Delphi 2010 How to modify TList < record > value ?
type TTest = record a,b,c:Integer end;
var List:TList<TTest>;
A:TTest;
P:Pointer;
....
....
List[10] := A; <- OK
List[10].a:=1; <- Here compiler error : Left side cannot be assined to
P:=@List[10]; <- Error: Variable requied
A := List[10];
A.a := 1;
list[10] := A;
You don't have to do this with objects because they're reference types, (accessed through a pointer which the compiler manages internally to keep it out of your hair,) but records are value types so it doesn't work that way.
You've hit upon a snag with using records.
Consider this code:
function Test: TTest;
begin
...
end;
Test.a := 1;
What your code looks like to the compiler is actually this:
TTest temp := Test;
temp.a := 1;
The compiler is telling you, with the error message, that the assignment is pointless, since it will only assign a new value to a temporary record value, which will be instantly forgotten.
Also, the @List[10]
is invalid because List[10]
again returns only a temporary record value, so taking the address of that record is rather pointless.
However, reading and writing the whole record is OK.
So to summarize:
List[10] := A; <- writing a whole record is OK
List[10].a:=1; <- List[10] returns a temporary record, pointless assignment
P:=@List[10]; <- List[10] returns a temporary record, its address is pointless
If you want to store records, dynamic arrays are more suited to handling them :
type TTest = record a,b,c : Integer end;
type TTestList = array of TTest;
var List:TTestList;
A:TTest;
P:Pointer;
....
....
SetLength( List, 20 );
List[10] := A; //<- OK
List[10].a := 1; //<- Ok
P := @List[10]; //<- Not advised (the next SetLength(List,xx) will blow the address away),
// but technically works
If you need to add methods to manipulate these data, you can store this array as the field of a class, and add your methods to this class.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With