# Generics

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## Introduction

The Free Generics Library or FGL is a native implementation of class templates. Generics are sometimes called parametrized types. FPC has had official support for generics since version 2.2.

## 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:

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';
// 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;
end;

Example of generic class implementation:

implementation

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) );
end.