Set

From Lazarus wiki

Deutsch (de) English (en) suomi (fi) français (fr) русский (ru)


Back to data types.

Back to Reserved words.


Introduction

A Set encodes many values from an enumeration into an Ordinal type.

For example let's consider this enumeration:

  TSpeed = (spVerySlow,spSlow,spAVerage,spFast,spVeryFast);

And this set:

  TPossibleSpeeds = set of TSpeed

Constant instances of TPossibleSpeeds can be defined using brackets to hold set elements:

  const
    RatherSlow = [spVerySlow,spSlow];
    RatherFast = [spFast,spVeryFast];

RatherSlow and RatherFast are some Set of TSpeed.

Manipulating sets

Two functions defined in the RTL System unit are used to manipulate a set: Include(ASet,AValue) and Exclude(ASet,AValue).

  var
    SomeSpeeds: TPossibleSpeeds;
  begin
    SomeSpeeds := [];
    Include(SomeSpeeds,spVerySlow);
    Include(SomeSpeeds,spVeryFast);
  end;

Sets cannot be directly manipulated if they are published. You usually have to make a local copy, change the local copy and then to call the setter.

  procedure TSomething.DoSomething(Sender: TFarObject);
  var
    LocalCopy: TPossibleSpeeds;
  begin
    LocalCopy := Sender.PossibleSpeeds; // getter to local
    Include(LocalCopy,spVerySlow);
    Sender.PossibleSpeeds := LocalCopy; // local to setter.
  end;

The reserved word in is also used to test if a value is in a set. It's usually used in this fashion:

  var
    CanBeSlow: Boolean;
  const
    SomeSpeeds = [Low(TSpeed)..High(TSpeed)];
  begin
    CanBeSlow := (spVerySlow in SomeSpeeds) or (spSlow in SomeSpeeds);
  end;

Bitmasks

Sets can be used to create bitmasks as shown in the example.

(*
  The set [FLAG_A, FLAG_C] will be stored like this:
  
  TFlags :   0000'0101
                   │││
  FLAG_C ──────────┘││
  FLAG_B ───────────┘│
  FLAG_A ────────────┘
*)

type
  TFlag = (FLAG_A, FLAG_B, FLAG_C);
  TFlags = set of TFlag;

var
  Flags: TFlags;

[..]
  Flags:= [FLAG_A, FLAG_C];
  if FLAG_A in Flags then ..  // check FLAG_A is set in flags variable


navigation bar: data types
simple data types

boolean byte cardinal char currency double dword extended int8 int16 int32 int64 integer longint real shortint single smallint pointer qword word

complex data types

array class object record set string shortstring