Generics: what changed

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Title: Generics

New		Line
1	+	Generics are sometimes called parameterized types.
2	+
3	+	## Introduction
4	+
5	+	[[FPC]] has had official support for generics in [[Mode ObjFPC\|{$mode ObjFPC}]] since version 2.2.0, and in [[Mode Delphi\|{$mode Delphi}]] since version 2.6.0.
6	+
7	+	The reason that FPC supports two different dialects is simply that FPC implemented generics several years before [[Delphi]].
8	+
9	+	It is possible to use [[Unit\|units]] written in `{$mode ObjFPC}` syntax in other units that use `{$mode Delphi}`, and vice versa.
10	+
11	+	The Free Generics Library or FGL is an FPC-native collection of generic containers written in `{$mode ObjFPC}`.
12	+
13	+	The rtl-generics package is a larger, somewhat more featureful collection of generic containers written in `{$mode Delphi}` that 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 as well.
14	+
15	+	Both FGL and rtl-generics can be used in both syntax modes.
16	+
17	+	## FGL unit
18	+
19	+	The easiest way to get started with generics is to use the [fgl unit](https://www.freepascal.org/docs-html/current/rtl/fgl/index.html), which is a prototype unit for base system generic [[Class\|classes]]. So far it contains a few basic classes:
20	+
21	+	- TFPGList
22	+	- TFPGObjectList
23	+	- TFPGInterfacedObjectList
24	+	- TFPGMap
25	+
26	+	### Getting started
27	+
28	+	The following simple example shows how to store multiple instances of a user defined class in a list:
29	+
30	+	``` pascal
31	+	{$mode objfpc}
32	+	uses fgl;
33	+
34	+	type
35	+	TMyClass = class(TObject)
36	+	fld1 : string;
37	+	end;
38	+
39	+	TMyList = specialize TFPGObjectList<TMyClass>;
40	+
41	+	var
42	+	list : TMyList;
43	+	c : TMyClass;
44	+
45	+	begin
46	+	// create the list and add an element
47	+	list := TMyList.Create;
48	+	c := TMyClass.Create;
49	+	c.fld1 := 'c1';
50	+	list.Add(c);
51	+	// retrieve an element from the list
52	+	c := list[0];
53	+	```
54	+
55	+	## Custom generic classes
56	+
57	+	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.
58	+
59	+	A generic class is defined using the [[Keyword\|keyword]] generic before the class name and use in class declaration:
60	+
61	+	``` pascal
62	+	type
63	+	generic TList<T> = class
64	+	Items: array of T;
65	+	procedure Add(Value: T);
66	+	end;
67	+	```
68	+
69	+	Here's an example of a generic class implementation:
70	+
71	+	``` pascal
72	+	implementation
73	+
74	+	procedure TList.Add(Value: T);
75	+	begin
76	+	SetLength(Items, Length(Items) + 1);
77	+	Items[Length(Items) - 1] := Value;
78	+	end;
79	+	```
80	+
81	+	A generic class, [[Object\|object]], [[Record\|record]], [[Interfaces\|interface]] or [[Method\|method]] can be simply specialized for a particular type by using the specialize keyword.
82	+
83	+	``` pascal
84	+	Type
85	+	TIntegerList = specialize TList<Integer>;
86	+	TPointerList = specialize TList<Pointer>;
87	+	TStringList = specialize TList<string>;
88	+	```
89	+
90	+	## Other points
91	+
92	+	1. The [[Compiler\|compiler]] parses a generic, but instead of generating code it stores all tokens in a token buffer inside the PPU file.
93	+	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\|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.
94	+
95	+	Therefore, in theory, there should be no speed differences between a "normal" class and a generic one.
96	+
97	+	## Example
98	+
99	+	## See also
100	+
101	+	- [[Templates]]
102	+	- [[Generics proposals]]
103	+	- [[Data Structures, Containers, Collections\|Data Structures, Containers and Collections]]
104	+
105	+	## External links
106	+
107	+	- [Free Pascal: Reference Guide – Generics](https://www.freepascal.org/docs-html/ref/refch8.html)
108	+
109	+	---
110	+	From the Free Pascal wiki, page "Generics", as it stood on 2024-03-28 after a change by Raskaton, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).

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