TryCast

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TryCast

Converts to an interface or a typed pointer if the object really is one, and answers null if it is not.

Syntax

result = trycast( interface, expression )
result = trycast( typename ptr, expression )

Parameters

• interface: the interface wanted.
• typename ptr: a pointer to the type wanted.
• expression: an interface value, or a pointer or reference to an object whose type extends object.

Return Value

With an interface: a value of that interface referring to the same object, or a null interface value (equal to nullptr) when the object does not implement it.

With a pointer type: a pointer to the object, or nullptr when the object is not of that type or one derived from it.

Description

trycast asks the object at run time. It is the checked counterpart of Cast: cast( Note ptr, v ) takes the programmer's word for it, and a wrong one is undefined behaviour; trycast( Note ptr, v ) looks, and answers nullptr when the object is something else.

From an interface value it does two things no other operator does:

• A cross cast: trycast( ISize, v ) asks whether the object behind an IShow also implements ISize, and if so gives an ISize for the same object.
• A downcast to the concrete type: trycast( Note ptr, v ).

interface IShow
    function Show( ) as ustring
end interface
interface ISize
    function Size( ) as int32
end interface

class Note
    implements IShow, ISize
    declare function Show( ) as ustring implements IShow.Show
    declare function Size( ) as int32 implements ISize.Size
    as ustring text = "remember the milk"
end class
function Note.Show( ) as ustring
    return text
end function
function Note.Size( ) as int32
    return len( text )
end function

class Label
    implements IShow
    declare function Show( ) as ustring implements IShow.Show
end class
function Label.Show( ) as ustring
    return "label"
end function

sub Describe( byval v as IShow )
    print v.Show( );
    dim as ISize sz = trycast( ISize, v )         '' does this object ALSO implement ISize?
    if sz <> nullptr then print ","; sz.Size( ); " characters";
    dim as Note ptr np = trycast( Note ptr, v )   '' is it a Note?
    if np <> nullptr then print " (a Note)";
    print
end sub

dim as Note n
dim as Label l
Describe( n )
Describe( l )

remember the milk, 17 characters (a Note)
label

Converting up never needs trycast: a Note converts to IShow implicitly, and an interface to any interface it inherits. Converting out of an interface implicitly is an error that suggests trycast (TKO1009).

Remarks

• The target must be an interface or a pointer to a type (TKO1021).
• trycast is a run-time search through a small table, so test once and keep the result rather than casting inside a hot loop.
• expression is typename is the yes/no form of the same question; see Operator Is. When the answer is known at compile time the compiler says so (TKO1016).

See also

• Cast
• Operator Is
• Interface
• Interfaces Tutorial

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Category: Interfaces Kind: operator