2CPP16 - STL

STANDARD TEMPLATE LIBRARIESMichael Heron

Introduction• The great promise of object orientation is reusability.• As of now, we haven’t done much re-using.• In the same way that Java provides comprehensive

libraries of basic structures, so too does C++• Through the mechanism of the Standard Template Library.

Standard Template Library

• The STL is not a part of the core C++ language.• It’s a set of extended libraries that have nearly

ubiquitous applicability.• They are defined in the C++ standard.

• It provides implementations of many of the standard data types available in other languages.

• It is driven by the power of templates (as discussed in the previous lecture).

Standard Template Library

• The STL subset that we are going to pay most attention to is that of collections.• Basic, core data structures.

• You’ll hear more about how these are internally implemented in a later part of the module.

• The collection classes allow us to make use of powerful, flexible data types without us having to write them ourselves.• There’s a considerable development burden in writing

objects that are genuinely reuseable.

Collections• Much like an array, a collection holds a number of discreet

elements.• Unlike an array, ordering cannot be assumed with many

collections.

• These classes are not designed to be base classes for more specialised derived classes.• Destructors for example are explictly designed to be non-virtual.

Collections• Collections break down into three main categories.• Sequential, where order is maintained.

• List• Vector

• Associative, where no order is guaranteed.• Set• Hash Map

• Adapter, which act as wrappers around other core structures.• Queue• Stack

The Vector• The Vector is defined in the std namespace.

• #include <vector> into your code.

• Vectors are resizeable arrays.• You can just keep adding things in and they’ll expand to meet your

requirements.

• Basic structure for interacting with a vector is common to all STL collections.

The Vector

#include <iostream>

#include <vector>

using namespace std;

int main() {

vector<int> *v = new vector<int>();

v->push_back (100);

v->push_back (200);

v->push_back (500);

for (int i = 0; i < v->size(); i++) {

cout << v->at (i) << endl;

}

return 0;

}

The Vector• Useful methods:

• push_back – push an element to the back of the structure• size – get the number of elements in the collection• at – pull the element out at that specific position.

• Memory management of a container done during accesses.

Vector Memory Management• Two measures for collections are available.

• capacity – how many elements can the collection hold before new space needs to be allocated.

• max_size – how many elements, in total, the collection can hold.• Determined by the system.

• resize() and reserve() allow for fine-grained control over capacity.

The List• The list implements a doubly-linked list for traversal.

• Can go forwards and backwards.

• Basic structure for adding to a list the same as with a vector.• Can also push_front

• Traversal of the list done through an iterator.• Possible for most of the STL classes.

Iterators• Iterators are a common interface across container

classes.• They represent an object that allows traversal through a collection.• Obtained by using the following syntax:

• Collection<type>::iterator• list<int>::iterator

• The iterator serves as the basis for more elegant output of state date.

Iterators

#include <iostream>

#include <list>

using namespace std;

int main() {

list<int> *l = new list<int>();

list<int>::iterator i;

l->push_back (100);

l->push_front(200);

for (i = l->begin(); i != l->end(); i++) {

cout << *i << endl;

}

return 0;

}

Reversal Traversal

#include <iostream>

#include <list>

using namespace std;

int main() {

list<int> *l = new list<int>();

list<int>::reverse_iterator i;

l->push_back (100);

l->push_front(200);

for (i = l->rbegin(); i != l->rend(); i++) {

cout << *i << endl;

}

return 0;

}

Collections• Most sequence collections also implement a sort function.

• Another time we need to overload an operator as part of C++• < operator must be overloaded to permit sorting of custom objects

based on developer requirements.

• Many other useful functions available.• Won’t cover these in the lecture – documentation available in many

places.

Algorithms• STL also contains implementations for common

algorithms.• Sort• Searches

• Must #include <algorithm> to get access to methods.• Many of these methods similarly require operators to be

overloaded.• Curse you C++.

Vector with Iterator, Sort and Search#include <iostream>

#include <vector>

#include <algorithm>

using namespace std;

int main() {

vector<int> *v = new vector<int>();

vector<int>::iterator i;

bool found;

v->push_back (200);

v->push_back (100);

v->push_back (500);

sort (v->begin(), v->end());

found = binary_search (v->begin(), v->end(), 200);

cout << "Value 200 found in Vector" << endl;

for (i = v->begin(); i < v->end(); i++) {

cout << *i << endl;

}

return 0;

}

Class Defined for Sorting

class Person {

private:

int age;

public:

Person();

Person (int x);

void set_age (int x);

int query_age();

bool operator< (Person &a);

};

Class Implementation

#include "Person.h"

Person::Person() :

age (18) {

}

Person::Person (int x) :

age (x) {

}

int Person::query_age() {

return age;

}

void Person::set_age (int na) {

age = na;

}

bool operator<(Person& a, Person& b) {

return a.query_age() < a.query_age();

}

Versus Java• It seems like much more is needed to implement a C++

class for collections than in Java.• The same amount of code is required for both.• Java requires an implementation of compareTo.

• C++ requires an implementation of an overloaded < operator.

Why Use STL Classes?• The STL classes are battle-hardened and battle-scarred

• They work.

• They make use of templates, and thus can handle any appropriately defined object you devise.

• Most of the common data structures are available as part of the STL.

Why Not Use Them?• Well, no real reason not to…• … for real code.• Array manipulation exercises and data structure creation are important ‘clarity’ topics.• It’s always worth understanding how data structures are

implemented.• Remember, C++ is all about how things are represented

internally.

• Best to learn how to ‘roll your own’ first.• Then use the STL versions because they are more

complete.

Summary• C++ provides implementations of most of the standard

data types.• And also corresponding implementations of default operations such

as searchs and sorts.

• Vectors and lists permit the traversal of ordered data.• Important to define user classes appropriately for sorting

and searching.