How do I test a generic type variable for equality with Default(T) in Delphi?

Question

I\'m trying to write a generic cached property accessor like the following but am getting a compiler error when trying to check whether the storage variable already contains

Answer 1

The problem is not the Default function, but the equality operator =.

You could constrain T to IEquatable and use the Equals method like this:

TMyClass = class
  function GetProp<T : IEquatable<T>>(var ADataValue: T; const ARetriever: 
end;
...
function TMyClass.GetProp<T>(var ADataValue: T; const ARetriever: TFunc<T>): T;
begin  
if ADataValue.Equals (Default(T)) then
  ADataValue := ARetriever();  
Result := ADataValue;
end;   

Answer 2

After a hint in the comments from Binis and digging around a little in Generics.Collections I came up with the following which appears to work just as I wanted it:

function TMyClass.GetProp<T>(var ADataValue: T; const ARetriever: TFunc<T>): T;
var
  lComparer: IEqualityComparer<T>;
begin
  lComparer := TEqualityComparer<T>.Default;
  if lComparer.Equals(ADataValue, Default(T)) then
    ADataValue := ARetriever();
  Result := ADataValue;
end;