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Article #1347: Manolo's Objective C "crash course"

Created at 13:22 Feb 06, 2014 by greg.ercolano

Manolo posted the following on fltk.general back on Dec 10, 2009:
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When you write in C++:
    
    object->memberfunction(arg1, arg2, arg3);
    
..in objective-c this would be:
    
    [object memberfunction:arg1 name-of-arg2:arg2 name-of-arg3:arg3];
    
..and this is called "sending the memberfunction:name-of-arg2:name-of-arg3: message to object".
This is declared by a prototype written as:
    
    @interface class-name : parent-class-name {
        class local variables;
    }
    - (return-type)memberfunction:(arg1-type)arg1-dummy  name-of-arg2:(arg2-type)arg2-dummy name-of-arg3:(arg3-type)arg3-dummy;
    @end
    
..and this stuff can, as in C++, return a value or not return anything.

All MAC OS defined class names begin by NS (which stands for "NextStep", Apple's older Unix-y OS),
and the subclasses I have defined by FL.

There is also the syntax:
    
    [classname memberfunction:arg1 name-of-arg2:arg2]
    
..that corresponds to a static member function:
    
    classname::memberfunction(arg1, arg2)
    
..and that usually allocates and returns an instance of this class (i.e., a class *).

One tends to nest things:
    
    [[NSEvent alloc] initWithData:data]
    
..reads, in C++:
    
    NSEvent *retval = NSEvent::alloc();
    retval->initWithData(data);
    
With this, you can read most of objective-c !
A tricky thing, though, is memory allocation.
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Erco adds the following.. not an Objective C programmer, just picking it up from Wikipedia and google,
so there may be errors here (if so, let me know in the comments, and I'll edit the OP):
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//
// EXAMPLE CLASS DEFINITION AND PROTOTYPES
//

@interface classname : superclassname {                  // C++: class classname : public superclassname { ..
   // INSTANCE VARIABLES GO HERE 
}
// THESE ARE "CLASS METHOD" PROTOTYPES (as denoted by '+')
+ classMethod1;                        // C++: static void* classMethod1(); 
+ (int)classMethod2;                   // C++: static int classMethod2(); 
+ (int)classMethod3:(float)arg1;       // C++: static int classMethod3(float arg1); 
//
// THESE ARE "INSTANCE METHOD" PROTOTYPES (as denoted by '-')
- (int)instanceMethod1:(float)arg1;                          // C++: int instanceMethod1(float arg1); 
- (int)instanceMethod2:(long)arg1 arg2_callName:(char)arg2;  // C++: int instanceMethod2(long arg1, char arg2) 
@end


//
// EXAMPLE CLASS IMPLEMENTATION
//

@implementation classname
- (int)instanceMethod1:(float)arg1
{
/* implementation of the method */
} 
                        
- (int)instanceMethod2:(long)arg1 arg2_callName:(char)arg2
{
/* implementation of the method */
} 
@end


CAVEATS
=======
AFAICT, there is no equivalent of default arguments for functions/methods in Objective C.
In other words, the C++ prototype:
    
    int someMethod(int a, int b, float gamma=2.0);
    
..has no equivalent in Objective C (since C doesn't support default args, neither does Objective C).
However, this can be emulated by defining two methods, one that takes the two arguments, and a second
that takes one, calling the first with the 'default' value, e.g.
    
    - (int)someMethod:(int)a b:(int)b gamma:(float)gamma { .. }
    - (int)someMethod:(int)a b:(int)b { [self someMethod:a b:b gamma:2.0]; }
    
Filename extensions:
    
    .m  -- Objective C (no C++ syntax allowed)
    .mm -- Objective C++ (allows a mix of C++ and Objective C, which is what we use in FLTK)
    

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