Mailing lists / fpc-pascal / 2021-06
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Why does TMemoryStream only implement one version of SetSize method?
The TStream class has two versions of SetSize method (virtual, overload)
- one with a parameter of LongInt type and the other with the Int64 type. Since function is virtual, child classes can override it and can be called from code, but then a version of the SetSize function not implemented for a given platform in the TMemoryStream class can be called.
Michał.
The base method checks which one is overridden.
So it does not matter which one you override in the child. The "other" one will always fall back to the method in TStream.
Michael.
W dniu 2021-06-14 o 21:27, Michael Van Canneyt via fpc-pascal pisze:
Unfortunately, on the 64bit platform it does not fully work. See the project below.
program Project1;
{$mode objfpc}
uses
Classes;
type
TMyMemStream = class(TMemoryStream)
procedure Foo;
procedure Bar;
end;
procedure TMyMemStream.Foo;
begin
SetSize(10);
end;
procedure TMyMemStream.Bar;
begin
SetSize(Int64(10));
end;
var
MS: TMyMemStream;
begin
MS := TMyMemStream.Create;
MS.Foo;
WriteLn(MS.Size);
MS.Bar;
WriteLn(MS.Size);
MS.Free;
ReadLn;
end.Michał.
Sorry, I was mixing up seek and setsize.
Indeed. You should not be calling the integer SetSize() directly.
People creating descendent classes are expected to know what to do, i.e. to use the 64-bit version.
Michael.
W dniu 2021-06-14 o 22:25, Michael Van Canneyt via fpc-pascal pisze:
You are absolutely right. But why do we have different function for different platforms? Wouldn't it be better to implement either one function with an Int64 parameter for both platforms, or even both variants of this function? The Size property is of type int64 on both platforms so why should SetSize (that is public in TMemoryStream) be platform dependent?
Michał.