In regular Delphi functions, the function return is actually a parameter passed by reference, even though syntactically it looks and feels like an ‘out’ parameter. You can test this out like so (this may be version dependent):
function DoNothing: IInterface;
begin
if Assigned(Result) then
ShowMessage('result assigned before invocation')
else
ShowMessage('result NOT assigned before invocation');
end;
procedure TestParameterPassingMechanismOfFunctions;
var
X: IInterface;
begin
X := TInterfaceObject.Create;
X := DoNothing;
end;
To demonstrate call TestParameterPassingMechanismOfFunctions()
Your code is failing because of a mismatch between Delphi and C++’s understanding of the calling convention in relation to the passing mechanism for function results. In C++ a function return acts like the syntax suggests: an out
parameter. But for Delphi it is a var
parameter.
To fix, try this:
function TestWideString: WideString; stdcall;
begin
Pointer(Result) := nil;
Result := 'TestWideString';
end;