Question 187615 in Json Question posted in
Our archive of expertly curated questions and answers provides insights and solutions to common problems related to this popular data interchange format. From parsing and manipulating JSON data to integrating it with various programming languages and web services, our archive has got you covered. Start exploring today and take your JSON skills to the next level

Delphi 11.3 – Unable to create/Add JSONs from variant

I am migrating some projects from Delphi 10.4 to Delphi 11.3.
I am facing some problems with the JSON library:

See the following codes:

First case:

function TestCreate;
var
  LValue: Variant;
  LJSON: TJSONNumber;
begin
  LValue := 100;
  LJSON := TJSONNumber.Create(LValue);
end;

Compiling this, it results in:

  [dcc64 Error] _Unit.pas(74): E2251 Ambiguous overloaded call to 'Create'
  System.JSON.pas(435): Related method: constructor TJSONNumber.Create(const string);
  System.JSON.pas(439): Related method: constructor TJSONNumber.Create(const Double);

Second case:

function TestAddPair;
var
  LValue: Variant;
  LJSON: TJSONObject;
begin
  LValue := 100;
  LJSON := TJSONObject.Create();
  LJSON.AddPair('Test', LValue);
end;

Compiling this, it results in:

  [dcc64 Error] _Unit.pas(77): E2251 Ambiguous overloaded call to 'AddPair'
  System.JSON.pas(661): Related method: function TJSONObject.AddPair(const string; const Boolean): TJSONObject;
  System.JSON.pas(651): Related method: function TJSONObject.AddPair(const string; const Double): TJSONObject;
  System.JSON.pas(636): Related method: function TJSONObject.AddPair(const string; const string): TJSONObject;

Both used to work properly on Delphi 10.4, but on Delphi 11.3 they do not compile.
Is there a way to let them compile? I’d prefer not to modify each command that uses a Variant to create/add a JSON.

Tags:

2

Answers


  1. 0

    A Variant can implicitly convert to both string or Double type without any preference for one of them.

    In R104 the constructor taking a string parameter is protected, while in 11.3 it is public.

    As Toon mentioned, you may not be able to avoid changing all the calls.

    Update: Alternatively you can create class helpers for TJSONNumber and TJSONObject with overrides for Variant. F.i. like this:

    type
  TJSONNumberHelper = class helper for TJSONNumber
  public
    constructor Create(const AValue: Variant); overload;
  end;

constructor TJsonNumberHelper.Create(const AValue: Variant);
begin
  inherited Create(Double(AValue));
end;
    Login or Signup to reply.
  2. 0

    Another alternative and different approach could be using mORMot, furthermore it’s very optimized to use as little memory and very fast JSON serialization / un-serialization.

    program Project1;

{$APPTYPE CONSOLE}

uses
  Syncommons;

var
  DoubleNumber : Double;
  VariantNumber : Variant;
  Json : variant;
begin
  DoubleNumber := 100;
  VariantNumber := 101;
  Json := _Obj(['Double' , DoubleNumber ,
                'Variant' , VariantNumber
               ]);
  assert(VariantSaveJson(Json)='{"Double":100,"Variant":101}');
end.
    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search