[SOLVED] Copy the contents of tStrings to another Tstrings.

[CM630]

I am trying to copy the contents of a ComboBox to a TStrings.

OldVals := ComboBox1.Items; does not copy them, but just creates a reference to the original one, so when ComboBox1.Items; is changed, OldVals is changed, too, which is not okay for the current purpose.

I also tried, the code below, but it results in RunError(211)... Call to abstract method.

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button1Click(Sender: TObject);
2. var
3.   OldVals: TStrings;
4. begin
5.   OldVals := TStrings.Create;
6.   OldVals.Assign(ComboBox1.Items);
7. end;  

There is no example here: https://www.freepascal.org/docs-html/rtl/classes/tstrings.assign.html
So I wonder what is the straight forward way.

[AlexTP]

Instead of

Code: Pascal  [Select][+][-]

1. OldVals := TStrings.Create;

Try

Code: Pascal  [Select][+][-]

1. OldVals := TStringList.Create;

[rvk]

As extra information from the manual:

TStrings implements an abstract class to manage an array of strings. It introduces methods to set and retrieve strings in the array, searching for a particular string, concatenating the strings and so on. It also allows an arbitrary object to be associated with each string.

It also introduces methods to manage a series of name=value settings, as found in many configuration files.

An instance of TStrings is never created directly, instead a descendant class such as TStringList should be created. This is because TStrings is an abstract class which does not implement all methods; TStrings also doesn't store any strings, this is the functionality introduced in descendants such as TStringList.

TStrings implements the IFPObserved interface: when the stringlist is changed, a ooChanged notification is sent to all observers.

https://freepascal.org/docs-html/rtl/classes/tstrings.html

So you should never create a TStrings directly byut always use a decendant of TStrings.

BTW. The variable itself (OldVals) is just a pointer so you could leave it as a TStrings because it can still hold a pointer to TStringList (but it doesn't hurt to make also make it a TStringList here just for clarity).

In TComboBox Items is declared as TStrings so it can also hold direct descendants from TStrings and doesn't necessarily need a descendant from TStringList.

[CM630]

Thanks for the clarification, it works now!

[Remy Lebeau]

So you should never create a TStrings directly byut always use a decendant of TStrings.

It has always annoyed me that Delphi and FreePascal allow abstract classes to be instantiated.  I have yet to encounter a scenario where that is a useful feature.

[simone]

It has always annoyed me that Delphi and FreePascal allow abstract classes to be instantiated.  I have yet to encounter a scenario where that is a useful feature.

Absolutely agree.

As stated in:

https://www.freepascal.org/docs-html/3.2.0/ref/refse35.html

this behavior is due to compatibility with Delphi, but I think it may be misleading, as in this case.

However, the compiler emits a warning message.

[Thaddy]

It is not always true:

Code: Pascal  [Select][+][-]

1. program testabstract;
2. {$ifdef fpc}{$mode delphi}{$warn CONSTRUCTING_ABSTRACT error}{$endif}
3. type
4.    TAbstractClass = class abstract
5.    public
6.      procedure testme;virtual;abstract;
7.    end;
8.  
9. var
10.   Test:TAbstractClass;
11. begin
12.   Test := TAbstractClass.Create;
13. end.

FreePascal can prevent you from compiling that. Half documented.
With the above provisions the code will not compile.

[simone]

@Thaddy: You're right. I forgot. It's documented here:

https://www.freepascal.org/docs-html/prog/progsu80.html

[Zoran]

It is not always true:

Code: Pascal  [Select][+][-]

1. program testabstract;
2. {$ifdef fpc}{$mode delphi}{$warn CONSTRUCTING_ABSTRACT error}{$endif}
3. type
4.    TAbstractClass = class abstract
5.    public
6.      procedure testme;virtual;abstract;
7.    end;
8.  
9. var
10.   Test:TAbstractClass;
11. begin
12.   Test := TAbstractClass.Create;
13. end.

FreePascal can prevent you from compiling that. Half documented.
With the above provisions the code will not compile.

Thank you for this, Thaddy!

[PascalDragon]

So you should never create a TStrings directly byut always use a decendant of TStrings.

And there should have been at least a warning from the compiler (the same warning that can be escalated to error using {$WARN CONSTRUCTING_ABSTRACT ERROR} as mentioned by Thaddy).