Casting

Casting is a process in which the data type of a variable or expression is explicitly converted to another type. Synergy DBL enables you to cast an object variable to a specific class type (including @class). You can also cast an integer value (for example, a Boolean) to or from an i4 variable (for example, an enumeration).

In Synergy .NET, to cast a negative number, the negative number must be in parentheses. For example, (@d)(-123).

Casting supports upcasting (converting a derived class to a more general parent class) and downcasting (converting from a parent class to a more specific class). For example, in figure 1, you can cast c2 to c1 or object and vice versa, but you can’t cast c3 to c2 or c1.

1. Class hierarchy.

The example below assumes ChildClass extends ParentClass and OtherClass doesn’t extend either class:

var1, @ParentClass
var2, @ChildClass
var3, @ChildClass
var4, @ParentClass
var5, @OtherClass
var1 = new ParentClass()
var2 = new ChildClass()
var4 = (ParentClass)var2          ;Cast allowed but not necessary 
var3 = (ChildClass)var2           ;Cast allowed but not necessary
var3 = (ChildClass)var1           ;Cast allowed (but if var1 contains an instance
                                  ; of a ParentClass object, fail at runtime)
var5 = (OtherClass)var1           ;Invalid cast

To reference a member of the object that has been cast, you need to wrap the cast part of the path in parenthesis. Two examples of the syntax are shown below:

((parent_class)object_var).field
((parent_class)object_var).method()

The first example demonstrates how to access a field from an object variable that has been cast. The second shows how to call a method on a cast object variable.

If you get an “Invalid statement at or near {%s}” error (INVSTMT) or an “Invalid expression at or near {%s}” error (INVEXPR) for an expression in a complex statement that contains the cast of a path, try enclosing the full path in parentheses. For example, change if (boolean)al[1] then to if ((boolean)al[1]) then Without the parentheses, the parser interprets (boolean) as the condition in the IF statement and a1[1] as the statement to be executed after an implied THEN statement. The parentheses ensure that the full path (boolean)a1[1] is interpreted as the condition.

To allow casting of objects to unrelated types, you must implement an op_Explicit operator in your class that has a return type to which you want to support conversion. op_Explicit cannot be invoked directly, only through casting. The following example uses op_Explicit to convert from i4 to myclass:

public static method op_Explicit, @myclass
    p1, i4
proc
    mreturn new myclass(p1)
end

Each conversion method must have one parameter that designates the source of the conversion and a return type to denote the destination for the conversion.

Also see Conversion operators for user-defined explicit conversions for classes.