STL in C++

Introduction to C++ STL

CodeClass[5]

Disclaimer: Slides taken from various sources and modified as required

C++ Generic Programming: Templates

#include <iostream>

#include <string>

using namespace std;

template <typename T>

inline T const& Max (T const& a, T const& b)

{

return a < b ? b:a;

}

int main ()

{

int i = 39;

int j = 20;

cout << "Max(i, j): " << Max(i, j) << endl;

double f1 = 13.5;

double f2 = 20.7;

cout << "Max(f1, f2): " << Max(f1, f2) << endl;

string s1 = "Hello";

string s2 = "World";

cout << "Max(s1, s2): " << Max(s1, s2) << endl;

return 0;

}

Output:Max(i, j): 39Max(f1, f2): 20.7Max(s1, s2): World

2

STL – Standard Template Library

Collections of useful classes for common data structures

Ability to store objects of any type (template)

Study of containers

Containers form the basis for treatment of data structures

Container – class that stores a collection of data

STL consists of 10 container classes: Sequence containers

Adapter containers

Associative containers

3

Why use STL?

STL offers an assortment of containers

STL publicizes the time and storage complexity of itscontainers

STL containers grow and shrink in size automatically

STL provides built-in algorithms for processing containers

STL provides iterators that make the containers andalgorithms flexible and efficient.

STL is extensible which means that users can add newcontainers and new algorithms such that:

STL algorithms can process STL containers as well as user definedcontainers

User defined algorithms can process STL containers as well userdefined containers

4

Standard Template Library

The standard template library (STL) contains Containers Algorithms Iterators

A container is a way that stored data is organized in memory, for example an array of elements.

Algorithms in the STL are procedures that are applied to containers to process their data, for example search for an element in an array, or sort an array.

Iterators are a generalization of the concept of pointers, they point to elements in a container, for example you can increment an iterator to point to the next element in an array

5

Strings

In C we used char * to represent a string.

The C++ standard library provides a common implementation of a string class abstraction named string.

6

Hello World - C

#include <stdio.h>

void main()

{

// create string ‘str’ = “Hello world!”

char *str = “Hello World!”;

//ORchar str1[] = “Hello World!”;

printf(“%s\n”, str);

}

7

Hello World – C++

#include <iostream> //for cout

#include <cstdio> //for printf

#include <string>


int main()

{

// create string ‘str’ = “Hello world!”

string str = “Hello World!”;

char str1[] = “Hello World!”;

cout << str << endl;

printf(“%s”,str1);

return 0;

}8

String

To use the string type simply include its header file.#include <string>

9

Creating strings

string str = “some text”;

or

string str(“some text”);

other ways:

string s1(7,‘a’);

string s2 = s1;

10

string length

The length of string is returned by itssize() operation.

#include <string>

string str = “something”;

cout << “The size of “

<< str

<< “is “ << str.size()

<< “characters.” << endl;

11

The size method

str.size() ???

In C we had structs containing only data, In C++, we have :

class string {

…

public:

…

unsigned int size();

…

};

12

String concatenation

concatenating one string to another is done by the ‘+’ operator.

string str1 = “Here ”;

string str2 = “comes the sun”;

string concat_str = str1 + str2;

13

String comparison

To check if two strings are equal usethe ‘==‘ operator.

string str1 = “Here ”;

string str2 = “comes the sun”;

if ( str1 == str2 )

/* do something */

else

/* do something else */

14

String assignment

To assign one string to another

use the “=“ operator.

string str1 = “Sgt. Pappers”;

string str2 = “lonely hearts club bend”;

str2 = str1;

Now : str2 equals “Sgt. Pappers”

15

Containers, Iterators, Algorithms

Container

AlgorithmIterator

Container

Iterator

Algorithm

Objects

Iterator

Iterator

Algorithm

Algorithms use iterators to interact with objectsstored in containers

16

Containers

A container is a way to store data, either built-in data

types like int and float, or class objects

The STL provides several basic kinds of containers <vector> : one-dimensional array (similar to C array, but dynamic)

<list> : double linked list

<deque> : double-ended queue

<queue> : queue

<stack> : stack

<set> : set of ordered keys

<map> : associative array (set of ordered key/value pairs)

17

Sequence Containers

A sequence container stores a set of elements in sequence, in other words each element (except for the first and last one) is preceded by one specific element and followed by another, <vector>, <list> and <deque> are sequential containers

In an ordinary C++ array the size is fixed and can not change during run-time, it is also tedious to insert or delete elements. Advantage: quick random access

<vector> is an expandable array that can shrink or grow in size, but still has the disadvantage of inserting or deleting elements in the middleVector should be your default choice, but choose wiselyBackward compatible with C : &v[0] points to the first element

18

Sequence Containers

<list> is a ’traditional’ double linked list (each element has points to its successor and predecessor), it is quick to insert or delete elements but has v.slow random access

<deque> is a double-ended queue, that means one can insert and delete elements from both ends, it is a kind of combination between a stack (last in first out) and a queue (first in first out) and constitutes a compromise between a <vector> and a <list>Offers random access, back and front insertion but slower than vectors, no C compatibilty

19

Associative Containers

An associative container is non-sequential but uses

a key to access elements. The keys, typically a number or a string, are used by the container to arrange the stored elements in a specific order,

for example in a dictionary the entries are ordered

alphabetically.

Offers O(log n) insertion and access

20


A <set> stores a number of items which contain keys. The keys are the attributes used to order the items, for example a set might store objects of the class. People which are ordered alphabetically using their name

A <map> stores pairs of objects: a key object and an associated value object. A <map> is somehow similar to an array except instead of accessing its elements with index numbers, you access them with indices of an arbitrary type.

<set> and <map> only allow one key of each value, whereas <multiset> and <multimap> allow multiple identical key values

21

Defining a new vector

Header file: <vector>

Syntax: vector<of what>

For example :vector<int> - vector of integers.vector<string> - vector of strings.vector<int * > - vector of pointers to integers.vector<Shape> - vector of Shape objects. Shape is a user defined class.

Two ways to use vector type: Array style, and STL style22

Operations on vector

iterator begin();

iterator end();

bool empty();

void push_back(const T& x);

iterator erase(iterator it);

iterator erase(iterator first, iterator last);

void clear();

….

23

../Local Settings/Temp/VECTOR_vectorCCbegin.htm#vector::begin

../Local Settings/Temp/VECTOR_vectorCCend.htm#vector::end

../Local Settings/Temp/VECTOR_vectorCCempty.htm#vector::empty

../Local Settings/Temp/VECTOR_vectorCCpush_back.htm#vector::push_back

../Local Settings/Temp/VECTOR_vectorCCerase.htm#vector::erase

../Local Settings/Temp/VECTOR_vectorCCerase.htm#vector::erase

../Local Settings/Temp/VECTOR_vectorCCclear.htm#vector::clear

Vector Container

12 7 9 21 13

int array[5] = {12, 7, 9, 21, 13 };

vector<int> v(array,array+5);

v.begin();

12 7 9 21

v.push_back(15);

12 7 9 21 15

12 7 9 21 15

v[3]

0 1 2 3 4

v.pop_back();

24

Vector Container

vector<int> v(3); // create a vector of ints of size 3

v[0]=23;

v[1]=12;

v[2]=9; // vector full

v.push_back(17); // put a new value at the end of array

for (int i=0; i<v.size(); i++) //member function size() of vector

printf(”%d ”,v[i]); // random access to i-th element

25

Vector Container

#include <vector>

#include <cstdio>

int arr[] = { 12, 3, 17, 8 }; // standard C array

vector<int> v(arr, arr+4); // initialize vector with C array

while ( ! v.empty()) // until vector is empty

{

printf(”%d ”, v.back() ); // output last element of vector

v.pop_back(); // delete the last element

}

26

Tip : vector<bool>

Meyers: "As an STL container, there are really only two things wrong with vector<bool>. First it's not an STL containers. Second it doesn't hold bools. Other than that, there's not much to object to." (Effective STL, p79)

vector<bool> does not conform to STL requirements

it stores bools in a packed representation (e.g. bitfield)

Accessing it returns proxy objects to bools, not true bools

Use a deque<bool> or a bitset to store bools

You won't get C compatibility (but C doesn't have bools anyways)

27

Tip : reserve()

Inserting elements may cause a memory reallocation container is doubled in capacity

elements are copied over

Both string and vector let you increase capacity in advance through reserve() May eliminate unnecessary reallocations

Keeps safety of dynamic data structures over C arrays

v.reserve(n)allocates memory for n objects

Distinguish capacity (memory) and size (objects) Writing beyond the size of the container is still bad

28

Tip : size() and empty()

You may check whether a container is empty by writing c.size() == 0 or c.empty()

However, with lists, which have a splice() function, if splice() is O(1), size() must be O(n) and conversely.

Therefore, while empty() will always run in O(1), size() may not. You should thus prefer calling empty() to checking size() against zero.

29

vector<int>

array_

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.

17

4

23

12

size_ 4

vector<int>::iterator

The iterator corresponding tothe class vector<int> is ofthe type vector<int>::iterator

30

Iterators

The member functions begin() and end() return an

iterator to the first and past the last element of a container

vector<int> v

array_ 17

4

23

12

size_ 4

v.end()

v.begin()

31

Iterators

One can have multiple iterators pointing to different or identical elements in the container

vector<int> v

array_ 17

4

23

12

size_ 4i3

i1

i2

32

Common Iterator Operations

* Return the item that the iterator currently references

++ Move the iterator to the next item in the list

-- Move the iterator to the previous item in the list

== Compare two iterators for equality

!= Compare two iterators for inequality

33

Iterators#include <vector>

#include <cstdio>


int main

{


vector<int> v(arr, arr+4); //initialize vector with C array

vector<int>::iterator iter=v.begin(); //iterator for class vector

// define iterator for vector and point it to first element of v

printf(”First element of v = %d\n”,*iter); // de-reference iter

iter++; // move iterator to next element

iter=v.end()-1; // move iterator to last element

}34

Iterators

int max(vector<int>::iterator start, vector<int>::iterator end)

{

int m=*start;

while(start != end)

{

if (*start > m)

m=*start;

++start;

}

return m;

}

printf(”max of v = %d”, max(v.begin(),v.end()));35

Iterators

vector<int> v;

for(int i=0;i<n;i++){

scanf(”%d”,&num);v.push_back(num);

}

for (vector<int>::iterator i=v.begin(); i!=v.end(); i++)

// initialize i with pointer to first element of v

// i++ increment iterator, move iterator to next element

{

printf(”%d ”,*i); // de-referencing iterator returns the

// value of the element the iterator points at

}

for(i=0;i<n;i++)

printf(”%d ”,v[i]);

36

Container Adaptors

There are a few classes acting as wrappers around other containers, adapting them to a specific interface

stack – ordinary LIFO queue – single-ended FIFO priority_queue – the sorting criterion can be specified

• They are implemented using the containers seen before- They do not provide actual data structure

• Container adapters do not support iterators

• The functions push and pop are common to all container adapters

37

Stack Container

They are implemented with vector, list, and deque(by default)

These containers restrict how elements enter and leave a sequence

Stack allows access at only one end of the sequence (top) Adds objects to container by pushing the object onto

the stack Removes objects from container by popping the stack LIFO ordering (last end, first out)

Header file: <stack>• Example of creating stacks

- A stack of int using a vector: stack < int, vector < int > > s1; - A stack of int using a list: stack < int, list < int > > s2; - A stack of int using a deque: stack < int > s3; 38

Queue Container

Queue Allows access only at the front and rear of the sequence Items enter at the rear and exit from the front Example: waiting line at a grocery store FIFO ordering (first-in first-out ) push(add object to a queue) pop (remove object from queue)

• Implemented with list and deque (by default)

• Header file <queue>

• Example:- A queue of int using a list: queue <int, list<int>> q1;

- A queue of int using a deque: queue <int> q2;

39

Priority Queue Container

Priority queue Operations are similar to those of a stack or queue

Elements can enter the priority queue in any order

Once in the container, a delete operation removes the largest (or smallest) value

Example: a filtering system that takes in elements and then releases them in priority order

8

18 133 15

27

40

Priority Queue Container

• Priority Queues are implemented with vector (by default) or deque

• The elements with the highest priority are removed first

- less<T> is used by default for comparing elements

• Header file <queue>

41


In an associative container the items are not arranged in sequence, but usually as a tree structure or a hash table.

The main advantage of associative containers is the speed of searching (binary search like in a dictionary)

Searching is done using a key which is usually a single value like a number or string

The value is an attribute of the objects in the container

The STL contains two basic associative containers sets and multisets

maps and multimaps

42

• Associative containers use keys to store and retrieve elements

• There are four types: multiset, set, multimap and map- all associative containers maintain keys in sorted order

- all associative containers support bidirectional iterators

- set does not allow duplicate keys

- multiset and multimap allow duplicate keys

- multimap and map allow keys and values to be mapped


43

Set Container

Set Collection of unique values, called keys or set members

Contains operations that allow a programmer to: determine whether an item is a member of the set

insert and delete items very efficiently

5 1 36 27 15

Buick FordJeep BMW

Set A Set B

44

Associative Containers: multiset

• Multisets are implemented using a red-black binary search tree for fast storage and retrieval of keys

• Multisets allow duplicate keys

• The ordering of the keys is determined by the STL comparator function object less<T>

• Keys sorted with less<T> must support comparison using the < operator

45

Sets and Multisets#include <set>

string names[] = {”Ole”, ”Hedvig”, ”Juan”, ”Lars”, ”Guido”};

set<string, less<string> > nameSet(names,names+5);

// create a set of names in which elements are alphabetically

// ordered string is the key and the object itself

nameSet.insert(”Patric”); // inserts more names

nameSet.insert(”Maria”);

nameSet.erase(”Juan”); // removes an element

set<string, less<string> >::iterator iter; // set iterator

string searchname;

cin >> searchname;

iter=nameSet.find(searchname); // find matching name in set

if (iter == nameSet.end()) // check if iterator points to end of set

printf(”%d not in set!”,searchname);

else

printf(”%d is in set!”,searchname); 46

Set and Multisets

string names[] = {”Ole”, ”Hedvig”, ”Juan”, ”Lars”, ”Guido”, ”Patric”, ”Maria”, ”Ann”};

set<string, less<string> > nameSet(names,names+7);

set<string, less<string> >::iterator iter; // set iterator

iter=nameSet.lower_bound(”K”);

// set iterator to lower start value ”K”

while (iter != nameSet.upper_bound(”Q”))

cout << *iter++ << endl;

// displays Lars, Maria, Ole, Patric

47

Maps and Multimaps

A map stores pairs <key, value> of a key object and associated value object.

The key object contains a key that will be searched for and the value object contains additional data

The key could be a string, for example the name of a person and the value could be a number, for example the telephone number of a person

48

Maps and Multimaps

#include <map>

string names[]= {”Ole”, ”Hedvig”, ”Juan”, ”Lars”, ”Guido”, ”Patric”, ”Maria”, ”Ann”};

int numbers[]= {75643, 83268, 97353, 87353, 19988, 76455, 77443,12221};

map<string, int, less<string> > phonebook;

map<string, int, less<string> >::iterator iter;

for (int j=0; j<8; j++)

phonebook[names[j]]=numbers[j]; // initialize map phonebook

for (iter = phonebook.begin(); iter !=phonebook.end(); iter++)

cout << (*iter).first << ” : ” << (*iter).second << endl;

cout << ”Lars phone number is ” << phonebook[”Lars”] << endl;

49

Algorithms

Implement simple, or not-so-simple loops on ranges

copy, find, but also partition, sort, next-permutation

Specify their need in terms of iterator categories They do not care about the exact class Must pay attention to the iterators provided by

containers

Often exist in several versions One uses default comparison, user-defined value Other calls user-provided predicate, function

Some impose requirement on the data binary_search needs sorted data

Tip: swap(c1,c2) would swap c1 and c2 irrespective of their data type 50

Fill and Generate

• fill(iterator1, iterator2, value);

fills the values of the elements between iterator1 and iterator2 with value

• fill_n(iterator1, n, value);

changes specified number of elements starting at iterator1 to value

• generate(iterator1, iterator2, function);

similar to fill except that it calls a function to return value

• generate_n(iterator1, n, function)

same as fill_n except that it calls a function to return value

51

Comparing sequences of values

• bool equal(iterator1, iterator2, iterator3);- compares sequence from iterator1 to iterator2 with the sequence

beginning at iterator3

- return true is they are equal, false otherwise

• pair mismatch(iterator1, iterator2, iterator3);- compares sequence from iterator1 to iterator2 with the sequence

starting at iterator3

- returns a pair object with iterators pointing to where mismatch occurred

- example of the a pair object

pair <<vector>::iterator, <vector>::iterator> par_obj;

52

Removing elements from containers

• iterator remove(iterator1, iterator2, value);- removes all instances of value in a range iterator1 to iterator2

- does not physically remove the elements from the sequence

- moves the elements that are not removed forward

- returns an iterator that points to the "new" end of container

• iterator remove_copy(iterator1, iterator2, iterator3, value);

- copies elements of the range iterator1-iterator2 that are not equal to value into the sequence starting at iterator3

- returns an iterator that points to the last element of the sequence starting at iterator3

53

Replacing elements (1)

• replace( iterator1, iterator2, value, newvalue );

replaces value with newvalue for the elements located in the range iterator1 to iterator2

• replace_if( iterator1, iterator2, function, newvalue );

replaces all elements in the range iterator1-iterator2 for which function returns true with newvalue

54

Replacing elements (2)

• replace_copy( iterator1, iterator2, iterator3, value, newvalue );

replaces and copies elements of the range iterator1-iterator2 to iterator3

• replace_copy_if( iterator1, iterator2, iterator3, function, newvalue );

replaces and copies elements for which function returns true where iterator3

55

Search algorithms

• iterator find(iterator1, iterator2, value)

returns an iterator that points to first occurrence of value

• iterator find_if(iterator1, iterator2, function)

returns an iterator that points to the first element for which function returns true.

56

Sorting algorithms

• sort(iterator1, iterator2)

sorts elements in ascending order

• binary_search(iterator1, iterator2, value)

searches in an ascending sorted list for value using a binary search

57

Copy and Merge

• copy(iterator1, iterator2, iterator3)

copies the range of elements from iterator1 to iterator2 into iterator3

• copy_backward(iterator1, iterator2, iterator3)

copies in reverse order the range of elements from iterator1 to iterator2 into iterator3

• merge(iter1, iter2, iter3, iter4, iter5) ranges iter1-iter2 and iter3-iter4 must be sorted in ascending order and copies both lists into iter5 in ascending order

58

Unique and Reverse order

• iterator unique(iterator1, iterator2) - removes (logically) duplicate elements from a sorted list

- returns iterator to the new end of sequence

• reverse(iterator1, iterator2)- reverses elements in the range of iterator1 to iterator2

59

Utility algorithms (1)

• random_shuffle(iterator1, iterator2) randomly mixes elements in the range iterator1-iterator2

• int count(iterator1, iterator2, value) returns number of instances of value in the range

• int count_if(iterator1, iterator2, function)counts number of instances for which functionreturns true

60


• iterator min_element(iterator1, iterator2)

returns iterator to smallest element

• iterator max_element(iterator1, iterator2)

returns iterator to largest element

• accumulate(iterator1, iterator2)

returns the sum of the elements in the range

61


• for_each(iterator1, iterator2, function)

calls function on every element in range

• transform(iterator1, iterator2, iterator3, function)

calls function for all elements in range iterator1-iterator2, and copies result to iterator3

62

Algorithms on sets (1)

• includes(iter1, iter2, iter3, iter4)

returns true if iter1-iter2 contains iter3-iter4. Both sequences must be sorted

• set_difference(iter1, iter2, iter3, iter4,iter5)

copies elements in range iter1-iter2 that do not exist in second range iter3-iter4 into iter5

• set_intersection(iter1, iter2, iter3, iter4, iter5)

copies common elements from the two ranges iter1-iter2 and iter3-iter4 into iter5

63

Algorithms on sets (2)

• set_symmetric_difference(iter1, iter2, iter3, iter4, iter5)

copies elements in range iter1-iter2 that are not in range iter3-iter4 and vice versa, into iter5. Both sets must be sorted

• set_union(iter1, iter2, iter3, iter4, iter5)

copies elements in both ranges to iter5. Both sets must be sorted

64

For_Each() Algorithm

#include <vector>

#include <algorithm>

#include <cstdio>

void show(int n)

{

printf(”%d ”,n);

}



for_each (v.begin(), v.end(), show); // apply function show

// to each element of vector v65

Find() Algorithm

#include <vector>


#include <cstdio>

int key;

int arr[] = { 12, 3, 17, 8, 34, 56, 9 }; // standard C array


vector<int>::iterator iter;

pritnf(”enter value :”);

scanf(”%d ”,key);

iter=find(v.begin(),v.end(),key); // finds integer key in v

if (iter != v.end()) // found the element

printf(”Element %d found\n”,key);

else

printf(”Element %d not in vector”, key);66

Find_If() Algorithm

#include <vector>


#include <cstdio>

Bool mytest(int n) { return (n>21) && (n <36); };



vector<int>::iterator iter;

iter=find_if(v.begin(),v.end(),mytest);

// finds element in v for which mytest is true

if (iter != v.end()) // found the element

printf(”Found %d\n”,iter);

else

printf(”Not found”);67

Count_If() Algorithm

#include <vector>


#include <cstdio>

Bool mytest(int n) { return (n>14) && (n <36); };



int n=count_if(v.begin(),v.end(),mytest);

// counts element in v for which mytest is true

printf(”found %d elements\n”, n );

68

List Container

An STL list container is a double linked list, in which

each element contains a pointer to its successor and

predecessor.

It is possible to add and remove elements from both

ends of the list

Lists do not allow random access but are efficient to

insert new elements and to sort and merge lists

69

List Container

12 7 9 21 13

int array[5] = {12, 7, 9, 21, 13 };

list<int> li(array,array+5);

7 9 21

li.push_front(8);

12 7 9 21 15

li.pop_front();

12 7 9 21

li.push_back(15);

12 7 9 21 15

li.pop_back();

8

7 12 17 21 23

li.insert()

70

Insert Iterators

If you normally copy elements using the copy algorithm you overwrite the existing contents

#include <list>

int arr1[]= { 1, 3, 5, 7, 9 };

int arr2[]= { 2, 4, 6, 8, 10 };

list<int> l1(arr1, arr1+5); // initialize l1 with arr1


copy(l1.begin(), l1.end(), l2.begin());

// copy contents of l1 to l2 overwriting the elements in l2

// l2 = { 1, 3, 5, 7, 9 }

71

Insert Iterators With insert operators you can modify the behavior of the copy algorithm

back_inserter : inserts new elements at the end front_inserter : inserts new elements at the beginning inserter : inserts new elements at a specified location

#include <list>

int arr1[]= { 1, 3, 5, 7, 9 };

int arr2[]= { 2, 4, 6, 8, 10 };



copy(l1.begin(), l1.end(), back_inserter(l2)); // use back_inserter

// adds contents of l1 to the end of l2 = { 2, 4, 6, 8, 10, 1, 3, 5, 7, 9 }

copy(l1.begin(), l1.end(), front_inserter(l2)); // use front_inserter

// adds contents of l1 to the front of l2 = { 9, 7, 5, 3, 1, 2, 4, 6, 8, 10 }

copy(l1.begin(), l1.end, inserter(l2,l2.begin());

// adds contents of l1 at the ”old” beginning of l2 = { 1, 3, 5, 7, 9, 2, 4, 6, 8, 10 }

72

Sort & Merge

Sort and merge allow you to sort and merge elements in a container

#include <list>

int arr1[]= { 6, 4, 9, 1, 7 };

int arr2[]= { 4, 2, 1, 3, 8 };



l1.sort(); // l1 = {1, 4, 6, 7, 9}

l2.sort(); // l2= {1, 2, 3, 4, 8 }

l1.merge(l2); // merges l2 into l1

// l1 = { 1, 1, 2, 3, 4, 4, 6, 7, 8, 9}, l2= {}

73

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 };


l1.sort(greater<int>()); // uses function object greater<int>

// for sorting in reverse order l1 = { 9, 7, 6, 4, 1 }74

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 };


int sum = accumulate(l1.begin(), l1.end() , 0, plus<int>());

int sum = accumulate(l1.begin(), l1.end(),0); // equivalent

int n = accumulate(l1.begin(), l1.end() , 100, minus<int>());

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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

76

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

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Education

STL in C++