Generalizing a Sort

C++ Library Equivalents

Callback

• A callback is a bit of computation that a function/method can call when appropriate in the implemented algorithm
• They permit customization of the computation in a function/method
• Not a new programming mechanism

Common Control Flow

Callback Control Flow

Control Flow Comparison

Callback Arguments

• free functions, function pointers
• lambda functions, anonymous functions
• functors, class that acts like a function
• methods, pointers to member functions

Example: Directory Traversal Iterator

Example: Directory Traversal Iterator

• Assume that Directory provides an iterator similar to std::vector
• The client has to know how to use the iterator
• Client calls Directory methods
• Directory methods do not call the client
• Have to know the methods available from the Directory

Example: Directory Traversal Callback

Example: Directory Traversal Callback

• Only one call to traverse the directory
• Each call to traverse can take a different callback
• Client calls traverse()
• The method traverse() calls the client's output()
• The callback is provided the filename via a parameter that the traverse() calls each pass
• Only the parameters are available to the callback
• Very useful when processing is quite complex, and it may be inefficient to save state and return each step
• The implementation of Directory::process() would probably not use an iterator but functions/methods native to the problem
• Processing of a directory is much more complex than this

Example: Process File

Callbacks

• Can be used with any level of complex processing
• Details of processing hidden inside the callback-enabled processor
• Client only has to provide the code to run when a particular event occurs
• Essential for asynchronous processing