Programming concept regarding objective-c

Question

I have a confusion regarding programming. What difference does it make in terms of memory if we do this in objective-c:

+(NSString *)getName {
  NSString *name = @"Hello";
  return name;
}

OR

+(NSString *)getName {
  return @"Hello";
}

Is both same or is there any difference in terms of speed and performance?

actually i thought that the 1st one will initiate an string object and then store info in it and then return it but the second will not create a string object. — Dhruv Jindal
– Dhruv Jindal, Commented Feb 4, 2014 at 11:52
That object is never used or referenced anywhere, so it's going to be removed during optimisation. — David Rönnqvist
– David Rönnqvist, Commented Feb 4, 2014 at 11:53
As a side note, getters in objective-C are named without 'get'. So your getter would be called -(NSString *)name; — Joride
– Joride, Commented Feb 4, 2014 at 11:55
You might find these wiki pages interesting: Program Optimization and Optimizing Compiler — David Rönnqvist
– David Rönnqvist, Commented Feb 4, 2014 at 11:58

DarkDust · Accepted Answer · 2014-02-04 12:09:04Z

8

The compiler will optimize the first example into the second example since the variable isn't used for anything else. So they are equivalent: none is faster, none saves any memory.

Edit:

So, I actually tried it and compared the assembler output.

Code used:

@implementation Test

- (NSString *)test1 {
        NSString *variable = @"Hello1";
        return variable;
}

- (NSString *)test2 {
        return @"Hello2";
}

@end

Compiler used:

Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn) Target: x86_64-apple-darwin13.0.0

With no optimization (-O0), test1 does indeed have code for the unused variable (movq %rax, -24(%rbp) and movq -24(%rbp), %rax, so one additional memory write and read). But already at -O1 the variable is optimized away (as are the reads for the internal self and _cmd variables).

So in other words: with -O0 (no optimization), test1 is indeed slower than test2. But if optimizations are switched on, they are equivalent and result in the same code.

edited Feb 4, 2014 at 12:09

answered Feb 4, 2014 at 11:59

DarkDust

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1 Comment

iPatel Over a year ago

Thanks for clear me ... :) i thinking that first one create object of string which useless.

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