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A modest STL tutorial

By Jak Kirman

I am using a software tool called hyperlatex to create this document. The tutorial is also available in

compressed postscript form. For those wanting to install a copy of these pages on their system, they

may be freely copied providing they are not modified in any significant way (other than, say, locale

changes). The file tutorial.tar.Z contains a tarred distribution of the pages. Please note that I will be

making modifications to this document in the coming months, so you may want to occasionally

check for changes. I will start putting in version numbers, and if I can manage to, changebars.

Disclaimer

I started looking at STL a little over a year ago; for a long time I could compile only minimal

subsets of the HP version of the library. More recently, the compilers used by the students I teach

have been able to support more of STL, and I have been adding it to the introductory and advanced

courses I teach.

I have been using C++ for six or seven years, and teaching C++ and object-oriented design courses

in industry for five. I really like the design philosophies in STL; I think that you can learn a great

deal about how generalization can simplify programming by understanding why STL is constructed

the way it is.

At the moment, books are starting to appear on STL; I haven’t yet bought any of them, but I have

looked at some. I would be very interested to hear peoples’ opinions of any books written on STL.

I haven’t seen very much online documentation on STL, apart from the good but rather dense paper

by Stepanov and Lee, I thought I would try to write something to give people a taste of what a good

library will be do for them.

Another reason for getting people to start trying out STL soon is to put pressure on the

compiler-writers to get their compilers patched up enough to take the strain it puts on them...

This is a fairly recent project (started April 1995) to which I can’t devote too much time, so it is still

very short, and pretty poorly organized.

I would greatly appreciate comments or suggestions from anyone.

Outline

STL contains five kinds of components: containers, iterators, algorithms, function objects and

allocators.

In the section Example I present a simple example, introducing each of the five categories of STL

components one at a time.

In the section Philosophy I explain the rationale behind the organization of STL, and give some

hints on the best ways to use it. (Not yet written)

The Components section goes into each category of component in more detail.

The section Extending STL shows how to define your own types to satisfy the STL requirements.

(Not yet written)

A first example

Most of you probably use some kind of auto-allocating array-like type. STL has one called vector.

To illustrate how vector works, we will start with a simple C++ program that reads integers, sorts

them, and prints them out. I will gradually replace bits of this program with STL calls.

Version 1: Standard C++

Here is a standard C++ program to read a list of integers, sort them and print them:

#include <stdlib.h>

#include <iostream.h>

// a and b point to integers. cmp returns -1 if a is less than b,

// 0 if they are equal, and 1 if a is greater than b.

inline int cmp (const void *a, const void *b)

{

int aa = *(int *)a;

int bb = *(int *)b;

return (aa < bb) ? -1 : (aa > bb) ? 1 : 0;

}

// Read a list of integers from stdin

// Sort (c library qsort)

// Print the list

main (int argc, char *argv[])

{

const int size = 1000; // array of 1000 integers

int array [size];

int n = 0;

// read an integer into the n+1 th element of array

while (cin >> array[n++]);

n--; // it got incremented once too many times

qsort (array, n, sizeof(int), cmp);

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

cout << array[i] << "\n";

}

Version 2: containers, iterators, algorithms

STL provides a number of container types, representing objects that contain other objects. One of

these containers is a class called vector that behaves like an array, but can grow itself as necessary.

One of the operations on vector is push_back, which pushes an element onto the end of the vector

(growing it by one).

A vector contains a block of contiguous initialized elements -- if element index k has been

initialized, then so have all the ones with indices less than k.

Notice that this means you cannot just bang things into a vector in a random order; it has to be

grown like a worm. If efficiency is not a primary concern, you could use a map from integers to

values in order to fill a vector in random order.

You can ask a vector how many elements it has with size. This is the logical number of elements

-- the number of elements up to the highest-indexed one you have used. There is a also a notion of

capacity -- the number of elements the vector can hold before reallocating.

Let’s read the elements and push them onto the end of a vector. This removes the arbitrary limit on

the number of elements that can be read.

Also, instead of using qsort, we will use the STL sort routine, one of the many algorithms provided

by STL. The sort routine is generic, in that it will work on many different types of containers. The

way this is done is by having algorithms deal not with containers directly, but with iterators.

Preview of iterators

I’ll go into iterators in detail later, but for now here is enough to get started.

Iterators provide a way of specifying a position in a container. An iterator can be incremented or

dereferenced, and two iterators can be compared. There is a special iterator value called

"past-the-end".

You can ask a vector for an iterator that points to the first element with the message begin. You

can get a past-the-end iterator with the message end. The code

vector<int> v;

// add some integers to v

vector::iterator i1 = v.begin();

vector::iterator i2 = v.end();

will create two iterators like this:

Operations like sort take two iterators to specify the source range. To get the source elements, they

increment and dereference the first iterator until it is equal to the second iterator. Note that this is a

semi-open range: it includes the start but not the end.

Two vector iterators compare equal if they refer to the same element of the same vector.

Putting this together, here is the new program:

#include <string.h>

#include <algo.h>

#include <vector.h>

#include <stdlib.h>

#include <iostream.h>

main ()

{

vector<int> v; // create an empty vector of integers

int input;

while (cin >> input) // while not end of file

v.push_back (input); // append to vector

// sort takes two random iterators, and sorts the elements between

// them. As is always the case in STL, this includes the value

// referred to by first but not the one referred to by last; indeed,

// this is often the past-the-end value, and is therefore not

// dereferenceable.

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

int n = v.size();

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

cout << v[i] << "\n";

}

Incidentally, this is much faster than qsort; at least a factor of 20 on the examples I tried. This is

presumably due to the fact that comparisons are done inline.

Version 3: iterator adaptors

In addition to iterating through containers, iterators can iterate over streams, either to read elements

or to write them.

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