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Generics are a native implementation of class templates. Generics are sometimes called parametrized types. FPC has had official support for generics since version 2.2.


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

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

Example of generic class implementation:

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

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

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

fgl unit

The fgl unit is a prototype unit for base system generic classes. So far it contains a few basic classes:

  • TFPGList
  • TFPGObjectList
  • TFPGInterfacedObjectList
  • TFPGMap

Technical details

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.

See also

External links

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