KPBecker
asked on
variable object in "with object do ..."
Hi,
is there a possibility to use a variable in the "with object do ..." construct for the 'object' ?
Thanks,
KPBecker
is there a possibility to use a variable in the "with object do ..." construct for the 'object' ?
Thanks,
KPBecker
maybe also
procedure WithAnyObjectOfaSpecificCl assDo(cons t AObject : TYourClassYouHave);
begin
if assigned(AObject) then
with AObject do
begin
.....
end;
end;
meikl ;-)
procedure WithAnyObjectOfaSpecificCl
begin
if assigned(AObject) then
with AObject do
begin
.....
end;
end;
meikl ;-)
btw.
WithAnyObjectOfaSpecificCl assDo
should be better named as
WithAnyInstanceOfaSpecific ClassDo
but of course its just a phantasy-name
meikl ;-)
WithAnyObjectOfaSpecificCl
should be better named as
WithAnyInstanceOfaSpecific
but of course its just a phantasy-name
meikl ;-)
With creates nothing but an anonymous variable which is local to the block of the with statement.
I took the question to mean "other than an object instance", thus providing the record example. But all of the above info is correct.
From the Delphi help:
A with statement is a shorthand for referencing the fields of a record or the fields, properties, and methods of an object. The syntax of a with statement is
with obj do statement
or
with obj1, ..., objn do statement
where obj is a variable reference denoting an object or record, and statement is any simple or structured statement. Within statement, you can refer to fields, properties, and methods of obj using their identifiers alone—without qualifiers.
For example, given the declarations
type TDate = record
Day: Integer;
Month: Integer;
Year: Integer;
end;
var OrderDate: TDate;
you could write the following with statement.
with OrderDate do
if Month = 12 then
begin
Month := 1;
Year := Year + 1;
end
else
Month := Month + 1;
This is equivalent to
if OrderDate.Month = 12 then
begin
OrderDate.Month := 1;
OrderDate.Year := OrderDate.Year + 1;
end
else
OrderDate.Month := OrderDate.Month + 1;
If the interpretation of obj involves indexing arrays or dereferencing pointers, these actions are performed once, before statement is executed. This makes with statements efficient as well as concise. It also means that assignments to a variable within statement cannot affect the interpretation of obj during the current execution of the with statement.
Each variable reference or method name in a with statement is interpreted, if possible, as a member of the specified object or record. If there is another variable or method of the same name that you want to access from the with statement, you need to prepend it with a qualifier, as in the following example.
with OrderDate do
begin
Year := Unit1.Year
...
end;
When multiple objects or records appear after with, the entire statement is treated like a series of nested with statements. Thus
with obj1, obj2, ..., objn do statement
is equivalent to
with obj1 do
with obj2 do
...
with objn do
statement
In this case, each variable reference or method name in statement is interpreted, if possible, as a member of objn; otherwise it is interpreted, if possible, as a member of objn–1; and so forth. The same rule applies to interpreting the objs themselves, so that, for instance, if objn is a member of both obj1 and obj2, it is interpreted as obj2.objn.
---------------
Russell
well russell, good explaination, but
let say
type To1 = Class(TObject)
f1 : AnyType;
end;
var
fo1, fo2 : To1;
begin
....creating
with fo1, fo2 do
f1 := f1; //what happened
end;
end;
or
var
fo1, fo2 : To1;
begin
....creating
with fo1 do
with fo2 do
f1 := f1; //what happened
end;
end;
end;
currently i'm avoid with ... do
its just for lazy coders (from my point of view)
and makes the code less readable
meikl ;-)
let say
type To1 = Class(TObject)
f1 : AnyType;
end;
var
fo1, fo2 : To1;
begin
....creating
with fo1, fo2 do
f1 := f1; //what happened
end;
end;
or
var
fo1, fo2 : To1;
begin
....creating
with fo1 do
with fo2 do
f1 := f1; //what happened
end;
end;
end;
currently i'm avoid with ... do
its just for lazy coders (from my point of view)
and makes the code less readable
meikl ;-)
Blame it on Borland (their doc, not mine...)
;-)
Russell
;-)
Russell
"with" considered harmful... =o)
"With" is useful as long as it's nuances are understood, and as long as one avoids ambiguous situations (such as that provided by Meikl). Another common mistake is to forget about the "Self" in an object method, eg:
// Label1 is of type TLabel
procedure TForm1.Something;
begin
// Intention is to assign the form caption to the label caption
with Label1 do
Caption:=Caption;
end;
But would in effect be the same as
Label1.Caption:=Label1.Cap tion;
As in Meikl's case, the variable just got assigned its own f1 field. Probably not what was intended. So in short, there are some gotcha's to it. When in doubt its probably best to skip the shorthand coding style.
my 2cents
Russell
// Label1 is of type TLabel
procedure TForm1.Something;
begin
// Intention is to assign the form caption to the label caption
with Label1 do
Caption:=Caption;
end;
But would in effect be the same as
Label1.Caption:=Label1.Cap
As in Meikl's case, the variable just got assigned its own f1 field. Probably not what was intended. So in short, there are some gotcha's to it. When in doubt its probably best to skip the shorthand coding style.
my 2cents
Russell
how about:
with Label1 do
Caption:=Name
...with can really be evil
with Label1 do
Caption:=Name
...with can really be evil
with can be *very* useful for ActiveX/COM.
If you have a COM object which is very deeply nested (many dereferencing dots a.b.c.d) then it gives you a performance boost.
The dereferencing is very costly for COM objects so a local variable helps. I got a tenfold boost once.
Of course a local variable is as good, but with is elegant.
If you have a COM object which is very deeply nested (many dereferencing dots a.b.c.d) then it gives you a performance boost.
The dereferencing is very costly for COM objects so a local variable helps. I got a tenfold boost once.
Of course a local variable is as good, but with is elegant.
ASKER
Hi,
reading the comments given above I think I should have been a bit more specific with my question.
Depending on "whatever" a lot of SQL-statements using "queryA" or "queryB" have to be performed
if whatever then
queryA.Parameters.ParamByN ame('pE'). Value:= MAPInfo[12];
queryA.Parameters.ParamByN ame('pA'). Value:= MAPInfo[13];
queryA.Parameters.ParamByN ame('pS'). Value:= MAPInfo[14];
queryA.Parameters.ParamByN ame('pK'). Value:= ABGInfo[4];
queryA.Parameters.ParamByN ame('pB'). Value:= MAPInfo[15];
queryA.Parameters.ParamByN ame('pM'). Value:= MAPInfo[16];
...
else
queryB.Parameters.ParamByN ame('pE'). Value:= MAPInfo[12];
queryB.Parameters.ParamByN ame('pA'). Value:= MAPInfo[13];
queryB.Parameters.ParamByN ame('pS'). Value:= MAPInfo[14];
queryB.Parameters.ParamByN ame('pK'). Value:= ABGInfo[4];
queryB.Parameters.ParamByN ame('pB'). Value:= MAPInfo[15];
queryB.Parameters.ParamByN ame('pM'). Value:= MAPInfo[16];
...
end;
What I would like to do is to use a "with query.Parameters do begin ..."
where "query.Parameters" is set to "queryA.Parameters" or "queryB.Parameters"
depending on "whatever"
KPBecker
reading the comments given above I think I should have been a bit more specific with my question.
Depending on "whatever" a lot of SQL-statements using "queryA" or "queryB" have to be performed
if whatever then
queryA.Parameters.ParamByN
queryA.Parameters.ParamByN
queryA.Parameters.ParamByN
queryA.Parameters.ParamByN
queryA.Parameters.ParamByN
queryA.Parameters.ParamByN
...
else
queryB.Parameters.ParamByN
queryB.Parameters.ParamByN
queryB.Parameters.ParamByN
queryB.Parameters.ParamByN
queryB.Parameters.ParamByN
queryB.Parameters.ParamByN
...
end;
What I would like to do is to use a "with query.Parameters do begin ..."
where "query.Parameters" is set to "queryA.Parameters" or "queryB.Parameters"
depending on "whatever"
KPBecker
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ASKER
Hello, meikl,
that's what I was looking for, thanks.
KPBecker
that's what I was looking for, thanks.
KPBecker
eg:
TPerson = record
age: integer;
name: string;
end;
var
person: TPerson;
begin
with person do
begin
age:=20;
name:='John';
end;
-------------
Regards,
Russell