Python for Bioinformatics: A peek into the Objective-C "runtime"

Thursday, January 27, 2011

A peek into the Objective-C "runtime"

I'm playing with Cocoa today. I ran across a post (here), reminding me that all our fancy "messages" get changed by the Objective-C "runtime." The real call is objc_msgSend and you can do that yourself if you want:

// gcc test.m -o test -framework Foundation
#import <Foundation/Foundation.h>

@interface MyClass:NSObject {}
-(NSString *) speak:(int) n;
@end

@implementation MyClass
-(NSString *) speak:(int) n {
    int i;
    NSMutableArray *mA;
    mA = [NSMutableArray arrayWithCapacity:n];
    for (i=0; i<n; i++) {
        [mA addObject:@"woof"];
    }
    return [mA componentsJoinedByString:@"-"];
    }
@end

int main(){
    NSAutoreleasePool *pool ;
    pool = [[NSAutoreleasePool alloc] init];
    MyClass *obj;
    obj = [[[MyClass alloc ] init] autorelease];
    NSLog(@"%@", [obj description]);
    NSLog(@"%@", [obj speak:2]);
    
    NSString *s;
    int count = 3;
    SEL sel = @selector(speak:);
    s = objc_msgSend(obj,sel,count);
    NSLog(@"%@", s);
    [pool drain];
    return 0;
}

Note: you have to use an int variable, you can't just pass an int directly to objc_msgSend.
Output:

> ./test
2011-01-27 13:21:37.890 test[38774:903] <MyClass: 0x10010cd20>
2011-01-27 13:21:37.892 test[38774:903] woof-woof
2011-01-27 13:21:37.893 test[38774:903] woof-woof-woof