objective c - Difference between literals and class methods for NSMutableArray and NSMutableDictionary -

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• is literal creation of nsmutabledictionary less efficient class helper method? 1 answer

when started osx/ios used

    nsmutablearray *        a1 = [nsmutablearray arraywithcapacity:123] ;     nsmutabledictionary *   d1 = [nsmutabledictionary dictionarywithcapacity:123] ; 

then discovered 'simpler' version

    nsmutablearray *        a2 = [nsmutablearray array] ;     nsmutabledictionary *   d2 = [nsmutabledictionary dictionary] ; 

i have moved to:

    nsmutablearray *        a3 = [@[] mutablecopy] ;     nsmutabledictionary *   d3 = [@{} mutablecopy] ; 

functionally, seem identical: once initialized either way, can used regardless of how created. differences?

in particular, should assume d3/a3 more similar d2/a2 d1/a1 in terms of memory pre-allocation (or lack thereof) ?

or matter of style?

form 1 class factory method optimization useful when know how large collection needs or know size constant content may change.

form 2 class factory method equivalent calling new. may or may not same alloc followed init, same.

form 3 implicitly same alloc followed initwitharray: or initwithdictionary: respectively. it's convenient generates unneeded immutable instance discarded under arc may not clear when discarded.

use form 1 or form 2 if not going ever use immutable instance again elsewhere.