Casting
Updated 531 Days AgoPublic

Fundamentally, casting is converting from one type to another. As demonstrated in the code snippet below, a Function that takes a grade and returns pass/fail can be thought of as a conversion from a Real to a Boolean .

Conversion Function
function IsPassing(grade : Real) : Boolean
{
  if(grade >= 70)
    return true;
  else
    return false;
}

The logic of that particular conversion is defined as a function and intended for a specific situation, but cast operations are more general-case and defined by Zilch.

Implicit and Explicit

There are two types of casting, implicit and explicit. Implicit casting occurs during the assignment of a variable of one type, with a value of a different type. Implicit casting is a mechanism that Zilch uses when a Variable of one type is set to data of another type. Explicit casting is a mechanic used by the user with the as keyword. Any cast that can happen implicitly can be done explicitly.

WARNING: Even with explicit casting, there are still cast operations that are invalid and will trigger compile-time exceptions.
Casting Implicitly and Explicitly
var myReal : Real = 1.0;
var myDoubleReal : DoubleReal = myReal; // implicit casting
var myInteger : Integer = myReal as Integer; // explicit casting

In the example above, myDoubleReal is being set to myReal, which invokes an implicit cast from Real to DoubleReal . When myReal as Integer is evaluated, explicit casting is invoked to convert a Real to an Integer . The format for explicit casting is <value> as <new type>.

NOTE: Explicit casting should only be used when one is aware of all of the nuances of the cast.

Casting Between Numeric and Boolean Types

The following base data types are considerered Numeric and Boolean:

All cast operations using only these types are valid, meaning either explicit or implicit. The code snippit below demonstates implicit casting between numeric types.

Implicit Numeric Casts
var myInteger : Integer = 1;

var myDoubleInteger : DoubleInteger = myInteger;
var myReal : Real = myInteger;
var myDoubleReal1 : DoubleReal = myInteger;

var myDoubleReal2 : DoubleReal = myDoubleInteger;
var myDoubleReal3 : DoubleReal = myReal;

Notice that implicit casting is allowed when converting to a numeric set, from a smaller data size to a larger one. The value of myInteger can be implicitly casted to a DoubleInteger , which holds twice the number of bits as an Integer . The value of myInteger can also be implicitly casted to a Real . Real values can hold whole numbers and a decimal portion, whereas integers can only be whole numbers.

From a Mathematics Perspective The integer set is a subset of the real set. For more information on mathematical sets, visit the Wikipedia Set (Mathematics) page.

Truncation

When a real value is converted to an integer value explicitly, the value will be truncated. This means the decimal part of the real value will be lost, and without rounding.

Implicit Numeric Casts
Console.WriteLine(1.1 as Integer);
Console.WriteLine(2.99 as Integer);
Console.WriteLine(-3.99 as Integer);
Console Window
1
2
-3

Upcasting and Downcasting

When casting between classes related through Inheritance, one class must be a base class of the other.

Classes with Inheritance
class A {}
class B : A {}
class C : B {}

The code snippet below uses the Classes with Inheritance snippet.

Upcasting and Downcasting
var a : A = A();
var b : B = B();
var c : C = C();

// implicit
var ba : A = b;
var ca : A = c;
var cb : B = c;

// explicit
var ab : B = a as B;
var ac : C = a as C;
var bc : C = b as C;

Notice how all the implicit casts involve converting from a derived class to a base class. This is known as upcasting. Downcasting involves taking a base class and casting it to a derived class, which must be done explicitly. Downcasting is also a type of dynamic casting, which is validated at runtime. Observe that the objects a, b, and c are all references being casted, and not values. If a base class reference is downcasted to a derived class reference and the value referenced is of the base class type, then the cast failed at runtime. A failed dynamic cast will result in the value null. The code snippet below uses the Classes with Inheritance snippet.

Dynamic Casting
var a_ref_b : A = B(); // upcast invoked implicitly
var b_ref_a : B = A() as B; // downcast invoked explicitly
var b_ref_b : B = a_ref_b as B; // downcast invoked explicitly
Console.WriteLine(a_ref_b == null);
Console.WriteLine(b_ref_a == null);
Console.WriteLine(b_ref_b == null);
Console Window
false
true
false

a_ref_b is a reference of type A to a value of type B, which uses an implicit upcast. b_ref_a is a reference of type B to a value of type A, which uses an explicit downcast. Notice how b_ref_a has the value null at runtime, the result of a failed dynamic cast. On the other hand, a_ref_b is successfully cast since it has a value of type B.

Null Casts

The value null can be cast to any reference type implicitly. The code snippet below uses the Classes with Inheritance snippet.

Null Casting
var a1 : A = null; // implicit cast
var a2 : A = null as A; // explicit cast (unnecessary)
NOTE: Interpreting null as an invalid state for object references is a common pattern in programming.

Any Casts

Any type can be cast implicitly to an any, and an any value can be implicitly cast to any type. The any type is used as a generic reference to any instantiated class or struct.

Any Casting
var integerOne : any = 1;
var myInteger : Integer = integerOne; // any cast
var myReal : Real = integerOne; // runtime exception

Notice how the attempt to cast integerOne, which is an any, to a Real results in a runtime exception. An any must first be cast to the type of the value it holds, even before other implicit conversions.

Same Casts

Same casts are casts that convert a value of one type to the same type. All same casts can be done implicitly and are unnecessary.

Same Casting
var myReal : Real = 1.0; // implicit same cast
var myInteger : Integer = 1 as Integer; // explicit same cast

Related Materials

Manual

Function
Variable
Inheritance

Code Reference

Integer
DoubleInteger
Real
DoubleReal
Boolean
any

Last Author
curtis.eichner
Last Edited
Aug 8 2017, 2:45 PM
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