Operator overloading (chapter 15, page 855) and generics are closely related. Imagine a generic class that has the following definition:
{$mode objfpc}
unit mya;
interface
type
Generic TMyClass<T> = Class(TObject)
Function Add(A,B : T) : T;
end;
Implementation
Function TMyClass.Add(A,B : T) : T;
begin
Result:=A+B;
end;
end.
When the compiler replays the generics macro, the addition must be possible. For a specialization like this:
TMyIntegerClass = specialize TMyClass<integer>;
This is not a problem, as the Add method would become:
Procedure TMyIntegerClass.Add(A,B : Integer) : Integer; begin Result:=A+B; end;
The compiler knows how to add two integers, so this code will compile without problems. But the following code:
Type TComplex = record Re,Im : Double; end; Type TMyIntegerClass = specialize TMyClass<TComplex>;
Will not compile, unless the addition of two TComplex types is defined. This can be done using record operators:
{$modeswitch advancedrecords}
uses mya;
Type
TComplex = record
Re,Im : Double;
class operator +(a,b : TComplex) : TComplex;
end;
class operator TComplex.+ (a,b : TComplex) : TComplex;
begin
Result.re:=A.re+B.re;
Result.im:=A.im+B.im;
end;
Type
TMyComplexClass = specialize TMyClass<TComplex>;
begin
// Code here
end.
Currently, due to an implementation restriction, it will not work using a global operator, i. e. the following does not work yet:
uses mya; Type TComplex = record Re,Im : Double; end; operator + (a,b : TComplex) : TComplex; begin Result.re:=A.re+B.re; Result.im:=A.im+B.im; end; Type TMyComplexClass = specialize TMyClass<TComplex>; begin // Code here end.
Support for this construct is expected in a future version of Free Pascal.