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Creating instances of a class in Objective C

  1. May 17, 2012 #1
    I'm relatively new to Objective C. I know I'm using C strings instead of NSStrings, but I'm more comfortable with them. And I know this is probably a bad example of Object-oriented programming, but this is for the sake of example.

    Code (Text):
    #import <Foundation/Foundation.h>

    @interface Greeter : NSObject
    {
        char* name;
    }
    -(void) setname : (char*) newname;
    -(char*) getname;
    -(void) greet;
    -(void) ask;
    @end

    @implementation Greeter
    -(void) setname : (char*) newname
    {
        name = newname;
    }
    -(char*) getname
    {
        return name;
    }
    -(void) greet
    {
        printf("Hello, %s!\n",name);
    }
    -(void) ask
    {
        printf("What's your name?\n");
        scanf("%s",name);
    }
    @end

    int main()
    {
        @autoreleasepool
        {
            Greeter* me;
            [me ask];
            [me greet];
        }
    }
    Nothing happens. I'm not completely sure why not, but again, I'm new to Objective C's approach to Object Oriented programming.
     
  2. jcsd
  3. May 17, 2012 #2

    jedishrfu

    Staff: Mentor

    did you try putting in the args for your main method

    int main ( int argc, const char *argv[] )

    that may solve your problem especially if the error returned was can't find main method.
     
  4. May 17, 2012 #3
    No, I didn't get an error message, and int main() is perfectly valid (though some use int argc,const char* argv[] since it's useful for running the program for the command line.) The problem is that when I run it, nothing happens.
     
  5. May 17, 2012 #4

    jedishrfu

    Staff: Mentor

    in many OO languages main() is different from main(...with args...) this is how they distinguish between methods for polymorphism to work ie the methodname+args is the signature that is searched for and called on a match.

    okay so put a pritf in the main method before the autorealeasepool stuff and see if it prints out:

    printf("Entering main()...");

    and

    maybe an exiting printf:

    printf("Exiting main()...");

    this will tell you if main is being called or not and that the autoreleasepool is the issue.
     
  6. May 17, 2012 #5
    No, the following has worked perfectly:

    Code (Text):
    #import <Foundation/Foundation.h>

    int main()
    {
        @autoreleasepool
        {
            printf("Hello world!");
        }
    }
     
  7. May 17, 2012 #6

    jedishrfu

    Staff: Mentor

    ahh okay do you need to get an instance of greeter?

    Greeter* me = [[Greeter alloc] init];
     
  8. May 17, 2012 #7
    Also, I obviously don't know how the scanf function works, since I'm getting an error, so a pointer?
     
  9. May 17, 2012 #8

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    I've never used Objective C and I don't understand the details of the syntax, but all your problems seem to have the same cause: you declared some pointers to objects, but you never created any objects.

    You defimed "me" as a pointer to a Greeter, but it doesn't actually point to a Greeter object. Simiilarly inside the Greeter, "name" is a pointer to a character, but you haven't allocated any space to hold the characters and set "name" to point to that space. So if you are lucky, "scanf" will fail because "name" isn't pointing to a location it can write characters to. If you are unlucky, it will be pointing to a random place that you CAN write characters to but shouldn't, and you have taken the first small step on the road to writing some malware!

    I think you need to get straightened out about a basic rule of C: pointers are not arrays, despite the fact that some of the syntax of C makes it look as if they might be.
     
  10. May 17, 2012 #9
    Yea, I need to get my understanding of scanf straightened out. And for some reason, from my experiences, it appears messages are sent to pointers to objects, which I don't understand, but seems to be the case.
     
  11. May 17, 2012 #10

    jedishrfu

    Staff: Mentor

    yes, in c code for pointers you either malloc memory for it:

    char *x = malloc(24); // assigns memory but you will have to copy something to it

    strcpy("hello world",x);

    or you assign it to a variable:

    int y=3;
    int *z = &y;

    or for arrays:

    int xxx[100];
    int *pxxx = xxx; // notice no & since xxx is an array; equivalent to --> int *pxxx = &xxx[0];

    each time you increment pxxx++ it will internally add 2 bytes to the pointer effectively making the pointer point to the next element of the array.

    Good catch AlephZero, I was focusing on the Greeter object and didn't look further. I mostly do Java coding and haven't done much C since maybe 20 years ago.
     
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