nil, Nil, NULL, NSNull
nshipster.com
nshipster.com
// For example, this expression...
if (name != nil && [name isEqualToString:@"Steve"]) { ... }
// ...can be simplified to:
if ([name isEqualToString:@"steve"]) { ... }
if ([name isDifferentFromString:@"bob"]) { ... }I'd argue this is the way most methods are naturally written, so it doesn't have too much of an impact.
I'm struggling to think of an example where logically returning true for a nil receiver is a much more natural than the other way around. Most of them, including your example of isEqualToString vs. isDifferenceFromString, are at best equal.
It's probably something that should be made more explicit in discussions about nil swallowing, such as the OP.
I always check for nil explicitly, personally. It sidesteps any question of whether the method makes sense in the nil case. And sending messages to nil can't work in general, because the return value could be a struct.
I agree there isn't really a sensible answer for [nil isEmpty].
There are two uses of isEmpty I can think of.
if ([container isEmpty]) {
[container addItem:item];
}
if (![container isEmpty]) {
Item item = [container getItemAt:0];
[item doSomething];
}
NSArray doesn't have an isEmpty method. I wonder if this is one of the reasons why, or if it's just something like wanting to keep the interface small.
[Edit] You can of course invert the logic. Using [container hasItems] in the two examples above would function just fine. While isEmpty seems to be the variant used everywhere, hasItems doesn't seem too unnatural.
As for how you'd use it, I've pretty much always used it for early outs:
if(items.empty()) return NULL;
return &items[items.size()%rand()];
if(items.empty()) return NULL;
if(cache.empty()) { /* fill cache */ }
assert(cache[index].underlying_item==items[index]);
return cache[index].some_other_data;
I can't think of a compare method in the Objective-C core lib (the biggest nil-swallowing lib I know of). Does anyone know of one? Would be interesting to see how it's handled and if it causes issues.
You could return an enum, which would allow you to use non-obvious values (Lower = 1, Equal = 2, Higher = 3), though this would make it a bit of a pain to interface with C-style libs.
Obviously the presence of a work-around doesn't mean nil-swallowing is a good idea, but rather cements the idea that library authors need to put in extra effort to make sure nil receivers are handled appropriately.
Maybe I misunderstood you, but there is plenty of compare:'s in Cocoa: http://cl.ly/image/1a112f1B1W1g.
They all return an enum that's either -1, 0 or 1.
Also, nils aren't allowed in the collection classes, so sorting with compare: will simply never encounter this case.
if (![missleLauncher isDisabled]) { /* declare thermonuclear war */ }
When missleLauncher is nil, thermonuclear war is still declared which may not be the programmer's intent.
In Objective-C, the question "should I name my method isEnabled or isDisabled?" has a reasonable answer - you consider what makes most sense (or any sense) when called on Nil.
This is probably a good rule for other languages too - "negative" booleans can lead to double-negatives and unreadable code.
I'm not saying the example is correct, I'm pointing out that "I've never heard of it" / "Works on my computer" are dangerous attitudes to have.
Of course, there is one place where you have to go the other way: -isNil: does not work, you need to use -isNotNil
Unless you use the runtime's (private) nil-hook:
+(void)setNilHandler
{
installed=YES;
_objc_setNilReceiver([self nsNil]);
}
static id idresult( id receiver, SEL selector, ... ) { return nil; }
+(BOOL)resolveInstanceMethod:(SEL)selector
{
class_addMethod(self, selector, (IMP)idresult , "@@:@");
return YES;
} NSString *val;
if(something){
val = @"hello";
}else if(somthingelse){
val = @"goodbye";
}
if(val){
[self display:val];
}
"Using ARC, strong, weak, and autoreleasing stack variables are now implicitly initialized with nil."
This doesn't affect plain C variables, but it would solve the issue you had above.
Member variables of a class are set to zero by (init or alloc? can't remember.)
package main
import "fmt"
func main() {
var z []int = make([]int, 0, 0) // jerf changed this line
fmt.Println(z, len(z), cap(z))
if z == nil {
fmt.Println("nil!")
} else {
fmt.Println("initialized slice is not nil")
}
// This won't even compile; "cannot convert nil
// to type int" at the if clause:
// var a int = 0
// if a == nil {
// fmt.Println("jerf is wrong; nil does == 0")
// }
}
"C represents nothing as 0 for primitive value"
That's not true. There is no "nothing" for integer types, and floats either also lack "nothing", or have NaN as their "nothing", depending on your perspective.
One of the (many) annoying features of C-style NULL is that it means that some types are effectively option types while other types aren't, and there's no decent way to make them so. An NSString* is inherently "pointer to NSString or nothing", but an int is always an int, and I have to either carve out a special value to mean "nothing" (e.g. -1 used as the error return from a lot of UNIX syscalls) or use a separate flag.
But yeah, point well-taken. Perhaps I'll take the opportunity to wax on the other special values used in Foundation & CoreFoundation sometime.
Or go to Wikipedia: 'The wThe word zero came via French zéro from Venetian zero, which (together with cypher) came via Italian zefiro from Arabic صفر, ṣafira = "it was empty", ṣifr = "zero", "nothing". ord zero came via French zéro from Venetian zero, which (together with cypher) came via Italian zefiro from Arabic صفر, ṣafira = "it was empty", ṣifr = "zero", "nothing".'
Now, you should step back for a second and think about what the value 0 means in a program. Take the following function:
int secondsUntilNextEvent()
An option type gives you a different state for "there is no value here, at all". Putting a zero into an int certainly does not signify "no value". It signifies a value, and that value is zero.
What number is greater than three but less than one? Nothing. There is no such number. "0" is not a correct answer.
You write: 'Putting a zero into an int certainly does not signify "no value". It signifies a value, and that value is zero.'
Again, the way you write this makes it seem like you say that this has always been the case and is the only way it could be, when in fact this idea of zero being "just another value" is a fairly recent one. And the way our machines handle scalar values (and where what you write is largely true) is even more recent and more of a special case.
'What number is greater than three but less than one? Nothing. There is no such number. "0" is not a correct answer.'
'Nothing' is also not the correct answer. What you have is a contradiction, which is not nothing. Or you could talk about the possible results as sets, in which case you have the empty set, which is also not nothing. It does have the cardinality 0, though.
"The empty set is not the same thing as nothing; rather, it is a set with nothing inside it and a set is always something. "
Of course, the empty set can be used to signify "nothing", just as the number zero can.
"The number zero is sometimes used to denote nothing. The empty set contains no elements."
Again, not arguing that you can't have contexts in which "nothing" and "zero" are distinct. Just pointing out that those contexts are hardly universal enough to justify a statement saying that nothing really isn't zero.
Things are a bit more complicated and less clear-cut than that.
Which is why I always liked C's and Objective-C's somewhat loose but intuitively (for me!) workable handling of NULL, 0, nil, false etc.
Zero is not nothing. Sometimes it means nothing, which is not the same thing.
C pointer types are always option types. They can hold "nothing" or a pointer. Code that deals with pointers always has to deal with this option nature of the types, whether it wants the feature or not.
C primitive types are never option types. They can never hold "nothing". Some of them can hold a value that is sometimes used to represent "nothing", but again, that is not the same thing. Code that deals with primitive types and wants to be able to represent "nothing" either has to keep a separate flag or borrow a value from the primitive type's range to indicate "nothing" without any language support for that.
In short: I object to C mixing option types with pointer types. Life would be simpler if they were separate. NULL (nil etc.) is how C indicates "none" for pointers. C has no built-in indicator for "none" for primitives.
Yes. In a computer, there is nothing that is nothing. You always have something that signifies nothing.
C pointers also do not hold "nothing". What you refer to as "nothing" is the address 0, which is a special value that the language makes certain guarantees about that you interpret as being "nothing" because of these guarantees.
Of course. The point is that pointers have that "special value", while primitives do not. The integer value 0 is not the same thing.
However, 'int *' is inherently "pointer to int or nothing" (in the same style as your NSString example), so I'm not sure I agree with your assertion that some types are option types while some aren't in C. Any type can be an option type (in your sense of the term, again a la your NSString example) by making it into a pointer.
Similarly, using an NSString without a pointer (which might not be possible in ObjC, but definitely possible for regular C/C++ objects) makes it not optional, like the 'int'.
(Using the prevailing terminology. I'm not sure I'm comfortable with this use of the term "option type", but I'm rolling with it for now.)
if(!this) return false; // Or whatever
I think you mean non-virtual method, not 'class method' or static method which wouldn't have a this pointer.
Naming things is always difficult, but I meant class method, although what I should have said was non-virtual class method, which is possible in C++. Both virtual and non-virtual have "this" pointers, the difference is whether the class method to call is looked up at runtime or compile-time, so I still would consider both "methods". I didn't learn OOP from Smalltalk so I may be abusing the terminology (it's not intentional).
"Class method" is usually used to refer to methods that class objects have themselves, as opposed to an "instance method", which is a method that instances of a class possess. C++ doesn't have class objects or class methods.
class X
{
int a;
public:
X() : a(123) { }
int get_a() { return this ? a : 1; }
};
class Y
{
int b;
public:
Y() : b(456) { }
int get_b() { return this ? b : 2; }
};
class Z : public X, public Y
{
int c;
public:
Z() : c(789) { }
int get_c() { return this ? c : 3; }
};
#include <iostream>
int main()
{
Z *z = NULL;
std::cout << z->get_a() << '\n';
std::cout << z->get_b() << '\n';
std::cout << z->get_c() << '\n';
}In the C Standard, an implementation can define NULL as either 0 or (void * ) 0. For an example of how that can go terribly wrong, consider the following:
printf ("%p\n", NULL)NULL should be defined as ((void * )0) for C always (and you should define your own NULL to enforce that if necessary in varargs functions). This is safe because C allows conversion from void * to other pointer types.
C++ does not allow conversion from void * to other pointer types, so what you were supposed to do was use plain 0, which introduced exactly the issue you mention (and which has bitten me before trying to use a C library from C++, where NULL was defined as plain 0).
With C++11 you should use nullptr, which does the right thing. Some implementations define NULL in C++ to a vendor-provided symbol which emulates nullptr so it might be safe to use NULL. If you have C++98 only and don't want to risk NULL then you should use static_cast<T*>(0) (where T is the actual pointer type), which is admittedly clunky.
So the bug you mention which bit you by using "C library from C++" was not caused by a difference between C and C++. It was caused by a difference in the definition of NULL between those two compilers. You could just as easily have found a C++ compiler which did work, or a C compiler which didn't.
class Foo { };
int main() {
Foo *foo = malloc(sizeof(Foo)); // returns void*
} Class _myClass;
// aClassName was previously defined
if((_myClass = NSClassFromString(aClassName)) == Nil)
[NSException raise:@"InvalidClass"
reason:@"Unknown class named %@.", aClassName);
id myObj = [[_myClass alloc] init];
[myObj whatever];Back when the book *"Effective Java" came out, a truly great advice was to use empty arrays everywhere you could instead of null.
The second enlightenment came to me when the smart brains at JetBrains decided to ship IntelliJ IDEA with the @NotNull annotation. I basically started to annotate as many methods returning something as @NotNull and then saw the number of NullPointerException go drastically down.
It's not possible everywhere but once you start to think about it, you realize that you don't need nearly as many null references as you think you did.
Then you started having IntelliJ IDEA warning you real-time (even on incomplete source file) about "reference xxx is never going to be null" (so the non-null check is pointless) or "warning: reference to xxx may be null". Great stuff. Years later there are still 99% of all the Java codebase not using the @NotNull annotation. Sad but true.
Now of course other language's take on the subject are interesting too: the maybe monad, the way Clojure deals with empty / nil sequences, etc.
Null as a valid value for every reference type by default is brain-dead.
It is one of my major gripes with modern C#.