(1) var modifier
Adding var is address transfer, which will modify the original variable
where
s: string;
begin
S := 'Hello';
ChangeSVar(s);
ShowMessage(S);
end;
// ChangeSVar definition
procedure TForm1.ChangeSVar (var A: string);
begin
A := A + 'World';
end;
The above will output Hello World, because it is an address, and the original A is modified
(2) without any modifiers
where
s: string;
begin
S := 'Hello';
ChangeS(s);
ShowMessage(S);
end;
// ChangeS definition
procedure TForm1.ChangeS(A: string);
begin
A := A + 'World';
end;
The above will output Hello, because the method ChangeS actually creates a new A, and the output is still the original A, and the value has not changed.
(3) out modifier
where
s: string;
begin
S := 'Hello';
ChangeSOut(s);// The value of S is 'Hello,' instead of 'Hello, World'!, the original value of S in the process Hello is discarded
ShowMessage(S);
end;
// ChangeSOut definition
procedure TForm1.ChangeSOut(out A: string);
begin
A := A + 'World';
end;
The above will output World, out just accepts the returned value, any input to out will be ignored. At the same time, the actual parameters passed to the procedure by out do not have to be initialized, such as the call to ChangeSOut:
Where
Tmp： string；
Begin
ChangeSOut(Tmp);//Compilation can also be done through
End;
(4) Const modifier
Parameters modified by Const are not allowed to be modified after being passed in
If the parameters are modified during the process, an error will be reported, for example:
Procedure xxxx.TestConst(const A: String);
Begin
A := 'ss'; //Attempt to modify the parameters modified by const, an error will be reported
End;