Dispatch

Selecting which implementation of an operation (method or function) to call

• static dispatch
• dynamic dispatch

Static Dispatch

Selecting which implementation of an operation (method or function) to call at compile time

• Which operation (method or function) will be called is determined at compile time
• Fast as a call can be made
• Preferred by the compiler for these reasons

C++ Static Dispatch

• The compiler uses static dispatch whenever it can:
• free functions
• non-virtual methods
• static methods
• method calls from non-pointer and non-reference variables
• Why? Less code to generate (program smaller)
• Why? Faster to call

Dynamic Dispatch

Selecting which implementation of an operation (method) to call at run time

• For virtual method calls whose object is a pointer or reference
• Requires more code than static dispatch
• Slightly slower to call
• Compilers have optimized dynamic dispatch, so the speed disadvantage is smaller than it used to be

Original class Shape

Problem for Dynamic Dispatch

• Methods through the pointer parameter pshape and the reference parameter shape calls may not exist yet
• The functions apply() are compiled so that for any class that inherits from the class Shape, the function calls the proper method
• Even though the parameter type at the time of compilation was a Shape* or a Shape&, the apply() functions must call the Shape methods for Shape objects and derived methods for objects of derived classes

Add class Circle

Problem for Dynamic Dispatch

• New methods now exist
• Original apply() code existed and was compiled long before class Circle was created
• The original apply() code in this case must call Circle::draw()

How? vtable

• At compile time, the virtual table or vtable is created to store the information required for the dynamic dispatch of virtual methods for a class
• For every object of a class that has a vtable, the object has a pointer to the class vtable
• Method calls go through the vtable for the class of an object
• We will examine the vtable structure in detail at a later date