Chapter 23

Allocators

Recall from Chapter 21 that every STL container takes an Allocator type as a template parameter, for which the default will usually suffice. For example, the vector template definition looks like this:

template <typename T, typename Allocator = allocator<T> > class vector;

The container constructors then allow you to specify an object of type Allocator. These extra parameters permit you to customize the way the containers allocate memory. Every memory allocation performed by a container is made with a call to the allocate() method of the Allocator object. Conversely, every deallocation is performed with a call to the deallocate() method of the Allocator object. The standard library provides a default Allocator class called allocator, which implements these methods simply as wrappers for operator new and operator delete.

If you want containers in your program to use a custom memory allocation and deallocation scheme, such as a memory pool, you can write your own Allocator class. Any class that provides allocate(), deallocate(), and several other required methods and typedefs can be used in place of the default allocator class. However, in our experience, this feature is rarely used, so we have omitted the details from this book. For more details, consult one of the books on the C++ Standard Library listed in Appendix B.

Iterator Adapters

The Standard Library provides three iterator adapters: special iterators that are built on top of other iterators. You’ll learn more about the adapter design pattern in Chapter 26. For now, just appreciate what these iterators can do for you. All three iterator adapters are declared in the <iterator> header.

You can also write your own iterator adapters. Consult one of the books on the Standard Library listed in Appendix B for details.

Reverse Iterators

The STL provides a reverse_iterator class that iterates through a bidirectional or random access iterator in reverse direction. Applying operator++ to a reverse_iterator calls operator-- on the underlying container iterator, and vice versa. Every reversible container in the STL, which happens to be every container that’s part of the standard, supplies a typedef reverse_iterator and methods called rbegin() and rend(). rbegin() returns a reverse_iterator starting at the last element of the container, and rend() returns a reverse_iterator starting at the first element of the container.

The reverse_iterator is useful mostly with algorithms in the STL that have no equivalents that work in reverse order. For example, the basic find() algorithm searches for the first element in a sequence. If you want to find the last element in the sequence, you can use a reverse_iterator instead. Note that when you call an algorithm like find() with a reverse_iterator, it returns a reverse_iterator as well. You can always obtain a normal iterator from a reverse_iterator by calling the base() method on the reverse_iterator. However, due to the implementation details of reverse_iterator, the iterator returned from base() always refers to one element past the element referred to by the reverse_iterator on which it’s called.

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Here is an example of find() with a reverse_iterator:

#include <algorithm> #include <vector> #include <iostream> #include <iterator> using namespace std;

//The implementation of populateContainer() is identical to that shown in

//Chapter 22, so it is omitted here.

int main(int argc, char** argv)

{

vector<int> myVector; populateContainer(myVector);

int num;

cout << “Enter a number to find: “; cin >> num;

vector<int>::iterator it1; vector<int>::reverse_iterator it2;

it1 = find(myVector.begin(), myVector.end(), num); it2 = find(myVector.rbegin(), myVector.rend(), num);

if (it1 != myVector.end()) {

cout << “Found “ << num << “ at position “ << it1 - myVector.begin() << “ going forward.\n”;

cout << “Found “ << num << “ at position “

<< it2.base() - 1 - myVector.begin() << “ going backward.\n”;

} else {

cout << “Failed to find “ << num << endl;

}

return (0);

}

One line in this program needs further explanation. The code to print out the position found by the reverse iterator looks like this:

cout << “Found “ << num << “ at position “

<< it2.base() - 1 - myVector.begin() << “ going backward.\n”;

As noted earlier, base() returns an iterator referring to one past the element referred to by the reverse_iterator. In order to get to the same element, you must subtract one.

Stream Iterators

As mentioned in Chapter 21, the STL provides adapters that allow you to treat input and output streams as input and output iterators. Using these iterators you can adapt input and output streams so that they can serve as sources and destinations, respectively, in the various STL algorithms. For example, you can use the ostream_iterator with the copy() algorithm to print the elements of a container with only one line of code:

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Chapter 23

#include <algorithm> #include <iostream> #include <iterator> #include <vector> using namespace std;

int main(int argc, char** argv)

{

vector<int> myVector;

for (int i = 0; i < 10; i++) { myVector.push_back(i);

}

// Print the contents of the vector.

copy(myVector.begin(), myVector.end(), ostream_iterator<int>(cout, “ “)); cout << endl;

}

ostream_iterator is a template class that takes the element type as a type parameter. Its constructor takes an output stream and a string to write to the stream following each element.

Similarly, you can use the istream_iterator to read values from an input stream using the iterator abstraction. An istream_iterator can be used as sources in the algorithms and container methods. It’s usage is less common than that of the ostream_iterator, so we don’t show an example here. Consult one of the references in Appendix B for details.

Insert Iterators

As mentioned in Chapter 22, algorithms like copy() don’t insert elements into a container; they simply replace old elements in a range with new ones. In order to make algorithms like copy() more useful, the STL provides three insert iterator adapters that actually insert elements into a container. They are templatized on a container type, and take the actual container reference in their constructor. By supplying the necessary iterator interfaces, these adapters can be used as the destination iterators of algorithms like copy(). However, instead of replacing elements in the container, they make calls on their container to actually insert new elements.

The basic insert_iterator calls insert(position, element) on the container, the back_insert_ iterator calls push_back(element), and the front_insert_iterator calls push_front(element).

For example, you can use the back_insert_iterator with the remove_copy_if() algorithm to populate a new vector with all elements from an old vector that are not equal to 100:

#include <algorithm> #include <functional> #include <iterator> #include <vector> #include <iostream>

using namespace std;

//The implementation of populateContainer() is identical to that shown in

//Chapter 22, so it is omitted here.

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Customizing and Extending the STL

int main(int argc, char** argv)

{

vector<int> vectorOne, vectorTwo; populateContainer(vectorOne);

back_insert_iterator<vector<int> > inserter(vectorTwo); remove_copy_if(vectorOne.begin(), vectorOne.end(), inserter,

bind2nd(equal_to<int>(), 100));

copy(vectorTwo.begin(), vectorTwo.end(), ostream_iterator<int>(cout, “ “)); cout << endl;

return (0);

}

As you can see, when you use insert iterators, you don’t need to size the destination containers ahead of time.

The insert_iterator and front_insert_iterator function similarly, except that the insert_iterator also takes an initial iterator position in its constructor, which it passes to the first call to insert(position, element). Subsequent iterator position hints are generated based on the return value from each insert() call.

One huge benefit of insert_iterator is that it allows you to use associative containers as destinations of the modifying algorithms. Recall from Chapter 22 that the problem with associative containers is

that you are not allowed to modify the elements over which you iterate. By using an insert_iterator, you can instead insert elements, allowing the container to sort them properly internally. Recall from Chapter 21 that associative containers actually support a form of insert() that takes an iterator position, and are supposed to use the position as a “hint,” which they can ignore. When you use an insert_iterator on an associative container, you can simply pass the begin or end iterator of the container to use as the hint. Here is the previous example modified so that the destination container is a set instead of a vector:

#include <algorithm> #include <functional> #include <iterator> #include <vector> #include <iostream> #include <set>

using namespace std;

//The implementation of populateContainer() is identical to that shown in

//Chapter 22, so it is omitted here.

int main(int argc, char** argv)

{

vector<int> vectorOne; set<int> setOne; populateContainer(vectorOne);

insert_iterator<set<int> > inserter(setOne, setOne.begin()); remove_copy_if(vectorOne.begin(), vectorOne.end(), inserter,

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