Question

i’m using JCL’s expression evaluator TEvaluator (a marvelous creation donated by barry kelly). (THANK YOU barry!)

background

i’ve used the AddFunc method.

function MyFunc:double; begin   // calculations here   Result:=1;   end; 

you can use the AddFunc method to make the function available:

  AddFunc('MyFunc', MyFunc); 

here’s the problem…

i need to call a method on an object instead of a standalone routine.

the reason is that i have a list of objects that provide the values.

say we have a list of vehicle objects. each object has a Weight function. i want to be able to make available each object’s weight available for use in the formula.

a silly example but it’s easy to explain:

type   TVehicle=class   private   public     function Weight:double;   end;  function StrangeCalculation:double; var   vehicle:TVehicle; begin   for iVehicle = 0 to Count - 1 do     begin       vehicle:=GetVehicle(iVehicle);       // E2250 There is no overloaded version of 'AddFunc' that can be called with these arguments       eval.AddFunc(vehicle.Name, vehicle.Weight);     end;    Result:=eval.Evaluate('JeepTJWeight + FordF150Weight * 2'); end; 

my options:

1. AddVar( ) or AddConst( ) — but that isn’t so great because i need to be able to raise an exception if the value is not available.

2. AddFunc( ) with standalone functions. can’t do that because the names of (and number of) variables is unknown until runtime.

3. modify the object to add a callback if the variable isn’t found. i have actually done this but needed to edit a copy of the source to call back to make it do this.

4. make an AddFunc( ) that’s able to use method functions.

option #3 is actually built but an additional AddFunc would be nicer. the trouble is i don’t know what method prototype to provide. i thought TMethod would be the way but my knowledge is too limited here… here was my unsuccessful attempt but i still get ‘E2250 There is no overloaded version of ‘AddFunc’ that can be called with these arguments’ at the eval.AddFunc() call like before.

TFloat64MethodFunc = function(c:pointer): TFloat64;  procedure TEasyEvaluator.AddFunc(const AName: string; AFunc: TFloat64MethodFunc); begin   FOwnContext.Add(TExprFloat64MethodFuncSym.Create(AName, AFunc)); end;  TExprFloat64MethodFuncSym = class(TExprAbstractFuncSym) private   FFunc: TFloat64MethodFunc; public   constructor Create(const AIdent: string; AFunc: TFloat64MethodFunc);   function Evaluate: TFloat; override; // not using   function Compile: TExprNode; override; end; 

thank you for your help!

mp

Answer 1

figured it out…

TFloat64MethodFunc = function: TFloat of object;