Question

I would like to be able to use TValue to store Data in a TList<>. Like in :

type
  TXmlBuilder = class
  type
    TXmlAttribute = class
      Name: String;
      Value: TValue; // TValue comes from Rtti
    end;

    TXmlNode = class
      Name: String;
      Parent: TXmlNode;
      Value: TXmlNode;
      Attributes: TList<TXmlAttribute>;
      Nodes: TList<TXmlNode>;
      function AsString(Indent: Integer): String;
    end;
  ...
  public
    ...
    function N(const Name: String): TXmlBuilder;
    function V(const Value: String): TXmlBuilder;
    function A(const Name: String; Value: TValue): TXmlBuilder; overload;
    function A<T>(const Name: String; Value: T): TXmlBuilder; overload;  
    ...
 end;     

implementation

function TXmlBuilder.A(const Name: String; Value: TValue): TXmlBuilder;
var
  A: TXmlAttribute;
begin
  A := TXmlAttribute.Create;
  A.Name := Name;
  A.Value := Value;
  FCurrent.Attributes.Add(A);
  Result := Self;
end;

function TXmlBuilder.A<T>(const Name: String; Value: T): TXmlBuilder;
var
  V: TValue;
begin
  V := TValue.From<T>(Value);
  A(Name, V);
end; 

And a bit later, in the main program, I use my “fluent” xml builder like this :

b := TXmlBuilder.Create('root');
b.A('attribute', 1).A('other_attribute', 2).A<TDateTime>('third_attribute', Now);

On the second call, the program raises an access violation exception.

It looks like the first TValue has been “freed”. Is it really possible to use TValue to store “Variant” data a runtime ?

I know that Variants exists in Delphi. My XML builder will be used to (de)serialize native delphi objects to XML using RTTI so I will be using TValue everywhere.

regards,

—
Pierre Yager

Answer 1

I found the answer. My mistake.

function TXmlBuilder.A<T>(const Name: String; Value: T): TXmlBuilder;
var
  V: TValue;
begin
  V := TValue.From<T>(Value);
  Result := A(Name, V); // I missed the return value
end; 

Sorry 😉