Testing for nil in Objective-C -- if(x != nil) vs if(x)

Most of the examples I found on the net write this:

if(x != nil)
    // ...

开发者_Python百科Is there any problems with this?

if(x)
    // ...

I tried both in a simple program and couldn't found any difference.

In Objective-C, nil is defined as a value called __DARWIN_NULL, which essentially evaluates to 0 or false in if-statements. Therefore, writing if (x == nil) is the same as writing if (!x) and writing if (x != nil) is equal to if (x) (since comparing to false creates a negation, and comparing to true keeps the condition the same).

You can write your code either way, and it really depends on which you think is more readable. I find if (x) to make more sense, but it depends on your style.

It's like comparing if (someCondition == true) versus if (someCondition).
It all depends on you, and who's going to be reading the code.

Edit: As Yuji correctly mentions, since Objective-C is a superset of C, any condition that evaluates to a value other than 0 is considered to be true, and therefore, if someCondition in the example above were to evaluate to an integer value of, say, -1, comparing it to true would result in false, and the if-statement would not be evaluated. Something to be aware of.

Both

if (x != nil)

and

if ( x )

are equivalent, so pick the variant that in your opinion makes your code more readable for you (and others who will read and support your code)

Both are the same and this is a style question and it boils down to whether you prefer:

1. if (something) { ... }

   versus

2. if (something != nothing) { ... }

I have always found #1 more clear but #2 is used extensively in documentation and hence the field so it is better to both know both forms and adapt to what a project uses and be stylistically consistent.

The best and safe way to check nil is
Make a common method, and add all these null :

+ (NSString *)trimWhiteSpaceAndNewLine:(NSString *)string {
    NSString *stringSource = [NSString stringWithFormat:@"%@",string];
    if ([stringSource isEqualToString:@"(null)"]) {
        stringSource = @"";
        return stringSource;
    }
    if ([stringSource isEqualToString:@"<null>"]) {
        stringSource = @"";
        return stringSource;
    }
    if ([stringSource isEqualToString:@"<nil>"]) {
        stringSource = @"";
        return stringSource;
    }
    if ([stringSource isKindOfClass:[NSNull class]]) {
        stringSource = @"";
        return stringSource;
    }
    if ([stringSource isEqualToString:@""]) {
        stringSource = @"";
        return stringSource;
    }
    if (stringSource == nil) {
        stringSource = @"";
        return stringSource;
    }
    NSString *stringFinal = [stringSource stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
    return stringFinal;
}  

And check

NSString *strUuid = [Common trimWhiteSpaceAndNewLine:[dict valueForKeyPath:@"detail.uuid"]];
        if (![strUuid isEqualToString:@""]) {
            // do your stuff
        }