Generics

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Introduction

FPC has had official support for generics in Objfpc syntax since version 2.2. and Delphi compatible syntax since version 2.6.0.
Generics are sometimes called parametrized types.
The reason that FPC supports two different dialects, Objfpc and Delphi, is because FPC had generics before Delphi. It is possible to use units written in Objfpc syntax in other units that use Delphi syntax and vice versa.

The Free Generics Library or FGL is a native implementation of class templates written in Objfpc generics syntax.
The rtl-generics package is an implementation of class templates written in Delphi generics syntax and tries to be compatible with the Delphi generics library. This package is provided as standard in FPC 3.1.1.+ but there is a version for FPC 3.0.4. available.
Both FGL and rtl-generics can be used in both modes.

fgl unit

The easiest way to get started with generics is to use the fgl unit, which is a prototype unit for base system generic classes. So far it contains a few basic classes:

  • TFPGList
  • TFPGObjectList
  • TFPGInterfacedObjectList
  • TFPGMap

Getting Started

The following simple example shows how to store multiple instances of a user defined class in a list:

{$mode objfpc}
uses fgl;
 
type
   TMyClass = class(TObject)
      fld1 : string;
   end;
 
   TMyList = specialize TFPGObjectList<TMyClass>;
 
var
   list : TMyList;
   c : TMyClass;
 
begin
   // create the list and add an element
   list := TMyList.Create;
   c := TMyClass.Create;
   c.fld1 := 'c1';
   list.Add(c);
   // retrieve an element from the list
   c := list[0];

Custom Generic Classes

If the generics defined in the fgl unit do not suit your needs, you may need to define your own generic classes from scratch using the underlying language primitives.

A generic class is defined using the keyword generic before the class name and use in class declaration:

type
  generic TList<T> = class
    Items: array of T;
    procedure Add(Value: T);
  end;

Example of generic class implementation:

implementation
 
procedure TList.Add(Value: T);
begin
  SetLength(Items, Length(Items) + 1);
  Items[Length(Items) - 1] := Value;
end;

A generic class can be simply specialized for a particular type by using the specialize keyword.

Type  
  TIntegerList = specialize TList<Integer>;
  TPointerList = specialize TList<Pointer>;
  TStringList = specialize TList<string>;

Other Points

  1. The compiler parses a generic, but instead of generating code it stores all tokens in a token buffer inside the PPU file.
  2. The compiler parses a specialization; for this it loads the token buffer from the PPU file and parses that again. It replaces the generic parameters (in most examples "T") by the particular given type (e.g. LongInt, TObject).

The code basically appears as if the same class had been written as the generic but with T replaced by the given type.

Therefore in theory there should be no speed differences between a "normal" class and a generic one.

Example

An example of how to use generics to write a function gmax() that takes the maximum of two not-yet-typed variables. Note that the functions are namespaced by the classname. A disadvantage may be that generics can't be overloaded.

program UseGenerics;
 
{$mode objfpc}{$H+}
 
type
  generic TFakeClass<_GT> = class
    class function gmax(a,b: _GT):_GT;
  end;
 
  TFakeClassInt = specialize TFakeClass<integer>;
  TFakeClassDouble = specialize TFakeClass<double>;
 
  class function TFakeClass.gmax(a,b: _GT):_GT;
  begin
    if a > b then 
      result := a
    else 
      result := b;
  end;
 
begin
    // show max of two integers
  writeln( 'Integer GMax:', TFakeClassInt.gmax( 23, 56 ) );
    // show max of two doubles
  writeln( 'Double GMax:', TFakeClassDouble.gmax( 23.89, 56.5) );
  readln();
end.


See also

External links