I’m a little confused by some references in the docs. Hopefully I can explain it clearly.

Looking at TFPGList in fgl. The Sort method refers to a TCompareFunc (it does not link to its definition). TFPGList says it’s methods are the same as TFPSList or classes.TFPList. So looking in these two other classes their Sort methods refer to TFPSListCompareFunc (in fgl) and TListSortCompare (in classes), respectively.

type TListSortCompare = function(
  Item1: Pointer;
  Item2: Pointer
):Integer;
type TFPSListCompareFunc = function(
  Key1: Pointer;
  Key2: Pointer
):Integer of object;

Two questions.
1) Should TCompareFunc in TFPGList.Sort be one of the above types, or is there another definition somewhere?
2) What is an “Integer of object” in TFPSListCompareFunc? (I expect it to be a typo but then I’m fairly new to free pascal so I might have missed the memo.)

Thanks,
Pete

For question 1, I find the definition of TCompareFunc here https://www.freepascal.org/docs-html/fcl/bufdataset/tcomparefunc.html

Just tried to compile a short program with that definition of compare: TCompareFunc, and that is the wrong type for TFPGList<String> in my program (yes there is a better class for strings but I just wanted a quick example program to compile). FPC error message says it wants: function(const ShortString;const ShortString):LongInt, so only two non-pointer input parameters. Must be another definition somewhere.

The compare function is declared right inside the class

  { TFPGList }

  generic TFPGList<T> = class(TFPSList)
  private
    type
      TCompareFunc = function(const Item1, Item2: T): Integer;

"T" is from the generic, so if you specialized with string, then T is string.

It is a function, not a method (i.e. not part of a class)

Of object belongs to the function (the entire function declaration before), not just the "integer".
So that function must be a method of a class.

Ah, Ok thanks!