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[[Category:Compiler]]
{{Warning|'''Warning''': Article under development}}

===Introduction===
Dynamic binding is a key feature behind object technology. In the following piece of code:

{|border="0" cellpadding="2" cellspacing="0" align="center"
|-valign="top" -halign="center"
|
<code>foo (a: A)
   do
      a.f
   end</code>
|}

It is not known at compile time, which feature has to be called for call a.f. This depends on the exact type of the object referenced by a, also called its dynamic type.

===Some Ids===
In the following sections several ids are introduced. Their explanation is based on the following example system (Further referenced as the ABC system):

{|border="0" cellpadding="0" cellspacing="0" align="center"
|-valign="top" -halign="center"
|[[Image:DynamicBindingABCExample.jpg]]
|}

====routine_id====
In a compiled Eiffel system, every feature that is a seed gets its own unique '''routine_id'''. Features that are just inherited or are a redeclaration have the same '''routine_id''' as their precursor (See [[Routine IDs]]). 

It is possible for a feature to have more than one '''routine_id''' when it has more than one precursor. This is the case for feature j of class C that redefines both feature g of A and feature h of B and thus gets both their '''routine_id''''s.

As it will be shown later, the chosen '''routine_id''' have an impact on the dynamic binding semantics. In class C the feature f_b gets the routine_id 1 since it is selected, the not selected feature f of C gets a new '''routine_id'''.

====body_id and real_body_id====
Every routine body, that means every declaration or redeclaration, gets its own unique '''body_id'''. The '''body_id''' of feature g in classes A and B are the same since g is not redefined in B.

A body of a generic class may need several translated versions, when there are different type instantiations of the class. 
For getting the right version of the correct type instantiation the '''real_body_id''' is used. 
There might be for example two translations for feature f of class A. One for each of the types A [INTEGER] and A [STRING]. Their '''real_body_id''' will thus be different.

====feature_id====
Every feature of a class gets a '''feature_id''' that is unique in the context of this class.

====class_id and type_id====

Every class gets a unique class_id an every type a unique '''type_id'''.

===Dynamic binding in finalized code===
In finalized code the dynamic binding is very simple. For every routine_id there exists an array, that maps dynamic type ids to function pointers. This array is called routine table.



The code snipped how the dynamic binding works: 

{|border="0" cellpadding="2" cellspacing="0" align="center"
|-valign="top" -halign="center"
|
<code>[c, N]char *(*Txiockf[6])();
void Txiockf_init () {
   // A [INTEGER] has type_id 50
   Txiockf[0] = (char *(*)()) Fcuus0f;
   // B [INTEGER] has type_id 50
   Txiockf[1] = (char *(*)()) Fcuus0g;
   // C [INTEGER] has type_id 50
   Txiockf[2] = (char *(*)()) Fcuwquc;
   Txiockf[3] = (char *(*)()) Fcuwque;
   Txiockf[4] = (char *(*)()) Fcuwquc;
   Txiockf[5] = (char *(*)()) Fcuwque;
}</code>
|
|
<code>[c,N]void Fcubdx0 (EIF_REFERENCE Current, EIF_REFERENCE arg1) {
   ...
   (FUNCTION_CAST(void, (EIF_REFERENCE)) 
      Txiockf[Dtype(arg1)-50])(arg1);
   ...
}</code>
|
|}


In this example the type_id for 
A body_id refers to a certain body of a class. The real_body_id refers a body of one specific generic derivation of a class. The real_body_id is very important in workbench mode. Once the real_body_id is resolved for a call, the rest is just melting ice stuff. In the call to a.f the routine_id of f is known.

Template used on this page:

Template:UnderConstruction (view source)

Return to Dynamic Binding.

Retrieved from "https://dev.eiffel.com/Dynamic_Binding"

View source for Dynamic Binding

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