There is only one dynamic array operator: +. This operator is available in Delphi mode, but must be enabled explicily using the modeswitch arrayoperators in objfpc mode:
{$mode objfpc}
{$modeswitch arrayoperators}
When enabled, its action is similar to the concatenation for strings: to concatenate the contents of the two arrays it acts on. The array element type must of course be identical for both arrays, i.e. the following will work:
{$mode objfpc}
{$modeswitch arrayoperators}
var
a,b, c : array of byte;
begin
a:=[0,1,2];
b:=[3,4,5];
c:=a+b;
writeln('C has ',length(c),' elements');
But the following will not work:
{$mode objfpc}
{$modeswitch arrayoperators}
var
b, c : array of byte;
a : array of integer;
begin
a:=[0,1,2];
b:=[3,4,5];
c:=a+b;
writeln('C has ',length(c),' elements');
The compiler will give an error when compiling this code.