Swift string through string literal versus initializer

Question

In other languages, such as Java, under the hood there is actually a difference between a string obtained using a string literal and an initializer. In Swift, are they equivalent under the hood?

eg.

var string:String = "" var string:String = String() 

Refer to this SO post for information on the differences between a literal and an object in Java.

Answer 1

Declarations are equivalent according to Apple docs:

Initialization of an empty line

To create an empty String value as a starting point for building a longer string, either assign an empty string literal to the variable, or initialize a new instance of String with the initializer syntax:

 var emptyString = "" // empty string literal var anotherEmptyString = String() // initializer syntax // these two strings are both empty, and are equivalent to each other 

Link: https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html

If we look at the assembly, we will see that the two constructors use the same instructions.

string.swift:

 let str = String() let str2 = "" 

Compiled assembly ( swiftc -emit-assembly string.swift ):

  .section __TEXT,__text,regular,pure_instructions .macosx_version_min 14, 3 .globl _main .align 4, 0x90 _main: .cfi_startproc pushq %rbp Ltmp0: .cfi_def_cfa_offset 16 Ltmp1: .cfi_offset %rbp, -16 movq %rsp, %rbp Ltmp2: .cfi_def_cfa_register %rbp subq $16, %rsp movq _globalinit_33_1BDF70FFC18749BAB495A73B459ED2F0_token4@GOTPCREL (%rip), %rax movq _globalinit_33_1BDF70FFC18749BAB495A73B459ED2F0_func4@GOTPCREL (%rip), %rcx xorl %edx, %edx movl %edi, -4(%rbp) movq %rax, %rdi movq %rsi, -16(%rbp) movq %rcx, %rsi callq _swift_once movq _globalinit_33_1BDF70FFC18749BAB495A73B459ED2F0_token5@GOTPCREL (%rip), %rdi movq _globalinit_33_1BDF70FFC18749BAB495A73B459ED2F0_func5@GOTPCREL (%rip), %rax xorl %r8d, %r8d movl %r8d, %edx movq __TZvOSs7Process5_argcVSs5Int32@GOTPCREL (%rip), %rcx movl -4(%rbp), %r8d movl %r8d, (%rcx) movq %rax, %rsi callq _swift_once movq __TZvO Ss7Process11_unsafeArgvGVSs20UnsafeMutablePointerGS0_VSs4Int8__@ GOTPCREL(%rip), %rax movq -16(%rbp), %rcx movq %rcx, (%rax) callq __TFSSCfMSSFT_SS leaq L___unnamed_1(%rip), %rdi xorl %r8d, %r8d movl %r8d, %esi movl $1, %r8d movq %rax, __Tv6string3strSS(%rip) movq %rdx, __Tv6string3strSS+8(%rip) movq %rcx, __Tv6string3strSS+16(%rip) movl %r8d, %edx callq __TFSSCfMSSFT21_builtinStringLiteralBp8byteSizeBw7isASCIIBi1__SS xorl %r8d, %r8d movq %rax, __Tv6string4str2SS(%rip) movq %rdx, __Tv6string4str2SS+8(%rip) movq %rcx, __Tv6string4str2SS+16(%rip) movl %r8d, %eax addq $16, %rsp popq %rbp retq .cfi_endproc .globl __Tv6string3strSS .zerofill __DATA,__common,__Tv6string3strSS,24,3 .globl __Tv6string4str2SS .zerofill __DATA,__common,__Tv6string4str2SS,24,3 .section __TEXT,__cstring,cstring_literals L___unnamed_1: .space 1 .no_dead_strip __Tv6string3strSS .no_dead_strip __Tv6string4str2SS .linker_option "-lswiftCore" .section __DATA,__objc_imageinfo,regular,no_dead_strip L_OBJC_IMAGE_INFO: .long 0 .long 512 .subsections_via_symbols 

Note that the declarations for str and str2 have the same instructions:

 xorl %r8d, %r8d movl %r8d, %esi movl $1, %r8d movq %rax, __Tv6string3strSS(%rip) movq %rdx, __Tv6string3strSS+8(%rip) movq %rcx, __Tv6string3strSS+16(%rip) movl %r8d, %edx # ... xorl %r8d, %r8d movq %rax, __Tv6string4str2SS(%rip) movq %rdx, __Tv6string4str2SS+8(%rip) movq %rcx, __Tv6string4str2SS+16(%rip) movl %r8d, %eax 

You can learn more about string literals by looking at the Apple documentation .