can be found in the NSDistributedNotificationCenter.
I have defined a protocol that all my plug-ins must implement. I would also like the plug-ins to all use certain strings, like MyPluginErrorDomain. With integers this is quite easily achieved in an enum, but I can't figure out how to do the same with strings. Normally, in classes I would define
extern NSString * const MyPluginErrorDomain;
in the .h file and in the .m file:
NSString * const MyPluginErrorDomain = #"MyPluginErrorDomain";
but that doesn't work very well in a protocol, because then each plug-in would have to provide its own implementation which defeats the purpose of having a constant.
I then tried
#define MYPLUGIN_ERROR_DOMAIN #"MyPluginErrorDomain"
but the implementing classes in the plug-in can't seem to see the #define. Who knows a good solution?
You can declare them in the header with the protocol (but outside the protocol interface itself), then define them in an implementation file for the protocol (obviously it wouldn't have an #implementation section - just your NSString definitions).
Or have a separate .h/.m pair that is just for the string constants (the protocol header can import the string constants header).
You keep the .h definition:
extern NSString * const MyPluginErrorDomain;
but put this part into a separate .m file that gets included in your framework:
NSString * const MyPluginErrorDomain = #"MyPluginErrorDomain";
So plug-ins can still implement the interface but when compiling they link or compile in your other .m file, so they will see the value of MyPluginErrorDomain.
In C++, I would declare them in a header like this:
const char * const MYPLUGIN_ERROR_DOMAIN = "MyPluginErrorDomain";
const char * const MYPLUGIN_FOO_DOMAIN = "MyPluginFooDomain";
Note that as the pointers are const, they will be local to the translation units the header is #included in, and so there will be no need to use extern to prevent multiple definition errors.
You should implement it as extern strings as in your example:
extern NSString * const MyPluginErrorDomain;
or provide extern functions which return static storage data. For example:
/* h */
extern NSString * MyPluginErrorDomain();
/* m */
NSString * MyPluginErrorDomain() {
static NSString * const s = #"MyPluginErrorDomain";
return s;
}
The reason is that strings and keys are often used and compared by pointer value or hash value, rather than true string comparison (isEqualToString:).
At the implementation level, there is a big difference between:
In code, that means that when the strings compared are defined in multiple binaries:
Say 'MyPluginErrorDomain' and 'key' have identical string values, but are defined in different binaries (i.e. on in the plugin host, one in the plugin).
/////// Pointer comparison (NSString)
BOOL a = [MyPluginErrorDomain isEqualToString:key];
BOOL b = MyPluginErrorDomain == key;
// c may be false because a may be true, in that they represent the same character sequence, but do not point to the same object
BOOL c = a == b;
/////// Hash use (NSString)
// This is true
BOOL d = [MyPluginErrorDomain hash] == [key hash];
// This is indicative if true
BOOL e = [MyPluginErrorDomain hash] == [someOtherStringKey hash];
// because
BOOL f = [MyPluginErrorDomain isEqualToString:someOtherStringKey];
// g may be false (though the hash code is 'generally' correct)
BOOL g = e == f;
It is therefore necessary to provide the keys in many cases. It may seem like a trivial point, but it is hard to diagnose some of the problems associated with the difference.
Hash codes and pointer comparisons are used throughout Foundation and other objc technologies in the internals of dictionary storage, key value coding... If your dictionary is going straight out to xml, that's one thing, but runtime use is another and there are a few caveats in the implementation and runtime details.