Intermediate C++ Game Programming Tutorial 16

In the video we take a look at a couple of bangers from <algorithm>, and we learn what lambda functions are and how to use them.

Topics Covered

• Algorithms std::sort and std::remove_if
• Concepts (as in, type concepts like Comparable)
• <functional> functors as predicates
• Lambda functions

Errata

In the video Chili states that std::remove does not preserve the relative order of elements in the container, but this is fake news! Relative order is preserved: [1]

Video Timestamp Index

Tutorial 16

• Generic Algorithms: how Templates and Iterators make Algorithms independent of Data Types and Containers 0:17
• The std::sort algorithm and its requirements 0:43
• Sorting a (forward) list std::list, using its member function std::list::sort() 2:16
• Introducing Concepts / named requirements of types (e.g., the Compare requirement of std::sort 2:54
• Modyfing sorting order by passing a comparison function object (functor) as a predicate, like so:
  std::sort(vec.begin(),vec.end(),std::greater<int>{}); 3:17
  Note: vec is a std::vector of integers
• Sort custom types, example code for sorting a custom class object by creating a functor, called like so:
  std::sort(vecObj.begin(),vecObj.end(),Obj::YLess{}); 4:07
• Removing items from a container, std::remove and std::remove_if 6:11
  - Create a functor as a unary predicate for the std::remove function to remove elements using a threshold variable 8:28
  - Use container member function erase() to correct the endpoint of the container iterator after the remove operation 9:51
• Introducing the Lambda function to replace custom functors 11:51
  - Using the Lambda capture to bring in variables into the Lambda function 13:34
  - Capture syntax explained: [varl],[&var],[=],[&],[newvar=var+10],[this], capture multiple variables, mix&match capture modes
• Stateful Lambdas; changing captured lambda variables using the mutable flag 15:59
• Some anxiety reducing consolation by Chili 16:40
• Syntax for a callable Lambda function 17:03
  - Make it callable once: [...](...){...}(vec.front());, functor is destroyed immediately
  - Keep the functor using auto mylamb = [...](...){...};, which can be called by bool result = mylamb(vec.front()), or std::remove_if(...,...,mylamb);
• The power of C++ is in its flexibility. Example code for converting a string to lowercase:
  std::transform(str.begin(),str.end(),str.begin(),::tolower);17:49
• Homework assignment 18:39
• Introducing the [Algorithm Gems] series 19:20

Homework

Solve the problems in Source.cpp attached below using the <algorithm> library and other parts of the standard library as extensively as possible. As a bonus problem, implement the sprite drawing effect shown at the end of the video using a lambda function.

• Source.cpp (homework file)
• Sprite Repo (for bonus problem)

Related Series

For an in-depth look into the <algorithm> library and beyond, check out STD Gems.

Links

Algorithms Library

See also

• Next in series (Tutorial 17)
• Intermediate C++ Game Programming Series