Assigning a value to id doesn't stick - objective-c

Question

I'm reading a plist from a file and want to set a number of properties to the values from the file, but only if they're not nil, so I came up with a little method, but it doesn't work as expected:
- (void)assignNonNullValue:(id)value to:(id)target
{
if (value) {
target = value;
NSLog(#"new value = %#", target);
}
}
// ...
NSString *foo = #"a";
[self assignNonNullValue:#"b" to:foo];
NSLog(#"foo=%#", foo);
I was expecting the following output:
new value = b
foo=b
but instead I got:
new value = b
foo=a
Why?

Answer 1

Here's another approach:
- (id)valueOrOriginalIfNull:(id)value original:(id)original {
if (value) {
return value;
} else {
return original;
}
}
// ...
NSString *foo = #"a";
foo = [self valueOrOriginalIfNull:#"b" original:foo];
NSLog(#"foo=%#", foo);
The reason your original approach fails is that parameters are passed by value. When you assign target to another value inside the method, it has no affect on the original value.

Answer 2

You're changing target value in your method. It won't affect value of foo in another scope. It want to change foo value, you need to pass your method where it points to. So that, when you change in a method, it'll affect foo value as you're changing the value it points to. To clarify my explanation, here is the code
- (void)assignNonNullValue:(id)value to:(id *)target
{
if(value != nil)
*target = value;
}
It's really really rare that you use id * as id is a already pointer to an object. .Passing id * as a parameter allows you to change the value where it points to. So that, it's going to be permanent.
After you've made the changes i pointed. You need to call this method as
[self assignNonNullValue:#"b" to:&foo];