Tutorial 6. Y. Bernat. Object Oriented Programming 2, Spring Y. Bernat Tutorial 6

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1 Tutorial 6 Y. Bernat Object Oriented Programming 2, Spring 2016

2 Outline Part I Today's Topics 1

3 Intro!= C++ Standard Library Standard Template Library 3 ingerdience/concepts Set of data structures that hold (contain) objects. E.g. std::vector, std::map, std::list Set of functions that can do many useful and common operations on (the) containers. E.g. std::count(), std::any_of(), std::max_element() An abstraction of pointers to connect between containers and algorithms

4 Operations At the minimum: dereferencing (*), comparing (==,!=) and increasing (++) Actually, the dereferencing is depended on the iterator category There are Output Iterator, Input Iterator and Forward Iterator categories Bidirectional Iterator allows decreasing ( ), too Random Iterator allows pointer arithmetic for jumping a few steps forward/backward at once iterator, const_iterator, reverse_iterator, const_reverese_iterator

5 Iterator invalidation It's important to be aware of iterator invalidation! Actually, it applies also for pointers and references to objects in the container Each container with its rules about what action on what part of the container invalidates which iterators/pointers/references... Understending the intended implementation under the container denition can help understanding the possible inuence on iterators etc. instead of just memorizing the rules And there are always good references...

6 Using iterators to mark a range It's obvious how we use iterator to point to a single object We use it for marking ranges too convention to do it is with right-open interval notation end() itself is not a part of the range In math notation: [begin(), end())

7 and their containers Usually, iterators know nothing about their containers You can't add new items to a container using an interator! (Usually... ) Only reading/changing current items is possible We'll see later special iterators that allow us adding new items to a container

8 Creating iterators Each container has begin(), end(), cbegin() and cend() member functions Most of them have also r-versions There are also free function versions (e.g. std::begin(), std::crend(), etc.) They can be used on arrays too, for example The exact return type of begin()/end()/rbegin()/rend() depends on the constness of the container (example code)

9 Range-based for-loops Implemented by using begin/end iterators (found by calling member begin/end or the non-member ones) It means that members of name begin/end are better to be public functions that return the appropriate iterators, if it makes sense to use our class with range-based for-loop Thus, iterator invalidation brakes the loop!

10 Container categories Sequence containers - Place objects by insertion order. Accessed by index or running over them sequentially Associative containers - Place objects ordered (sorted). Typically accessed by a key associated with the stored value Unordered associative containers - Similar to the associative containers but without an order on the objects (new to C++11) Adaptors - Wrappers around other containers to expose a specic interface Understanding the usual implementation helps reasoning about the available operations, main characteristics, eciency and iterator type and invalidation

11 Sequence containers std::vector, std::list, std::deque (and since C++11, std::forward_list and std::array) The template argument is what type we want it to hold Vector should be the default choice Vector is ecient for insertion/deletion at the end Deque - for insertion/deletion at both sides List - everywhere (assuming you already have an iterator to that place... ) Vector and deque items can be accessed randomly, list - only sequencially Iterator invalidation dierences (Deque is just weird... ) Performance considerations std::vector::reserve() for stabilizing (and better performance)

12 Associative containers std::map, std::set, std::multimap, std::multiset Ordered The template arguments for (multi-)map are two types, type of the key and the mapped type For (multi-)set there is only one type argument, the value type Another argument to the template says how to sort (default is less-than, <) operator[] vs. nd() and insert()/emplace() Performance Sorted vector as a replacement

13 Unordered associative containers std::unordered_map, std::unordered_set, std::unordered_multimap, std::unordered_multiset Performance

14 How to choose which container to use The most natural for the problem If there is a performance issue and the proler shows it's around the container access: Try converting to vector Use reserve() for better vector performance (But rst think about your algorithms and access patterns!)

15 Intro Not part of, part of the language (since C++11, with great enhancements in C++14) Needed as a tool for the next topic for, algorithms The problem: Sometimes it's very useful to get a function as a parameter The trivial choice is a function pointer Example code Initial solution: Functor (function object) Example code

16 Lambda parts: [] - Lambda function introducer (and capture list) Capture by value, by reference, this, default capturing init-capture (C++14) The parameter list (can be ommitted if empty; can use auto since C++14) mutable The return type (trailing return type style) - usually not needed The (lambda) function body

17 Summary Lambda function is a functor that is written easily and inline All the logic is local, no need to nd the correct scope to dene the function when writing and nd the function when reading the code Don't overuse/abuse! If it's long, it's probably better as a regular function or functor

18 Overview Most of them in <algorithm>, some in <numeric>, and specic groups in <functional> or even <iterator> Implement common algorithms in a generic way Generic AKA using iterators What if we need an action to be done on the container? 3 options: 1 Specic algorithm version as a member (e.g. sort() and remove() for std::list) 2 Using iterator that knows how to do it (e.g. std::back_insert_iterator) 3 Giving us the needed info and we'll do it (e.g. std::remove())

19 Examples std::reverse() - Takes two iterators, rst and last, and replace every pair of values between rst and last-1 std::count() - Takes two iterators and a value and counts how many times the value appears in the range std::count_if() - Takes two iterators and a predicate and counts for how many values the predicate returns true std::all_of() / std::any_of() / std::none_of() - Take two iterators and a predicate and returns if the predicate holds for all/any/none of the values in the range. Note the dierences about empty range!

20 Examples 2 std::remove() - Takes two iterators and a value and removes this value from the range Returns the new end Be careful! It doesn't do what we'd think it does! It doesn't know anything about the container, so it just takes all the not-to-be-removed items to the front, optionally overwriting some to-be-removed items, and tells us where is the new end Now we can use the returned information to really delete the unneeded items Remove-erase idiom std::remove_if() - Of course...

21 Examples 3 std::copy() / std::copy_if() - Takes two input iterators and an output iterator (and predicate, in case of the _if version) We CAN'T just give it an iterator for the container. It doesn't know how to add new objects there! Two approaches: 1 Prepare the container to have enough size to overwrite with the new copied objects 2 Use an iterator that knows how to insert new objects, e.g. std::back_inserter_iterator

22 Outline Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ Part II Backlog 2 Templates - cont. 3 More SFML Stu 4 Conversion to/from Text 5 Inheritance 6 Class Special Functions Class Special Member Functions 7 Common Linker Errors 8 C++

23 Misc Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ typename for template-depended types For default value, use c-tor calling syntax / brace initialization instead of a simple assignment (of 0, for example) for allowing types with explicit c-tors Use of global (not-in-class) template operators Declaration should be in class (as friend), if we want implicit conversion to work (e.g. for operator* for Zp class)

24 Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ Template specializations Template explicit specialization (full specialization) - relevant for classes and functions (but see below) Template partial specialization - relevant for classes but not for functions For functions, it may seems like a partial specialization, but it's overloading But explicit specialization is not part of the overloading resolution! Better to use only overloading (without specialization) or specialize inside a struct

25 sf::view, sf::viewport Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ sf::view controlls the displayed part of the window Denes which part of the window coordinate system is displayed sf::viewport controlls which part of the window will be used to show the relevant sf::view Converting mouse click position to window coordinates is a must if you start using views and viewports

26 Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ Converting values (numbers) to/from text Converting to text is easy: std::to_string() Before C++11 we used std::(o)stringstream. It's still useful in some cases Converting text to values: Don't use atoi() or even strtoll()! (or sscanf()) Use the safe alternative: stoi() etc. The second argument, pos, can become handy std::(i)stringstream is more useful in many cases

27 Inheritance Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ One of the main concepts of OOP Will be more eective when we'll see polymorphism (which is one of OOP principles) Inheritance vs. real world categories Pinguin example Rectangle and square example

28 Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ Polymorphism virtual functions Understand how it works (usually) compile-time dispatch vs. run-time dispatch Possible pitfalls Using dierent arguments (override keyword) Usage in c-tor/d-tor Overriding of non-virtual functions Default arguments Overriding part of an overloading set of virtual functions (using) Virtual d-tor nal keyword (but only for virtual functions) Abstract Base Class principle assignment issue

29 Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ Class special member functions Class Special Member Functions Destructor Frees the class resources Copy c-tor Controls the creation of a new object from another (e.g. for deep copy) Assignment operator Controls copying one object over an existing one (again, for deep copy and freeing the old resources) Move c-tor Controls the creation of a new object from another, when we can steal resources from the original Move assignment operator Controls the copying one object over an existing one, when we can steal resources from the original Default c-tor Controls the creation of an object when no parameter has given

30 Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ More about special member functions Class Special Member Functions Explicitly defaulting or deleting special functions Copy/Move sometimes suppressed by RVO/NRVO Rule-of-thumb to identify when move can't happen: if there are two names to access the same object e.g. returning a function static variable

31 Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ A common misconcept about c-tors Class Special Member Functions Calling a c-tor from within another c-tor body doesn't do what you maybe think it does! It isn't the same as calling a c-tor from another c-tor member initializer list (the term for such a call is delegation)

32 Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ Compiler errors vs. Linker errors Compiler errors include source line number; Linker errors - only (object) le name Linker errors are usually more complicated and less understandable

33 Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ 2 most common Linker errors Double denition Unresolved externals (unresolved reference)

34 Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ C++ vs. C Line comments (// vs. block comments - /* comment */) References (so don't use pointers, until we'll have to) const (ported back to C, too) - use it wisely wherever you can; don't use #dene Function overloading Default values for function arguments Namespaces Exceptions OOP (class, public, private, etc.) Generic programming (Templates) Much richer standard library (including )

35 Introduction Templates - cont. More SFML Stu Conversion to/from Text Inheritance Class Special Functions Common Linker Errors C++ C++11/14 Feels like a new language (Bjarne Stroustrup) We'll see where there are diereneces and what the C++98/03 way to do things was VS2015 implements almost all C++11/14 and even a bit from C++17 Pros and Cons Course objectives

Tutorial 7. Y. Bernat. Object Oriented Programming 2, Spring Y. Bernat Tutorial 7

Tutorial 7. Y. Bernat. Object Oriented Programming 2, Spring Y. Bernat Tutorial 7 Tutorial 7 Y. Bernat Object Oriented Programming 2, Spring 2016 Exercise 4 STL Outline Part I Today's Topics 1 Exercise 4 2 STL Containers - continue Lambda functions Algorithms Exercise 4 STL Exercise