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MODULE 31a_1

THE C++ STL ITERATOR PART 4

What do we have in this session?

1. operator- prototype

2. iterator, operator- program example

3. The Iterator Template Classes

4. The random_access_iterator_tag Template Class

5. iterator, template class

6. The iterator_traits Template Class

7. iterator, template class

8. An insert_iterator Template Class

9. An insert_iterator Template Class Members

10. insert_iterator, container_type program example

11. insert_iterator, container_reference program example

12. An insert_iterator::insert_iterator

13. insert_iterator, insert_iterator program example

14. An insert_iterator::operator*

15. insert_iterator, operator* program example

16. iterator, advance() – g++ program example

17. insert_iterator, insert_iterator – g++ program example

The Iterator Template Classes

31.5  Some Of The Iterator Template Class program examples

Therandom_access_iterator_tag Template Class

• A class that provides a return type for iterator_category function that represents a random-access iterator.

struct random_access_iterator_tag : public bidirectional_iterator_tag { };

• The category tag classes are used as compile tags for algorithm selection. The template function needs to find the most specific category of its iterator argument so that it can use the most efficient algorithm at compile time. For every iterator of type Iterator, iterator_traits<Iterator>::iterator_category must be defined to be the most specific category tag that describes the iterator's behavior.

• The type is the same as iterator<Iter>::iterator_category when Iter describes an object that can serve as a random-access iterator.

// iterator, template class

#include <iterator>

#include <vector>

#include <iostream>

#include <list>

using namespace std;

int main()

{

    vector<int> vec1;

    vector<char> vec2;

    list<char> lst;

    iterator_traits<vector<int>::iterator>::iterator_category cati;

    iterator_traits<vector<char>::iterator>::iterator_category catc;

    iterator_traits<list<char>::iterator>::iterator_category catlst;

    // both are random-access iterators

    cout<<"The type of iterator for vector<int> is\nidentified by the tag:\n "<<typeid(cati).name()<<endl;

    cout<<"The type of iterator for vector<char> is \nidentified by the tag:\n "<<typeid(catc).name()<<"\n";

    cout<<"\nOperation: typeid(cati) == typeid(catc)\n";

    if(typeid(cati) == typeid(catc))

        cout<<"The iterators type are the same."<<endl<<endl;

    else

        cout<<"The iterators type are not the same."<<endl<<endl;

    // but the list iterator is bidirectional, not random access

    cout<<"The type of iterator for list<char> is\nidentified by the tag:\n"<<typeid(catlst).name()<<endl;

    cout<<"\nOperation: typeid(vec1.begin())==typeid(vec2.begin())\n";

    if(typeid(vec1.begin()) == typeid(vec2.begin()))

        cout<<"The iterators type are the same."<<endl;

    else

        cout<<"The iterators type are not the same."<<endl;

    return 0;

}

Output:

Theiterator_traits Template Class

• A template helper class providing critical types that are associated with different iterator types so that they can be referred to in the same way.  Note that, from the following three class templates you can see how thetypedef defined in the template class as well asstruct usage.

template<class Iterator>

   struct iterator_traits

{

   	typedef typename Iterator::iterator_category iterator_category;

   	typedef typename Iterator::value_type value_type;

   	typedef typename Iterator::difference_type difference_type;

   	typedef typename Iterator::pointer pointer;

   	typedef typename Iterator::reference reference;

   };

template<class Type>

   struct iterator_traits<Type*> 

{

   	typedef random_access_iterator_tag iterator_category;

  	typedef Type value_type;

   	typedef ptrdiff_t difference_type;

   	typedef Type *pointer;

   	typedef Type& reference;

   };

template<class Type>

   struct iterator_traits<const Type*>

{

   	typedef random_access_iterator_tag iterator_category;

   	typedef Type value_type;

   	typedef ptrdiff_t difference_type;

   	typedef const Type *pointer;

   	typedef const Type& reference;

   };

• The template class defines the following member types:

// iterator, template class

#include <iostream>

#include <iterator>

#include <vector>

#include <list>

using namespace std;

template<class ite>

// create a function of template class type...

void funct(ite i1, ite i2)

{

    iterator_traits<ite>::iterator_category cat;

    cout<<"Test the iterator type...\n";

    cout<<typeid(cat).name()<<endl;

    // print the container data

    cout<<"The data: ";

    while(i1 != i2)

    {

        iterator_traits<ite>::value_type p;

        p = *i1;

        cout<<p<<" ";

        i1++;

    };

    cout<<endl<<endl;

};

int main()

{

    // declare containers vector and list

    vector<char> vec(9, 'T');

    list<int> lst(8, 7);

    // function call...

    funct(vec.begin(), vec.end());

    funct(lst.begin(), lst.end());

    return 0;

}

Output:

template <class Container>

An insert_iterator Template Class Members

Typedefs

• A type that represents the container into which a general insertion is to be made.

typedef Container container_type;

• The type is a synonym for the template parameter Container.

// insert_iterator, container_type

#include <iterator>

#include <list>

#include <iostream>

using namespace std;

int main()

{

    list<int> lst1;

    insert_iterator<list<int> >::container_type lst2 = lst1;

    inserter(lst2, lst2.end()) = 12;

    inserter(lst2, lst2.end()) = 17;

    inserter(lst2, lst2.begin()) = 24;

    inserter(lst2, lst2.begin()) = 9;

    list<int>::iterator veciter;

    cout<<"The list lst2 data: ";

    for(veciter = lst2.begin(); veciter != lst2.end(); veciter++)

        cout<<*veciter<<" ";

    cout<<endl;

    return 0;

}

Output:

• A type that provides a reference to an element in a sequence controlled by the associated container.

typedef typename Container::reference reference;

• The type describes a reference to an element of the sequence controlled by the associated container.

// insert_iterator, container_reference

#include <iterator>

#include <list>

#include <iostream>

using namespace std;

int main()

{

    list<int> lst;

    insert_iterator<list<int> > iivIter(lst, lst.begin());

    *iivIter = 12;

    *iivIter = 21;

    *iivIter = 9;

    *iivIter = 31;

    list<int>::iterator lstIter;

    cout<<"The list lst data: ";

    for(lstIter = lst.begin(); lstIter != lst.end(); lstIter++)

        cout<<*lstIter<<" ";

    cout<<endl;

    cout<<"\nOperation: refirst = *(lst.begin())\n";

    insert_iterator<list<int> >::reference refirst = *(lst.begin());

    cout<<"The first element in the list lst is: "<<refirst<<endl;

    return 0;

}

Output:

Member Functions

An insert_iterator::insert_iterator

• Constructs an insert_iterator that inserts an element into a specified position in a container.

insert_iterator(Container& _Cont, typename Container::iterator _It);

Parameters

• All containers have the insert member function called by theinsert_iterator.

• For associative containers the position parameter is merely a suggestion. The inserter function provides a convenient way to insert to values.

// insert_iterator, insert_iterator

#include <iterator>

#include <list>

#include <iostream>

int main()

{

    using namespace std;

    int i;

    list <int>::iterator lstiter;

    list<int> lst;

    for(i = 10; i<15; ++i) 

        lst.push_back(i);

    cout<<"The list lst data: ";

    for(lstiter = lst.begin(); lstiter != lst.end(); lstiter++)

        cout<<*lstiter<<" ";

    cout<<endl;

    // using the member function to insert an element

    cout<<"\nOperation: inserter(lst, lst.begin()) = 21...";

    inserter(lst, lst.begin()) = 21;

    inserter(lst, lst.begin()) = 27;

    cout<<"\nAfter the insertions, the list lst data:\n";

    for(lstiter = lst.begin(); lstiter != lst.end(); lstiter++)

        cout<<*lstiter<<" ";

    cout<<endl;

    // alternatively, using the template version

    cout<<"\nOperation: Iter(lst, lst.end()) and *Iter = 9...";

    insert_iterator< list < int> > Iter(lst, lst.end());

    *Iter = 9;

    *Iter = 33;

    cout<<"\nAfter the insertions, the list lst data:\n";

    for(lstiter = lst.begin(); lstiter != lst.end(); lstiter++)

        cout<<*lstiter<<" ";

    cout<<endl;

    return 0;

}

Output:

Operators

An insert_iterator::operator*

• Dereferences the insert iterator returning the element is addresses.

insert_iterator& operator*();

• The return value is the member function returns the value of the element addressed.

• Used to implement the output iterator expression *Iter = value. If Iter is an iterator that addresses an element in a sequence, then *Iter = value replaces that element with value and does not change the total number of elements in the sequence.

// insert_iterator, operator*

#include <iterator>

#include <list>

#include <iostream>

using namespace std;

int main()

{

    int i;

    list<int>::iterator lstiter;

    list<int> lst;

    for(i = 10; i<=15; ++i) 

        lst.push_back(i);

    cout<<"The original list lst data: ";

    for(lstiter = lst.begin(); lstiter != lst.end(); lstiter++)

        cout<<*lstiter<<" ";

    cout<<endl;

    cout<<"\nOperation: Iter(lst, lst.begin()) and *Iter = 21... \n";

    insert_iterator< list < int> > Iter(lst, lst.begin());

    *Iter = 21;

    *Iter = 9;

    *Iter = 34;

    cout << "After the insertions, the list lst data:\n";

    for(lstiter = lst.begin(); lstiter != lst.end(); lstiter++)

        cout<<*lstiter<<" ";

    cout<<endl;

    return 0;

}

Output:

• The following is program example compiled usingg++.  g++ will prompt you if old STL constructs that do not comply to standard, used in your programs such as examples presented at the beginning of this Module.

// *****iteratoradvance.cpp**********

// iterator, advance()

#include <iterator>

#include <list>

#include <iostream>

using namespace std;

int main()

{

    int i;

    list<int> lst;

    for(i = 1; i <= 10; ++i) 

        lst.push_back(i);

    list<int>::iterator lstIter, lstpos = lst.begin();

    cout<<"The lst list data: ";

    for(lstIter = lst.begin(); lstIter != lst.end(); lstIter++)

        cout<<*lstIter<<" ";

    cout<<endl;

    cout<<"The the first element pointed by iterator lstpos is: "<<*lstpos<<endl;

    advance(lstpos, 5);

    cout<<"Advanced lstpos 5 steps forward pointing to the "<<*lstpos<<endl;

    advance(lstpos, -4);

    cout<<"Moved lstpos 4 steps backward pointing to the "<<*lstpos<<endl;

    advance(lstpos, 8);

    cout<<"Finally, the last element pointed by iterator lstpos is: "<<*lstpos<<endl;

    return 0;

}

[bodo@bakawali ~]$ g++ iteratoradvance.cpp -o iteratoradvance

[bodo@bakawali ~]$ ./iteratoradvance

The lst list data: 1 2 3 4 5 6 7 8 9 10

The the first element pointed by iterator lstpos is: 1

Advanced lstpos 5 steps forward pointing to the 6

Moved lstpos 4 steps backward pointing to the 2

Finally, the last element pointed by iterator lstpos is: 10

// *******insertiterator.cpp********

// insert_iterator, insert_iterator

#include <iterator>

#include <list>

#include <iostream>

int main()

{

    using namespace std;

    int i;

    list <int>::iterator lstiter;

    list<int> lst;

    for(i = 10; i<15; ++i) 

        lst.push_back(i);

    cout<<"The list lst data: ";

    for(lstiter = lst.begin(); lstiter != lst.end(); lstiter++)

        cout<<*lstiter<<" ";

    cout<<endl;

    // using the member function to insert an element

    cout<<"\nOperation: inserter(lst, lst.begin()) = 21...";

    inserter(lst, lst.begin()) = 21;

    inserter(lst, lst.begin()) = 27;

    cout<<"\nAfter the insertions, the list lst data:\n";

    for(lstiter = lst.begin(); lstiter != lst.end(); lstiter++)

        cout<<*lstiter<<" ";

    cout<<endl;

    // alternatively, using the template version

    cout<<"\nOperation: Iter(lst, lst.end()) and *Iter = 9...";

    insert_iterator< list < int> > Iter(lst, lst.end());

    *Iter = 9;

    *Iter = 33;

    cout<<"\nAfter the insertions, the list lst data:\n";

    for(lstiter = lst.begin(); lstiter != lst.end(); lstiter++)

        cout<<*lstiter<<" ";

    cout<<endl;

    return 0;

}

[bodo@bakawali ~]$ g++ insertiterator.cpp -o insertiterator

[bodo@bakawali ~]$ ./insertiterator

The list lst data: 10 11 12 13 14

Operation: inserter(lst, lst.begin()) = 21...

After the insertions, the list lst data:

27 21 10 11 12 13 14

Operation: Iter(lst, lst.end()) and *Iter = 9...

After the insertions, the list lst data:

27 21 10 11 12 13 14 9 33

tenouk C++ STL tutorial

Further C++ STL iterators related reading:

1. The source code in text for this tutorial is available inC++ STL Iterator source code.

2. C++ Templates programming tutorials.

3. Acomplete C Standard Library.

4. Acomplete C++ Standard Library documentation that includes STL.

5. Check thebest selling C / C++ and STL books at Amazon.com.

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