Editing Virtual method table (section)

==Implementation==
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An object's virtual method table will contain the [[Memory address|addresses]] of the object's dynamically bound methods. Method calls are performed by fetching the method's address from the object's virtual method table. The virtual method table is the same for all objects belonging to the same class, and is therefore typically shared between them. Objects belonging to type-compatible classes (for example siblings in an inheritance hierarchy) will have virtual method tables with the same layout: the address of a given method will appear at the same offset for all type-compatible classes. Thus, fetching the method's address from a given offset into a virtual method table will get the method corresponding to the object's actual class.<ref>Ellis & Stroustrup 1990, pp. 227–232</ref>

The [[C++]] standards do not mandate exactly how dynamic dispatch must be implemented, but compilers generally use minor variations on the same basic model.

Typically, the compiler creates a separate virtual method table for each class. When an object is created, a pointer to this table, called the '''virtual table pointer''', '''vpointer''' or '''VPTR''', is added as a hidden member of this object. As such, the compiler must also generate "hidden" code in the [[Constructor (object-oriented programming)|constructor]]s of each class to initialize a new object's virtual table pointer to the address of its class's virtual method table.

Many compilers place the virtual table pointer as the last member of the object; other compilers place it as the first; portable source code works either way.<ref>
Danny Kalev.
[http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=196 "C++ Reference Guide: The Object Model II"].
2003.
Heading "Inheritance and Polymorphism" and "Multiple Inheritance".
</ref>
For example, [[g++]] previously placed the pointer at the end of the object.<ref>{{cite web |url=http://www.codesourcery.com/public/cxx-abi/cxx-closed.html |title=C++ ABI Closed Issues |access-date=2011-06-17 |url-status=bot: unknown |archive-url=https://web.archive.org/web/20110725153606/http://www.codesourcery.com/public/cxx-abi/cxx-closed.html |archive-date=25 July 2011}}</ref>