Swift string via string literal vs 初始值设定项

Question

在其他语言中，例如 Java，在幕后，通过字符串文字与初始化程序获得的字符串实际上是有区别的。在 Swift 中，它们在底层是等价的吗？

例如

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

请参阅此 SO post 了解有关 Java 中文字和对象之间差异的信息。

Answer 1

根据 Apple 文档，这些声明是等效的：

Initializing an Empty String

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

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

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

如果我们查看程序集，我们会发现两个构造函数使用相同的指令。

string.swift:

let str = String()
let str2 = ""

编译程序集（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    , %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    __TZvOSs7Process11_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    , %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    , %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

请注意 str 和 str2 的声明具有相同的说明：

xorl    %r8d, %r8d
movl    %r8d, %esi
movl    , %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

您可以通过查看 Apple's documentation.

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