Objective-C中圆括号的奇怪行为

Question

i was wondering why my code not work as i suppose to. Please consider the following code:

- (NSString *)bake {
    return [NSString stringWithFormat:(@"Take %@, put it in oven for 30 minutes, add 250 gramm of %@ cheese, many %@ toppings and little drop of %@ sauce", [self dough], [self cheese], [self toppings], [self sauce])];
}

- (NSString *)cut {
    return [NSString stringWithFormat:(@"Cut in 4 pieces")];
}

- (NSString *)box {
    return [NSString stringWithFormat:(@"Box pizza. Ready for sale!")];
}

- (NSString *)orderString {
    return [NSString stringWithFormat:@"%@ %@ %@", [self bake], [self cut], [self box]];
}

Pretty straightforward. Now, output result in console is (that is what orderString contain):

Tobasko Cut in 4 pieces Box pizza. Ready for sale!

Look like for some reason, we got only last word from text contained in bake NSString . And compiler even warn this line of code with 2 "yellow" warnings:

Format string is not a string literal (potentially insecure)

Expression result unused

I solve that problem simply by remove round brackets like that:

return [NSString stringWithFormat:@"Take %@, put it in oven for 30 minutes, add 250 gramm of %@ cheese, many %@ toppings and little drop of %@ sauce", [self dough], [self cheese], [self toppings], [self sauce]];

However, i don't understand, why compiler truncate my string when i simply put statement in round brackets?

Answer 1

The parentheses (not round brackets) group everything inside it into an expression. It useful to change the precedence of certain operators or to make code more readable.

In C based languages (including Objective-C), the value of a series of comma separated expressions is the value of the last expression in the list.

The parentheses in your line end up grouping the separate parameters to the stringWithFormat: method into a single argument instead of the expected separate set of arguments.

So the value of:

(@"Take %@, put it in oven for 30 minutes, add 250 gramm of %@ cheese, many %@ toppings and little drop of %@ sauce", [self dough], [self cheese], [self toppings], [self sauce])

is just the value of the last one:

[self sauce]

So in affect, your line is really just:

return [NSString stringWithFormat:[self sauce]];

which is why you get the two warnings and why it only returns Tobasko .

Further, the use of NSString stringWithFormat: should always have at least two parameters. The first should be a string with at least one format specifier and the remaining parameters should correspond to each of the format specifiers in the format string.

So code like:

return [NSString stringWithFormat:(@"Cut in 4 pieces")];

Has two problems. 1. Pointless parentheses and 2. There's no need to use stringWithFormat: . Just use:

return @"Cut in 4 pieces";

Answer 2

I think to understand what is happening, we need to break it down into 3 aspects:

1. [NSString stringWithFormat:] takes 1 or more arguments. You can pass in just a string, with no other arguments, and it will return just that string. If you have format descriptors in the string (eg %@ , %d , etc.) then a matching number of additional parameters are required. But again, a single string with no format descriptors is a valid parameter.

2. The parentheses () signify that the contents inside the parentheses should first be evaluated and then used as a single expression

3. The comma operator in C means, roughly, "evaluate these expressions from left to right and return the value of the rightmost expression"

http://crasseux.com/books/ctutorial/The-comma-operator.html

So what is happening in your "bake" method is:

1. Evaluate the contents of the parentheses first as a single expression
2. Since the contents of the parentheses are comma-separated, evaluate each expression left to right, and return the value of the rightmost expression (in this case [self sauce] , which evaluates to @"Tabasko" )
3. Use this single expression as the parameter for [NSString stringWithFormat:] , ie [NSString stringWithFormat:@"Tabasko"];

All of your other methods before "orderString" are just single string parameters anyway, so the parentheses don't make a difference in the final result.