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 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

• Assume that Directory provides an iterator similar to std::vector
• 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 Directory

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 outputFilename()
• 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::traverse() would probably not use an iterator, but native functions/methods to the problem

Example: Directory Traversal Callback Constructor

• Only one call to traverse the directory
• Callback is set up in the constructor, so only one callback is possible per object
• Client calls intermediate method DirectoryProcessor::process()
• DirectoryProcessor::process() calls traverse()
• The method traverse() calls the client's outputFilename()
• So DirectoryProcessor code can work without knowing the code in the outputFilename()

Example Comparison