Hairy Pixels wrote 16 lines
Another question I just ran in to. Why isn’t "array of TMyClass” compatible with “array of TObject” open array? TMyClass is a descendant of TObject and dynamic arrays are compatible with open arrays right?
======================
type
TMyClass = class
end;
procedure MyProcedure(values: array of TObject);
begin
end;
var
myArray: array of TMyClass;
begin
MyProcedure(myArray); // error: Incompatible type for arg no. 1: Got "{Dynamic} Array Of TMyClass", expected "{Open} Array Of TObject"
Rougly said:
Arrays are considered equal if the element type is equal. And equal does not mean 'inherits from'.
One reason we do not allow descendents is that if you have a var array, the returned value may contain classes of a type that the caller does not expect. (This is the same reason why (var value: TObject) requires an exact match in type)
But a case could be made to allow it in case the parameter is passed by value.
You can force it with a typecast:
Type
TObjectArray = Array of Object;
var
myArray: array of TMyClass;
begin
MyProcedure(TObjectArray(myArray));
end.
But then you're on you own, as you are deliberately bypassing the compiler's typechecking (as is the case with all typecasts...).
Michael.
