What is the difference between Delphi string comparsion functions?

5
votes

There's a bunch of ways you can compare strings in modern Delphi (say 2010-XE3):

  • '<=' operator which resolves to UStrCmp / LStrCmp
  • CompareStr
  • AnsiCompareStr

Can someone give (or point to) a description of what those methods do, in principle?

So far I've figured that AnsiCompareStr calls CompareString on Windows, which is a "textual" comparison (i.e. takes into account unicode combined characters etc). Simple CompareStr does not do that and seems to do a binary comparison instead.

But what is the difference between CompareStr and UStrCmp? Between UStrCmp and LStrCmp? Do they all produce identical results? Do those results change between versions of Delphi?

I'm asking because I need a comparison which will always produce the same results, so that indexes in app built with one version of Delphi remain consistent with code built with another.

1
delphistring-comparison

1 Answers

7
votes

AnsiCompareStr is specified as taking locale into account, and should return identical results regardless of Delphi version, but may return different results based on Windows version and/or settings.. CompareStr is a pure binary comparison: "The comparison operation is based on the 16-bit ordinal value of each character and is not affected by the current locale" (for the CompareStr(const S1, S2: string) overload). UStrCmp also uses a pure binary comparison: "Strings are compared according to the ordinal values that make up the characters that make up the string." So there should not be a difference between the latter two. The way they return the result is different, so two implementations are needed (although it would be possible to make one rely on the other).

As for the differences between LStrCmp and UStrCmp, LStrCmp takes AnsiStrings, UStrCmp takes UnicodeStrings. It's entirely possible that two characters (let's say A and B) are ordered in the misnamed "ANSI" code page as A < B, but are ordered in Unicode as A > B. You should almost always just use the comparison appropriate for the data you have.