Objective-c – Attribute introspection to get a variable in Objective-C

objective c

Given a variable id x and a string NSString *s how can I get the instance attribute with name s for variable x?

ie. If we write NSString *s=@"a", then we want x.a

Best Answer

Ivar class_getInstanceVariable(Class cls, const char * name)

which returns an opaque type representing an instance variable in a class. You then pass that to

id object_getIvar(id object, Ivar ivar)

to get the actual instance variable. So you could say

#import <objc/runtime.h>

id getInstanceVariable(id x, NSString * s)
{
    Ivar ivar = class_getInstanceVariable([x class], [s UTF8String]);
    return object_getIvar(x, ivar);
}

if the instance variable is an object. However, if the instance variable is not an object, call

Ivar object_getInstanceVariable(id obj, const char * name, void ** outValue)

passing in a pointer to a variable of the right type. For example, if the instance variable is an int,

int num;
object_getInstanceVariable(x, [s UTF8String], (void**)&num);

will set num to the value of the integer instance variable.

An option:

[NSString stringWithFormat:@"%@/%@/%@", one, two, three];

Another option:

I'm guessing you're not happy with multiple appends (a+b+c+d), in which case you could do:

NSLog(@"%@", [Util append:one, @" ", two, nil]); // "one two"
NSLog(@"%@", [Util append:three, @"/", two, @"/", one, nil]); // three/two/one

using something like

+ (NSString *) append:(id) first, ...
{
    NSString * result = @"";
    id eachArg;
    va_list alist;
    if(first)
    {
        result = [result stringByAppendingString:first];
        va_start(alist, first);
        while (eachArg = va_arg(alist, id)) 
        result = [result stringByAppendingString:eachArg];
        va_end(alist);
    }
    return result;
}

Ios – Constants in Objective-C

You should create a header file like

// Constants.h
FOUNDATION_EXPORT NSString *const MyFirstConstant;
FOUNDATION_EXPORT NSString *const MySecondConstant;
//etc.

(you can use extern instead of FOUNDATION_EXPORT if your code will not be used in mixed C/C++ environments or on other platforms)

You can include this file in each file that uses the constants or in the pre-compiled header for the project.

You define these constants in a .m file like

// Constants.m
NSString *const MyFirstConstant = @"FirstConstant";
NSString *const MySecondConstant = @"SecondConstant";

Constants.m should be added to your application/framework's target so that it is linked in to the final product.

The advantage of using string constants instead of #define'd constants is that you can test for equality using pointer comparison (stringInstance == MyFirstConstant) which is much faster than string comparison ([stringInstance isEqualToString:MyFirstConstant]) (and easier to read, IMO).

Best Answer

Related Solutions

Objective-c – Shortcuts in Objective-C to concatenate NSStrings

An option:

Another option:

Ios – Constants in Objective-C

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