Question

I am new to iOS so take me slow. When i declare an object in my .h view controller named “_a” and i declare a property “a” and when i synthesize in the .m file

@synthesize a=_a;

must i use “a” or “_a” when i modify that object ? ( “a” is a UINavigationController in my case).

In another question, does my compiler automatically draw a connection from a object declared “ob” to a “_ob” declaration ?

Again, sorry for the poor explanation but this environment isn’t quite something i am use to.

Answer 1

An object declared like this:

@interface Example : NSObject {
    NSObject *_a;
}
@property (retain) NSObject *a;
@end

And implemented like this:

#import "Example.h"

@implementation Example
@synthesize a = _a;
@end

Makes an ivar named _a and two accessor methods in the Example object. The accessor methods have these signatures:

- (NSObject *)a;
- (void)setA:(NSObject *)theA;

Method a returns the object in the _a ivar. Method setA releases the object stored in _a (if not nil), assigns the parameter to _a, and sends the parameter an retain message.

These methods may also be access through dot notation:

Example *e = [[Example alloc] init];
// These two are equivalent. 
e.a = anotherNSObject;
[e setA:anotherNSObject];
// These two are equivalent.
anotherNSObject = e.a;
anotherNSObject = [e a];

Accessing _a directly will circumvent the accessor methods, potentially causing problems such as memory leaks. For example if _a holds the only reference to an object and a new object reference is assigned to _a the old object will become a leaked object.

To directly answer your two questions:

You may use either a or _a. In most cases you’ll be better off using _a when reading the value within methods of the object declaring a, and setA (or a in dot notation) when setting the value of _a. Objects that use Example objects should use the accessor methods (with or without dot notation).

The complier does not automatically make a connection between ob and _ob declarations. In this example the @synthesize a = _a; statement makes the connection with the optional = _a. The ivar may have any name. @synthesize a = george; would also be valid. Without the = _a part the compiler would make an ivar named a and two accessor methods.

One further note: You may omit the declaration of _a in the interface, which restricts the scope of the _a ivar to just the implementation of the Example object. Adding the optional = _a to the @synthesize statement will make as ivar of the same type as the property declared in the interface.