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### 8.5 A word about scope

It should be stressed that all identifiers other than the template placeholders should be known when the generic class is declared. This works in 2 ways. First, all types must be known, that is, a type identifier with the same name must exist. The following unit will produce an error:

unit myunit;
interface
type
Generic TMyClass<T> = Class(TObject)
Procedure DoSomething(A : T; B : TSomeType);
end;
Type
TSomeType = Integer;
TSomeTypeClass = specialize TMyClass<TSomeType>;
Implementation
Procedure TMyClass.DoSomething(A : T; B : TSomeType);
begin
// Some code.
end;
end.

The above code will result in an error, because the type TSomeType is not known when the declaration is parsed:

home: >fpc myunit.pp
myunit.pp(8,47) Error: Identifier not found "TSomeType"
myunit.pp(11,1) Fatal: There were 1 errors compiling module, stopping

The second way in which this is visible, is the following. Assume a unit

unit mya;
interface
type
Generic TMyClass<T> = Class(TObject)
Procedure DoSomething(A : T);
end;
Implementation
Procedure DoLocalThings;
begin
Writeln(’mya.DoLocalThings’);
end;
Procedure TMyClass.DoSomething(A : T);
begin
DoLocalThings;
end;
end.

and a program

program myb;
uses mya;
procedure DoLocalThings;
begin
Writeln(’myb.DoLocalThings’);
end;
Type
TB = specialize TMyClass<Integer>;
Var
B : TB;
begin
B:=TB.Create;
B.DoSomething(1);
end.

Despite the fact that generics act as a macro which is replayed at specialization time, the reference to DoLocalThings is resolved when TMyClass is defined, not when TB is defined. This means that the output of the program is:

home: >fpc -S2 myb.pp
home: >myb
mya.DoLocalThings

This is dictated by safety and necessity:

1. A programmer specializing a class has no way of knowing which local procedures are used, so he cannot accidentally ’override’ it.
2. A programmer specializing a class has no way of knowing which local procedures are used, so he cannot implement it either, since he does not know the parameters.
3. If implementation procedures are used as in the example above, they cannot be referenced from outside the unit. They could be in another unit altogether, and the programmer has no way of knowing he should include them before specializing his class.

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