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STL Overview

Prof. Brinton

Gordon College

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Standard Template Library (STL)

• Part of ISO-OSI Standard C++ Library 1998 • Object oriented programming is about reuse

– STL has many reusable components– Divided into

1. Containers - manage collections of objects

2. Iterators - step through the elements of collections

3. Algorithms - used to process the elements of collections

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Introduction to Containers

• Three types of containers 1. sequence containers - ordered collections

vector, deque, list

2. associative containers - sorted collectionsset, multiset, map, multimap

3. container adapters Stacks, queues, priority queues

• Near-containers - similar to containers, without all the capabilities– C-like arrays– string– bitset - for maintaining sets of 1/0 flag values – valarray - high-speed mathematical vector operations

• The containers have similar functions

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Introduction to Containers

• vector direct access to any element• deque rapid insertion at front or back

direct access to any element• list rapid insertion and deletion anywhere

• set rapid lookup, no duplicates• multiset rapid lookup, duplicates• map, multimap store key/value pairs

• stack last-in-first-out• queue first-in-first-out• priority_queue highest priority element is first

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Introduction to Containers

• Common member functions of all STL containers:constructors, destructorsassignment, all comparison operatorsswapemptymax_sizesize

• Use if(c.empty()) not if(c.size() == 0)

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Introduction to Iterators

• Iterators are similar to pointers– point to element in a container– iterator operators uniform for all containers

• * dereferences, ++ advances pointer• container.begin() returns iterator pointing to first element• container.end() returns iterator pointing after last element

for(it = c.begin(); it != c.end(); it++){ *it…}

it

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Iterator Categories

• Bidirectional iterators – Iterate in 2 directions: p++ or p--– list, set, multiset, map, multimap– Use != instead of < or > in loops

• Random access iterators– Have all properties of bidirectional iterators– Can also perform random access

• +, -, <, >

– vector, deque, strings

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Constness of Iterators

• Examples

vector<int>::iterator it = v.begin();vector<int>::const_iterator cit = v.begin();

*cit = 24; // does not compile *it = 24; // ok

void fct(const vector<int>& v)

{ vector<int>::iterator it; it = v.begin(); // does not compile}

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Constness of Iterators

• Examples (Fixed)

vector<int>::iterator it = v.begin();vector<int>::iterator cit = v.begin();

*cit = 24; // ok

*it = 24; // ok

void fct(const vector<int>& v)

{ vector<int>::const_iterator it; it = v.begin(); // ok

}

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Iterator Adapters

• Everything that behaves like an iterator is an iterator– Insert iterator (inserters)

– Stream iterator - allow you to use a stream as a source or dest of an algorithm.

– Reverse iterators

• Example:

list<Point>::reverse_iterator r1 = L.rbegin(), r2 = L.rend();

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Iterator Adapters vector<int> coll; back_insert_iterator<vector<int> > iter(coll); *iter = 1; iter++; *iter = 2; iter++; *iter = 3; PRINT_ELEMENTS(coll); back_inserter(coll) = 44; back_inserter(coll) = 55; PRINT_ELEMENTS(coll); coll.reserve(2*coll.size()); copy (coll.begin(), coll.end(), // source back_inserter(coll)); // destination PRINT_ELEMENTS(coll);

Results:1 2 31 2 3 44 551 2 3 44 55 1 2 3 44 55

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Introduction to Algorithms

• STL 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

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Sequence Containers

• Three sequence containers– vector - based on arrays– deque - based on arrays– list - robust linked list

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vector Sequence Container

• vector – #include <vector>– data structure with contiguous memory locations– use the subscript operator []– used when data must be sorted and easily accessible– when memory exhausted

• allocates a larger, contiguous area of memory• copies itself there• deallocates the old memory

– has a random access iterator

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vector Sequence Container

• Declarations – vector <type> v;– vector <type> v(7812); – template <class It>vector<type> v(It begin, It end);

– type - int, float, Point, whatsoever

• Iterators: – vector<type>::const_iterator it; vector<type>::iterator it = v.begin();

*(it1 + 5) = 34;

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vector Sequence Container (II)

• vector functions, for vector object vv.push_back(value) - add element to endv.size() - current size of vectorv.capacity() - how much vector can hold before reallocationv.reserve(n) - allow vector to avoid reallocation

v.insert( pointer, value ) - inserts value before pointer.

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vector Sequence Container (III)

• vector functions and operationsv.erase( pointer )

• remove element from container

v.erase( pointer1, pointer2 ) • remove elements starting from pointer1 and up to (not including) pointer2.

v.clear()• erases entire container.

v[elementNumber] = value; • assign value to an element

v.at[elementNumber] = value;• as above, with range checking

• throws out_of_bounds exception

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list Sequence Container

• list container – #include <list>– efficient insertion/deletion anywhere in container– doubly-linked list– bidirectional iterators

• There exists also a non-standard slist– singly-linked list– forward iterator

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list Sequence Container (II)

• list functions for listObject and otherObjectlistObject.sort()

• sorts in ascending order

listObject.splice(iterator, otherObject);• inserts values from otherObject before location of iterator

listObject.merge(otherObject)• removes otherObject and inserts it into listObject, sorted

listObject.unique()• removes duplicate elements

listObject.swap(otherObject); • exchange contents

listObject.assign(iterator1, iterator2)• replaces contents with elements in range of iterators

listObject.remove(value)• erases all instances of value

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deque Sequence Container

• deque ("deek") - double-ended queue – #include <deque>– indexed access using []– efficient insertion/deletion in front and back– non-contiguous memory: "smarter" pointers

• same basic operations as vector – adds push_front / pop_front - insertion/deletion at

beginning of deque

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Associative Containers

• Associative containers – provide direct access to store and retrieve elements via keys

(search keys)

• 4 types: multiset, set, multimap and map– keys in sorted order– multiset and multimap allow duplicate keys– multimap and map allow keys and associate values

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Excursion: Function Objects

template <class T>struct less { bool operator()(const T& lhs, const T& rhs) const { return lhs < rhs; }};

less<int> is_smaller;if(is_smaller(45, 34)){..}

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multiset Associative Container

• multiset - fast storage, retrieval of keys– allows duplicates

• Ordering by comparator function objectless<type> - sorts keys in ascending ordermultiset< int, less<int> > integers;

• CGAL comparator function objectstypedef CGAL::Cartesian<double> K;multiset< K::Point_2, K::Compare_xy_2 > points;

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multiset Associative Container (II)

• Functions for multiset object msObject– msObject.insert(value) - inserts value– msObject.find(value) - returns iterator to first

instance of value, or msObject.end()– msObject.lower_bound(value)- returns iterator to first

location of value – msObject.upper_bound(value)- returns iterator to

location after last occurrence of value– for a pair object p

p = msObject.equal_range(value)• sets first and second to lower_bound and upper_bound

for a given value

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set Associative Container

• set– implementation identical to multiset– unique keys - duplicates ignored and not inserted– supports bidirectional iterators (but not random access)– #include <set>

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map Associative Container

• map– fast storage/retrieval of unique key/value pairs– #include <map>– one-to-one mapping (duplicates ignored)– use [] to access values

for map<int, double> M;

M[30] = 4000.21;

sets the value of key 30 to 4000.21

– if subscript not in map, creates new key/value pair– Efficiency remark: Use insert to insert, operator[] to update

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Non Standard Associative Container

• hash_(multi)set, hash_(multi)map – Not part of the standard, but provided by some STLs

– REALLY fast storage/retrieval of unique key/value pairs

– Based on hashing and not on binary trees

– No order, but provide iterators

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Container Adapters

• container adapters: stack, queue, priority_queue – not first class containers– do not support iterators– do not provide actual data structure– programmer can select implementation of the container adapters– have member functions push() and pop()

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stack Adapter

• stack– insertions and deletions at one end – last-in-first-out data structure– implemented with vector, list, and deque (default)– #include <stack>

• Declarationsstack<type, vector<type> > myStack;stack<type, list<type> > myOtherStack;stack<type> anotherStack;

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queue Adapter

• queue - insertions at back, deletions at front– first-in-first-out data structure– implemented with list or deque– #include <queue>

• Functions– push(element) - (push_back) add to end– pop(element) - (pop_front) remove from front– empty() - test for emptiness– size() - returns number of elements

• Example:queue <double> values; //create queuevalues.push(1.2); // values: 1.2values.push(3.4); // values: 1.2 3.4values.pop(); // values: 1.2

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priority_queue Adapter

• insertions in sorted order, deletions from front– implemented with vector or deque– highest priority element always removed first

• heapsort puts largest elements at front• less<T> by default, programmer can specify another

• Functions– push - (push_back then reorder elements)– pop - (pop_back to remove highest priority element)– size– empty

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Algorithms• Before STL

– class libraries were incompatible among vendors– algorithms built into container classes

• STL separates containers and algorithms– easier to add new algorithms– not member functions of containers, global functions that operate with iterators

(reduces the amount of code)• Not necessarily intuitive…not all combinations possible.

– General fundamental services on containers:• Searching• Sorting• Copying• Reordering• Modifying• Numeric processing

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Algorithms Example#include <iostream>#include <vector>#include <algorithm>using namespace std;

int main(){ vector<int> coll; vector<int>::iterator pos; coll.push_back(2); coll.push_back(5); coll.push_back(4); coll.push_back(1); coll.push_back(6); coll.push_back(3);

pos = min_element (coll.begin(), coll.end()); cout << "min: " << *pos << endl; pos = max_element (coll.begin(), coll.end()); cout << "max: " << *pos << endl;

sort (coll.begin(), coll.end());

pos = find (coll.begin(), coll.end(), 3); reverse (pos, coll.end());

}

2 5 4 1 6 3

1

6

1 2 3 4 5 6

1 2 6 5 4 3

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Algorithms• Ranges

pos = min_element (coll.begin(), coll.end());

• Multiple Rangesif (equal (coll1.begin(), coll1.end(), coll2.begin()) { … }

Algorithm Categories:– Nonmodifying - count, for_each, min_element…– Modifying - copy, transform, merge, replace, fill…– Removing - remove, unique…– Mutating - reverse, rotate, random_shuffle…– Sorting - sort_heap, make_heap, sort, …– Algorithms for Sorted Ranges - binary_search, merge, …– Numeric - accumulate, inner_product, …

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Non-Modifying Algorithm: for_eachvoid print (int elem){ cout << elem << ' ';}

int main(){ vector<int> coll;

INSERT_ELEMENTS(coll,1,9);

// call print() for each element for_each (coll.begin(), coll.end(), print);

cout << endl;}

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Modifying Algorithm: copy{ vector<int> coll1; list<int> coll2; INSERT_ELEMENTS(coll1,1,9);

copy (coll1.begin(), coll1.end(), back_inserter(coll2)); copy (coll2.begin(), coll2.end(),

ostream_iterator<int>(cout," ")); cout << endl;

copy (coll1.rbegin(), coll1.rend(), coll2.begin());

}

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Modifying Algorithm: unique int source[] = { 1, 4, 4, 6, 1, 2, 2, 3, 1, 6, 6, 6, 5, 7, 5, 4, 4 }; int sourceNum = sizeof(source)/sizeof(source[0]); list<int> coll;

copy (source, source+sourceNum, back_inserter(coll));

list<int>::iterator pos; pos = unique (coll.begin(), coll.end());

copy (coll.begin(), pos, ostream_iterator<int>(cout," " // source)); cout << "\n\n"; copy (source, source+sourceNum, coll.begin());

coll.erase (unique (coll.begin(), coll.end(), greater<int>()), coll.end());

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accumulate, transform

• list<int> L(..);int sum = accumulate(L.begin(), L.end(), 0, plus<int>());

• transform(istream_iterator<Point>(cin), istream_iterator<Point>(), ostream_iterator<double>(cout), mem_fun_ref(Point::x));

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Basic Searching Algorithms

• find(iterator1, iterator2, value)– returns iterator pointing to first instance of value

• find_if(iterator1, iterator2, function)– like find, but returns an iterator when function returns true.

• binary_search(iterator1, iterator2, value)– searches an ascending sorted list for value using a binary search

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Sorting Algorithms

• sort(begin, end) • partial_sort(begin, begin+N, end)

– finds first N and sorts them

• nth_element(begin, begin+N, end)– finds first N, not sorted

• partition(begin, end, function)– splits in two intervals

• stable_sort, stable_partition

• Remarks– All take optionaly a comparison function– std::sort is faster than clib sort

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equal, mismatch,lexicographical_compare

• Functions to compare sequences of values

• equal – returns true if sequences are equal (uses ==)– returns false if of unequal lengthequal(iterator1, iterator2, iterator3);

– compares sequence from iterator1 up to iterator2 with the sequence beginning at iterator3

– Containers can be of different types

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fill, fill_n, generate, generate_n

• STL functions, change containers.

• fill - changes a range of elements (from iterator1 to iterator2) to value

– fill(iterator1, iterator2, value);

• fill_n - changes specified number of elements, starting at iterator1– fill_n(iterator1, quantity, value);

• generate - like fill, but calls a function for value– generate(iterator1, iterator2, function);

• generate_n - like fill_n, but calls function for value– generate(iterator1, quantity, value)

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swap, iter_swap and swap_ranges

• swap(element1, element2) - exchanges two valuesswap( a[ 0 ], a[ 1 ] );

• iter_swap(iterator1, iterator2) - exchanges the values to which the iterators refer

• swap_ranges(iterator1, iterator2, iterator3) - swap the elements from iterator1 to iterator2 with the elements beginning at iterator3

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copy_backward, merge, unique, reverse

• copy_backward(iterator1, iterator2, iterator3)– copy the range of elements from iterator1 to iterator2 into

iterator3, but in reverse order.

• merge(iter1, iter2, iter3, iter4, iter5) – ranges iter1-iter2 and iter3-iter4 must be sorted in ascending

order. – merge copies both lists into iter5, in ascending order.

• unique(iter1, iter2) - removes duplicate elements from a sorted list, returns iterator to new end of sequence.

• reverse(iter1, iter2)- reverses elements in the range of iter1 to iter2.

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Remove does (not) Remove

• Member function:L.remove(Point(0,0));

• Algorithm moves at end:list<Point> L(istream_iterator<Point>(cin), istream_iterator<Point>()); list<Point>::iterator eit;

eit = remove(L.begin(), L.end(), Point(0,0));

L.erase(eit, L.end());

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Other Features of the STL

• Streams• strings, wide strings• locales (for internationalization)• numerical• valarray

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References

• N.M. Josuttis, The C++ Standard Library, Addison-Wesley 2001