The Standard Template Library

The Standard Template Library • provides the framework for building generic,highly reusable algorithms and data structures • A reference implementation of STL has been put into the public domain by Hewlett-Packard • Bjarne Stroustrup AT&T : "large, systematic, clean, formally sound, comprehensible, elegant, and efficient framework” • Pamela Seymour Leiden University: "STL looks like the machine language macro library of an anally retentive assembly language programmer"

Design goals • Generality + Efficiency • Well structured, comprehensive library of useful components • Every component is as abstract as theoretically possible and as efficient as its hand-coded, non-abstract version in C

The past 25 years have seen attempts to revolutionize programming by reducing all programs to a single conceptual primitive. Functional programming, for example, made everything into a function; the notions of states, addresses, and side effects were taboo. Then, with the advent of object-oriented programming (OOP), functions became taboo; everything became an object (with a state). STL is heavily influenced by both functional programming and OOP. But it's not a single-paradigm library; rather, it's a library for general-purpose programming of von Neumann computers. STL is based on an orthogonal decomposition of component space. For example, an array and a binary search should not be reduced to a single, fundamental notion. The two are quite different. An array is a data structure -- a component that holds data. A binary search is an algorithm -- a component that performs a computation on data stored in a data structure. As long as a data structure provides an adequate access method, you can use the binary-search algorithm on it. Alexander Stepanov (BYTE 1995)

Standard Template Library (STL) • object oriented programming - reuse, reuse, reuse • STL has many reusable components • Divided into • containers • iterators • algorithms • This is only an introduction to STL, a huge class library

STL (Standard Template Library) A library of class and function templates based on work in generic programming done by Alex Stepanov and Meng Lee of the Hewlett Packard Laboratories in the early 1990s. It has three components: 1. Containers: Generic "off-the-shelf" class templates for storing collections of data 2. Algorithms: Generic "off-the-shelf" function templates for operating on containers 3. Iterators: Generalized "smart" pointers that allow algorithms to operate on almost any container Iterators Container Classes Algorithms begin() end() vector sort()

Standard Template Library (STL) algorithms containers sort() vector iterators function libraries class libraries begin() end() classtemplates functiontemplates classes overloadedfunctions user-defined(enums, structs, etc.) specific functions inline Code data types Data + Algorithms = Programs The Evolution of Reusability/Genericity

Containers, Iterators, Algorithms Algorithms use iterators to interact with objects stored in containers Container Container Iterator Algorithm Iterator Objects Algorithm Iterator Iterator Algorithm

STL's Containers In 1994, STL was adopted as a standard part of C++. There are 10 containers in STL: Kind of ContainerSTL Containers Sequential:deque, list, vector Associative:map, multimap, multiset, set Adapters:priority_queue,queue, stack Non-STL:bitset, valarray, string

Organizacja STL (2)

Operations Constructors: vector<T> v, // empty vector v1(100), // contains 100 elements of type T v2(100, val), // contains 100 copies of val v3(fptr,lptr); // contains copies of elements in // memory locations fptr up to lptr Copy constructor Destructor v.capacity()Number of elements v can contain without growingv.max_size() Upper limit on the size and capacity v.size() Number of elements v actually containsv.reserve(n) Increase capacity (but not size) to n vector v.empty() Check if v is empty v.push_back(val) Add val at end v.pop_back() Remove value at endv.front(), v.back(), Access first value, last value, v[i], v.at(i)i-th value without / with range checking(at throws out-of-range exception) Relational operators Lexicographic order is used Assignment (=) e.g., v1 = v2;v.swap(v1) Swap contents with those of vector v1

Iterators • Iterators are similar to pointers • point to first element in a container • iterator operators uniform for all containers • * dereferences, ++ points to next element • begin() returns iterator pointing to first element • end() returns iterator pointing to last element • use iterators with sequences (ranges) • containers • input sequences - istream_iterator • output sequences - ostream_iterator

Iterators • Iterators are pointer-like entities that are used to access individual elements in a container. • Often they are used to move sequentially from element to element, a process called iterating through a container. vector<int> array_ 17 vector<int>::iterator 4 23 The iterator corresponding to the class vector<int> is of the type vector<int>::iterator 12 size_ 4

Iterators – generalized pointers template<typename T> T T The other operations require knowledge of iterators. For example: v.begin() Returns iterator positioned at first elementv.end() Returns iterator positioned immediately after last element v.insert(it, val) Inserts val at position specified by iterator itv.erase(it) Removes the element at position specified by iterator itNote:insert() moves all the elements from position it and following one position to the right to make room for the new one. erase() moves all the elements from position it and following one position to the left to close the gap. An iterator declaration for vectors has the form: vector<T>::iterator it; Example: Function to display the values stored in a vector of doubles: ostream & operator<<(ostream & out, const vector<double> & v){ for (int i = 0; i < v.size(); i++) out << v[i] << " "; return out;} or using an iterator: for (vector<double>::iterator it = v.begin(); it != v.end(); it++) out << *it << " "; Go to 54

Przykład – vector (1)

vector inefficiences • When its capacity must be increased, • it must copy all the objects from the old vector to the new vector. • it must destroy each object in the old vector. • a lot of overhead! • With deque this copying, creating, and destroying is avoided. • Once an object is constructed, it can stay in the same memory locations as long as it exists (if insertions and deletions take place at the ends of the deque). • Unlike vectors, a deque is not stored in a single varying-sized block of memory, but rather in a collection of fixed-size blocks (typically, 4K bytes). • One of its data members is essentially an array map whose elements point to the locations of these blocks.

STL’s stack container STL includes a stack container. Actually, it is an adapter, as indicated by the fact that one of its type parameters is acontainer type. Sample declaration: stack<int, vector<int> > st; Basically, it is a class that acts as a wrapper around another class,providing anew user interface for that class. A container adapter such as stack uses the members of the encapsulated container to implement what looks like a new container. For a stack<T, C<T> >, C<T> may be any container that supports push_back() and pop_back() in a LIFO manner.In particular C may be a vector, a deque, or a list.

Basic Operations Constructorstack< T, C<T> > st; creates an empty stack st of elements of type T; it uses a container C<T>to store the elements. Note 1: The space between the two >s must be there to avoid confusing the compiler (else it treats it as >>); for example, stack< int, vector<int> > s;not stack< int, vector<int>> s; Note 2: The default container is deque; that is, if C<T> is omitted as instack<T> st; a deque<T> will be used to store the stack elements. Thus stack<T> st; is equivalent to stack<T, deque<T> > st; Destructor Assignment, relational Operators size(),empty(), top(), push(), pop()

STL's queue container In queue<T, C<T> >, container type C may be list or deque. Why not vector? You can't remove from the front efficiently! The default container is deque. queue has same member functions and operations as stack except: push() adds item at back (our addQ()) front() (instead of top()) retrieves front item pop() removes front item (our removeQ()) back() retrieves rear item

queue Example

Contd.

Deques As an ADT, a deque — an abbreviation for double-ended queue — is a sequential container that functions like a queue (or a stack) on both ends. It is an ordered collection of data items with the property that items can be added and removed only at the ends. Basic operations are: Construct a deque (usually empty): Check if the deque is empty Push_front: Add an element at the front of the deque Push_back: Add an element at the back of the deque Front: Retrieve the element at the front of the deque Back: Retrieve the element at the back of the deque Pop_front: Remove the element at the front of the deque Pop_back: Remove the element at the back of the deque

STL's deque Class Template Has the same operations as vector<T> except that there is no capacity() and no reserve() Has two new operations: d.push_front(value);Push copy of value at front of d d.pop_front(value);Remove value at the front of d Like STL's vector, it has several operations that are not defined for deque as an ADT: [] insert and delete at arbitrary points in the list, same kind of iterators. But insertion and deletion are not efficient and, in fact, take longer than forvectors.

vector vs. deque Capacity of a vector must be increased  it must copy the objects from the old vector to the newvector  it must destroy each object in the old vector  a lot of overhead! With deque this copying, creating, and destroying is avoided. Once an object is constructed, it can stay in the same memory locationsas long as it exists (if insertions and deletions take place at the ends of the deque). Unlike vectors, a deque isn't stored in a single varying-sized block of memory, but rather in a collection of fixed-size blocks (typically, 4K bytes). One of its data members is essentially an array map whose elements point to the locations of these blocks.

Algorithms • Before STL • class libraries were incompatible among vendors • algorithms built into container classes • STL separates containers and algorithms • easier to add new algorithms • more efficient, avoids virtual function calls • STL provides algorithms used generically across containers • operate on elements indirectly through iterators • often operate on sequences of elements defined by pairs of iterators • algorithms often return iterators, such as find() • premade algorithms save programmers time and effort

STL algorithms

STL's algorithms (§7.5) Another major part of STL is its collection of more than 80 genericalgorithms. They are not member functions of STL's container classes and do not access containers directly. Rather they are stand-alone functions that operate on data by means of iterators . This makes it possible to work with regular C-style arrays as well as containers. We illustrate one of these algorithms here: sort. Sort 1: Using < template <typename ElemType> void Display(ElemType arr, int n) { for (int i = 0; i < n; i++) cout << arr[i] << " "; cout << endl; } #include <iostream> #include <algorithm>using namespace std; // Add a Display() template for arrays int main(){ int ints[] = {555, 33, 444, 22, 222, 777, 1, 66}; // must supply start and "past-the-end" pointers sort(ints, ints + 8); cout << "Sorted list of integers:\n"; Display(Ints, 8);

double dubs[] = {55.5, 3.3, 44.4, 2.2, 22.2, 77.7, 0.1}; sort(dubs, dubs + 7); cout << "\nSorted list of doubles:\n"; Display(Dubs, 7); string strs[] = {"good","morning","cpsc","186","class"}; sort(strs, strs + 5); cout << "\nSorted list of strings:\n"; Display(strs, 5);} Output:Sorted list of integers:1 22 33 66 222 444 555 777Sorted list of doubles:0.1 2.2 3.3 22.2 44.4 55.5 77.7Sorted list of strings:186 class cpsc good morning

Sorting a vector of stacks using < (defined for stacks) #include <iostream> #include <algorithm> #include <vector> using namespace std; #include "StackT.h" /* Add operator<() to our Stack class template as a member function with one Stack operand or as a friend function with two Stacks as operands. Or because of how we're defining < for Stacks here, st1 < st2 if top of st1 < top of st2 we can use the top() access function and make operator<() an ordinary function */ template <typename StackElement> bool operator<(const Stack<StackElement> & a, const Stack<StackElement> & b) { return a.top() < b.top();}

int main() { vector< Stack<int> > st(4); // vector of 4 stacks of ints st[0].push(10); st[0].push(20); st[1].push(30); st[2].push(50); st[2].push(60); st[3].push(1); st[3].push(999); st[3].push(3); sort(st.begin(), st.end()); for (int i = 0; i < 4; i++) { cout << "Stack " << i << ":\n"; st[i].display(); cout << endl; } } Output Stack 0: 3 999 1 Stack 1: 20 10 Stack 2: 30 Stack 3: 60 50

For_Each() Algorithm #include <vector> #include <algorithm> #include <iostream> void show(int n) { cout << n << ” ”; } int arr[] = { 12, 3, 17, 8 }; // standard C array vector<int> v(arr, arr+4); // initialize vector with C array for_each (v.begin(), v.end(), show); // apply function show // to each element of vector v

Function Objects • function objects- contain functions invoked off the object using operator() • header <functional>

Functions Objects • Some algorithms like sort, merge, accumulate can take a function object as argument. • A function object is an object of a template class that has a single member function : the overloaded operator () • It is also possible to use user-written functions in place of pre-defined function objects #include <list> #include <functional> int arr1[]= { 6, 4, 9, 1, 7 }; list<int> l1(arr1, arr1+5); // initialize l1 with arr1 l1.sort(greater<int>()); // uses function object greater<int> // for sorting in reverse order l1 = { 9, 7, 6, 4, 1 }

Function Objects • The accumulate algorithm accumulates data over the elements of the containing, for example computing the sum of elements #include <list> #include <functional> #include <numeric> int arr1[]= { 6, 4, 9, 1, 7 }; list<int> l1(arr1, arr1+5); // initialize l1 with arr1 int sum = accumulate(l1.begin(), l1.end() , 0, plus<int>()); int sum = accumulate(l1.begin(), l1.end(),0); // equivalent int fac = accumulate(l1.begin(), l1.end() , 0, times<int>());

User Defined Function Objects class squared _sum // user-defined function object { public: int operator()(int n1, int n2) { return n1+n2*n2; } }; int sq = accumulate(l1.begin(), l1.end() , 0, squared_sum() ); // computes the sum of squares

User Defined Function Objects template <class T> class squared _sum // user-defined function object { public: T operator()(T n1, T n2) { return n1+n2*n2; } }; vector<complex> vc; complex sum_vc; vc.push_back(complex(2,3)); vc.push_back(complex(1,5)); vc.push_back(complex(-2,4)); sum_vc = accumulate(vc.begin(), vc.end() , complex(0,0) , squared_sum<complex>() ); // computes the sum of squares of a vector of complex numbers

Standard C++ Exceptions (1)

The Standard Template Library