Double Dispatch
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This document discusses using double dispatch to determine which method to invoke based on both the receiver and an argument. It provides examples of printing documents using type checks, which has drawbacks when new document types are added. Double dispatch solves this by having the receiver send a message back to the argument, passing itself as an argument, allowing the most specific method to be invoked based on both objects. It also provides a number coercion example where double dispatch avoids type checks by having integers and floats dispatch to primary operations on each other.
![S.Ducasse 3
Type Checking for Dispatching
How to invoke a method depending on the
receiver and an argument?
A not so good solution:
PSPrinter>>print: aDocument
^ aDocument isPS
ifTrue: [self printFromPS: aDocument]
ifFalse: [self printFromPS: aDocument asPS]
PSPrinter>>printFormPS: aPSDoc
<primitive>
PdfPrinter>>print: aDocument
^ aDocument isPS
ifTrue: [self printFromPDF: aDocument asPDF]
ifFalse: [self printFromPDF: aDocument]
PdfPrinter>>printFormPS: aPdfDoc
<primitive>
3](https://image.slidesharecdn.com/stoop-302-doubledispatch-101026133729-phpapp01/85/Double-Dispatch-3-320.jpg)
![S.Ducasse 7
Let’s Step Back
Example: Coercion between Float and Integer
Not a really good solution:
Integer>>+ aNumber
(aNumber isKindOf: Float)
ifTrue: [ aNumber asFloat + self]
ifFalse: [ self intAddPrimitive: aNumber]
Float>>+ aNumber
(aNumber isKindOf: Integer)
ifTrue: [aNumber asFloat + self]
ifFalse: [self floatAddPrimitive: aNumber]
Here receiver and argument are the same, we can](https://image.slidesharecdn.com/stoop-302-doubledispatch-101026133729-phpapp01/85/Double-Dispatch-7-320.jpg)
Double Dispatch
- 1. Stéphane Ducasse 1 Stéphane Ducasse stephane.ducasse@inria.fr http://stephane.ducasse.free.fr/ Double Dispatch?
- 2. S.Ducasse 2 How to invoke a method Depending on **both** the receiver and an argument… Type check are evil Overloading is plain bad Remember Java lookup? Use Double Dispatch
- 3. S.Ducasse 3 Type Checking for Dispatching How to invoke a method depending on the receiver and an argument? A not so good solution: PSPrinter>>print: aDocument ^ aDocument isPS ifTrue: [self printFromPS: aDocument] ifFalse: [self printFromPS: aDocument asPS] PSPrinter>>printFormPS: aPSDoc <primitive> PdfPrinter>>print: aDocument ^ aDocument isPS ifTrue: [self printFromPDF: aDocument asPDF] ifFalse: [self printFromPDF: aDocument] PdfPrinter>>printFormPS: aPdfDoc <primitive> 3
- 4. S.Ducasse 4 Drawbacks ofTypecheck • Adding new kinds of documents requires changes everywhere • Adding new documents requires changes everywhere • No dynamic (without recompilation) possibilities
- 5. S.Ducasse 5 Double Dispatch Solution: use the information given by the single dispatch and redispatch with the argument (send a message back to the argument passing the receiver as an argument)
- 6. S.Ducasse 6 Double Dispatch (a) PSPrinter>>print: aDoc aDoc printOnPSPrinter: self (b) PdfPrinter>>print: aDoc aDoc printOnPdfPrinter: self (c) PSDoc>>printOnPSPrinter: aPSPrinter <primitive> (d) PdfDoc>>printOnPdfPrinter: aPSPrinter aPSprinter print: self asPS (e) PSDoc>>printOnPSPrinter: aPdfPrinter aPdfPrinter print: self asPdf (f) PdfDoc>>printOnPdfPrinter:aPdfPrinter <primitive> SomeTests: psprinter print: psdoc =>(a->c) pdfprinter print: pdfdoc => (b->f) psprinter print: pdfdoc => (a->d->b->f) pdfprinter print: psdoc => (b->e->b->f)
- 7. S.Ducasse 7 Let’s Step Back Example: Coercion between Float and Integer Not a really good solution: Integer>>+ aNumber (aNumber isKindOf: Float) ifTrue: [ aNumber asFloat + self] ifFalse: [ self intAddPrimitive: aNumber] Float>>+ aNumber (aNumber isKindOf: Integer) ifTrue: [aNumber asFloat + self] ifFalse: [self floatAddPrimitive: aNumber] Here receiver and argument are the same, we can
- 8. S.Ducasse 8 Double Dispatch on Numbers (a) Integer>>+ aNumber ^ aNumber sumFromInteger: self (b) Float>>+ aNumber ^ aNumber sumFromFloat: self (c) Integer>>sumFromInteger: anInteger <primitive: 40> (d) Float>>sumFromInteger: anInteger ^ anInteger asFloat + self (e) Integer>>sumFromFloat: aFloat ^aFloat + self asFloat (f) Float>>sumFromFloat: aFloat <primitive: 41> Some Tests: 1 + 1: (a->c) 1 + 2.0 1.0 + 1.0: (b->f) 1 + 1.0: (a->d->b->f) 1.0 + 1: (b->e->b->f
- 9. S.Ducasse 9 Double Dispatching • Three Kinds of Messages – Primary operations – Double dispatching methods – Forwarding operations
- 10. S.Ducasse 9 Double Dispatching • Three Kinds of Messages – Primary operations – Double dispatching methods – Forwarding operations