[SOLVED] Linked list inside a class.

[seany]

my adventure on making a markdown renderer with slightly more function continues  .. I have this linked list and class definition :

Code: Pascal  [Select][+][-]

1. unit TDocument;
2.  
3. {$mode ObjFPC}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, TRenderObjects, TBlockIdentifierEngine;
9.  
10. type
11.     SectionNodePTR = ^NodeStruct;
12.     NodeStruct  = packed record
13.       rendObj   : Variant;
14.       ID        : Integer;
15.       Parent    : SectionNodePTR;
16.       Children  : Array of SectionNodePTR;
17.       objType   : TBlockIdentifierEngine.classificationEnum;
18.       end;
19.     TNodeArray = Array of NodeStruct;
20.  
21.  
22.   { markedDocument }
23.  
24.   markedDocument = class
25.     // Sections : Array of TRenderObjects.Section;
26.   public
27.       nodeTree                   : NodeStruct;
28.       constructor Create();
29.       procedure AppendNode(var ChildObj : NodeStruct);
30.       procedure AppendObject_asNode(var insertionObject : variant; var ID : integer; var objType : TBlockIdentifierEngine.classificationEnum);
31.  
32.  
33.   end;    
34.  

The classificationEnum is defined as :

Code: Pascal  [Select][+][-]

1. classificationEnum = (localCommand, globalCommand, renderTarget, unknown, highLightCommand);  

- this definition is in a different file: tblockidentifierengine.pas . This works.

I want to add every block of renderable text (starting and ending with a newline) as well as every command block (starting with a . or period and ending with a ; or semicolon) as document nodes. The nodes will be marked either as renderTarget or as various commands. So The function is defined as:

Code: Pascal  [Select][+][-]

1. procedure markedDocument.AppendObject_asNode(var insertionObject: variant;
2.   var ID: integer; var objType: TBlockIdentifierEngine.classificationEnum);
3. var
4.    newNode                       : NodeStruct;
5.    currNode                      : NodeStruct;
6. begin
7.  
8.    newNode.rendObj      := insertionObject;
9.    newNode.ID           := ID;
10.    newNode.objType      := objType;
11.  
12.    if nodeTree = nil then
13.    begin
14.      nodeTree    := newNode;
15.    end
16.    else
17.    begin
18.      currNode    := nodeTree;
19.      while length(currNode.Children) <> 0 do
20.      begin
21.        currNode  := currNode.Children[0];
22.      end;
23.      SetLength(currNode.Children, length(currNode.Children) + 1);
24.      currNode.Children[length(currNode.Children) -1] := newNode;
25.    end;
26.  
27. end;      
28.    

This is called as:

classInstance.AppendObject_asNode("some text",0,a tesst enum value);

But this does not compile. It tells me:

In the line:

Code: Pascal  [Select][+][-]

1.  if nodeTree = nil then  

Error: Operator is not overloaded: "NodeStruct" = "Pointer"

I was hoping that, I can find out if the root NodeStruct in the class instance is populated or not via comparing it with nil. My plan is to add every block (text or command as described above) as a node in a linked list. In this list, each element is a Nodestruct. The first element of the linked list is denoted as nodeTree in the class. So if I see that the nodeTree is not set in an instance of the class, it will mean that that first element of the linked list is not set. So we can set the newly created NodeStruct with the incoming block directly as the nodeTree.

Moreover, I also tried with:

Code: Pascal  [Select][+][-]

1.  if markedDocument.nodeTree = nil then  

In that case, It tells me, that in the same line: Only class methods, class properties and class variables can be referred with class references




Also, in this line:

Code: Pascal  [Select][+][-]

1. currNode  := currNode.Children[0];

I get an error: Error: Incompatible types: got "SectionNodePTR" expected "NodeStruct".

I read that objects are already pointers, so there is no need to dereference them. But seems that is not the case?

I have a similar error in this line.

Code: Pascal  [Select][+][-]

1. currNode.Children[length(currNode.Children) -1] := newNode;

But I will be able to address that as soon as I have addressed the #2 error.


Please help. Thank you.

[Blaazen]

This is pretty complex question. Anyway, record cannot be nil. Pointer to record can.

Declare

Code: Pascal  [Select][+][-]

1.  nodeTree: SectionNodePTR;

and try

Code: Pascal  [Select][+][-]

1. if nodeTree = nil then
2.    begin
3.      new(nodeTree);
4.      nodeTree^    := newNode;
5.    end
6.    else
7.    begin

Also here: correct is

Code: Pascal  [Select][+][-]

1. currNode  := currNode.Children[0]^;

("Correct" means that both sides are of the same type; if it will do what you want is another story)

[seany]

That needed a little bit of work.

So this is how I did it.

The class:

Code: Pascal  [Select][+][-]

1. uses
2.   Classes, SysUtils, TRenderObjects, Dialogs, TBlockIdentifierEngine;
3.  
4. type
5.     genNodePtr = ^NodeStruct;
6.     NodeStruct  = packed record
7.       rendObj   : Variant;
8.       ID        : Integer;
9.       Parent    : genNodePtr;
10.       Children  : Array of genNodePtr;
11.       objType   : TBlockIdentifierEngine.classificationEnum;
12.       end;
13.     TNodeArray = Array of NodeStruct;
14.      
15.   markedDocument = class
16.   public
17.       nodeTree                   : genNodePtr;
18.       constructor Create();
19.       procedure AppendNode(var ChildObj : NodeStruct);
20.       procedure AppendObject_asNode(var insertionObject : variant; var ID : integer; var objType : TBlockIdentifierEngine.classificationEnum);
21.  
22.  
23.   end;  

Then the Insertion Program

Code: Pascal  [Select][+][-]

1. procedure markedDocument.AppendObject_asNode(var insertionObject: variant;
2.   var ID: integer; var objType: TBlockIdentifierEngine.classificationEnum);
3. var
4.    newNode                       : ^NodeStruct;
5.    currNode                      : ^NodeStruct;      // I made them pointers
6.  
7.    argStr                        : String;
8. begin
9.  
10.    newNode               := New(genNodePtr);
11.  
12.    newNode^.rendObj      := insertionObject;
13.    newNode^.ID           := ID;
14.    newNode^.objType      := objType;
15.  
16.    // pointer is dereferenced with the caret after it's name, and then filled up
17.  
18.    if nodeTree = nil then
19.    begin
20.      new(nodeTree);
21.      nodeTree     := newNode;
22.    end
23.    else
24.    begin
25.  
26.      currNode    := nodeTree;
27.  
28.      while length(currNode^.Children) <> 0 do                       // the last item in the list has no children, so children.length is 0
29.      begin
30.        
31.        currNode  := currNode^.Children[0];                          // Dereference and pick the first child, because at this stage the size of children is exactly ONE
32.        
33.      end;
34.      
35.      SetLength(currNode^.Children, length(currNode^.Children) + 1);      // increase arraysize from zero to one
36.  
37.      currNode^.Children[length(currNode^.Children) -1] := newNode;       // Fill up first child. If i instead had currnode and newnode as records themselves, then
38.                                                                          //  currNode.Children[length(currNode.Children) -1]^ := newNode compiles,
39.                                                                          //  but throws a sigsev error in runtime
40.    end;
41.  
42. end;

This works, I can look in the list, and it contains the stuff as i want them to...

QUESTION

Why the sigsev error (as shown in the code by a comment)?

[Blaazen]

I'm surprised that this even compile:

Code: Pascal  [Select][+][-]

1.  newNode := New(genNodePtr);

It's not even documented: https://www.freepascal.org/docs-html/rtl/system/new.html

I always used:

Code: Pascal  [Select][+][-]

1. new(newNode);

BTW what is genNodePtr?

[seany]

My bad.

Code: Pascal  [Select][+][-]

1. genNodePtr = ^NodeStruct;

Also adjusted my previous post in this thread

[seany]

It's not even documented: https://www.freepascal.org/docs-html/rtl/system/new.html

About documentation: I followed it here:

https://forum.lazarus.freepascal.org/index.php?topic=9521.0

The example code there looks like:

Code: Pascal  [Select][+][-]

1. type
2.   TMailingListRecord = record
3.     FirstName: string;
4.     // [...]
5.   end;
6.   PMailingListRecord = ^TMailingListRecord;
7. var
8.   ptr: PMailingListRecord;
9. begin
10.   ptr := New(PMailingListRecord);                       // MY COMMENT: ptr is of type pointer to record, and is initialized as a new pointer to the same type of record
11.   ptr^.FirstName := 'Per';
12.   // [...]
13.   Dispose(ptr);
14. end;
15.