Dealing with overloaded functions that have ambiguous parameters

Question

Take this small example class (not my real code, but it exposes the problem):

Convert = class(TObject)
  public
    class function ToString(value: Double): String; overload;
    class function ToString(value: TDateTime): String; overload;
  end;

It compiles fine until you try to use the Double or TDateTime functions As In:

var
  d: Double;
begin
  d := 99.99;
  ShowMessage(Convert.ToString(d));

You will get this compile error: Ambiguous overloaded call to 'ToString'. The problem boils down to the fact that TDateTime is a type of Double

My Question: how do You deal with this type of problem?

EDIT - I am NOT looking for a solution for the example given

I have found 3 Solutions so far:

• Rename one of the 2 functions
• Add a "Dummy" parameter to one of the 2 functions
• Change the parameters to Var types, this has the disadvantage that I can no longer call this function with constants

are there any other solutions out there?

Answer 1

Overloaded methods can be very effective. However, as soon as there is a hint of ambiguity they become a liability. A good example of this are the new TStream overloads introduced in XE3. It's not hard to fall into a trap where the compiler chooses an overload that you weren't expecting. At least in your code the compiler stopped. In that sense you were lucky.

So my advice, in your situation, is to abandon overloads. Express the different input types in the method name. Yes it's a little more verbose, but you won't make any mistakes, and you code will compile!

Answer 2

Your posted example compiles and executes fine in XE.

In a comment you give this example instead:

ShowMessage( Convert.ToString( 99.99 ));  // <- gives compiler error 2251

In this particular case the solution is to explicitly define the type( I thought):

ShowMessage( Convert.ToString( Double(99.99) )); // <- E2089, Invalid Typecast

Looking into the documentation:

This error message is issued for type casts not allowed by the rules. The following kinds of casts are allowed:

• Ordinal or pointer type to another ordinal or pointer type
• A character, string, array of character or pchar to a string
• An ordinal, real, string or variant to a variant
• A variant to an ordinal, real, string or variant
• A variable reference to any type of the same size.

So, to explicitly tell the compiler to select the Double overloaded function:

ShowMessage( Convert.ToString( Double(Variant(99.99)))); // Ok

A bit convoluted perhaps. But for the other overloaded function it is simpler:

ShowMessage( Convert.ToString( EncodeDate(2013,1,5));

---

Update

---

To make this a generic solution working for all classes, consider adding class functions to resolve your ambiguous types.

Convert = Class(TObject)
  ...
  class function AsDouble( value: Double) : Double; inline; static;
  class function AsTDateTime( value: TDateTime) : TDateTime; inline; static;
end;

class function Convert.AsDouble(value: Double): Double;
begin
  Result := Value;
end;
class function Convert.AsDateTime(value: TDateTime): TDateTime;
begin
  Result := Value;
end;

Now you can call your overloaded class function with constants:

ShowMessage( Convert.ToString( Convert.AsDouble(99.99)));