What is the better way to check for an empty string in Delphi?

Question

A common condition that all programs should do is to check if string are empty or not.

Take the below statements:

(1)

if Length(Str)=0 then   // do something 

(2)

if Str='' then   // do something 

Answer 1

In XE2, if str = '' compiles to a better faster code for Ansi and Unicode strings. if Length(str) = 0 compiles to a better faster code for Wide strings.

Test program:

{$A8,B-,C+,D+,E-,F-,G+,H+,I+,J-,K-,L+,M-,N-,O+,P+,Q-,R-,S-,T-,U-,V+,W-,X+,Y+,Z1} program Project172;  {$APPTYPE CONSOLE}  {$R *.res}  var   sa1,sa2: AnsiString;   sw1,sw2: WideString;   su1,su2: UnicodeString;  begin   if Length(sa1) = 0 then     ;   if sa2 = '' then     ;   if Length(sw1) = 0 then     ;   if sw2 = '' then     ;   if Length(su1) = 0 then     ;   if su2 = '' then     ; end. 

Compiled code:

Project172.dpr.14: if Length(sa1) = 0 then 004050E2 A19C9B4000       mov eax,[$00409b9c] 004050E7 85C0             test eax,eax 004050E9 7405             jz $004050f0 004050EB 83E804           sub eax,$04 004050EE 8B00             mov eax,[eax] 004050F0 85C0             test eax,eax  Project172.dpr.16: if sa2 = '' then 004050F2 833DA09B400000   cmp dword ptr [$00409ba0],$00  Project172.dpr.18: if Length(sw1) = 0 then 004050F9 A1A49B4000       mov eax,[$00409ba4] 004050FE 85C0             test eax,eax 00405100 7407             jz $00405109 00405102 83E804           sub eax,$04 00405105 8B00             mov eax,[eax] 00405107 D1E8             shr eax,1 00405109 85C0             test eax,eax  Project172.dpr.20: if sw2 = '' then 0040510B A1A89B4000       mov eax,[$00409ba8] 00405110 33D2             xor edx,edx 00405112 E839E8FFFF       call @WStrEqual  Project172.dpr.22: if Length(su1) = 0 then 00405117 A1AC9B4000       mov eax,[$00409bac] 0040511C 85C0             test eax,eax 0040511E 7405             jz $00405125 00405120 83E804           sub eax,$04 00405123 8B00             mov eax,[eax] 00405125 85C0             test eax,eax  Project172.dpr.24: if su2 = '' then 00405127 833DB09B400000   cmp dword ptr [$00409bb0],$00 

The difference is even bigger if optimization is disabled.

Project172.dpr.14: if Length(sa1) = 0 then 004050E2 A19C9B4000       mov eax,[$00409b9c] 004050E7 8945EC           mov [ebp-$14],eax 004050EA 837DEC00         cmp dword ptr [ebp-$14],$00 004050EE 740B             jz $004050fb 004050F0 8B45EC           mov eax,[ebp-$14] 004050F3 83E804           sub eax,$04 004050F6 8B00             mov eax,[eax] 004050F8 8945EC           mov [ebp-$14],eax 004050FB 837DEC00         cmp dword ptr [ebp-$14],$00  Project172.dpr.16: if sa2 = '' then 004050FF 833DA09B400000   cmp dword ptr [$00409ba0],$00  Project172.dpr.18: if Length(sw1) = 0 then 00405106 A1A49B4000       mov eax,[$00409ba4] 0040510B 8945E8           mov [ebp-$18],eax 0040510E 837DE800         cmp dword ptr [ebp-$18],$00 00405112 740D             jz $00405121 00405114 8B45E8           mov eax,[ebp-$18] 00405117 83E804           sub eax,$04 0040511A 8B00             mov eax,[eax] 0040511C D1E8             shr eax,1 0040511E 8945E8           mov [ebp-$18],eax 00405121 837DE800         cmp dword ptr [ebp-$18],$00  Project172.dpr.20: if sw2 = '' then 00405125 A1A89B4000       mov eax,[$00409ba8] 0040512A 33D2             xor edx,edx 0040512C E81FE8FFFF       call @WStrEqual  Project172.dpr.22: if Length(su1) = 0 then 00405131 A1AC9B4000       mov eax,[$00409bac] 00405136 8945E4           mov [ebp-$1c],eax 00405139 837DE400         cmp dword ptr [ebp-$1c],$00 0040513D 740B             jz $0040514a 0040513F 8B45E4           mov eax,[ebp-$1c] 00405142 83E804           sub eax,$04 00405145 8B00             mov eax,[eax] 00405147 8945E4           mov [ebp-$1c],eax 0040514A 837DE400         cmp dword ptr [ebp-$1c],$00  Project172.dpr.24: if su2 = '' then 0040514E 833DB09B400000   cmp dword ptr [$00409bb0],$00 

Answer 2

Semantically they are identical and there will be no discernible performance difference. So we are left looking at clarity for the reader of the code.

if Str='' then 

is the readable version in my opinion. Look at the title of your question:

What is the better way to check for an empty string?

In your head you view this as the empty string as opposed to the string with length 0. So write the code the way that matches your viewpoint.