Operator overloading (chapter 15, page 759) and generics are closely related. Imagine a generic
class that has the following definition:
{$mode objfpc}
unit mya;
interface
type
Generic TMyClass<T> = Class(TObject)
Function Add(A,B : T) : T;
end;
Implementation
Function TMyClass.Add(A,B : T) : T;
begin
Result:=A+B;
end;
end.
When the compiler replays the generics macro, the addition must be possible. For a specialization
like this:
TMyIntegerClass = specialize TMyClass<integer>;
This is not a problem, as the Add method would become:
Procedure TMyIntegerClass.Add(A,B : Integer) : Integer;
begin
Result:=A+B;
end;
The compiler knows how to add 2 integers, so this code will compile without problems. But the
following code:
Type
TComplex = record
Re,Im : Double;
end;
Type
TMyIntegerClass = specialize TMyClass<TComplex>;
Will not compile, unless the addition of 2 TComplex types is defined. This can be done using record
operators:
{$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.