(1) C++ Focus On STL – Day 1, Part 1

layout: true
name: blank
styling: styling.css
styling-by: Martin Weitzel

.stylehint[
Styled with [{{styling}}]({{styling}}) by {{styling-by}}
]

---
layout: true
name: plain
copyright: (CC) BY-SA
branding:  [Dipl.-Ing. Martin Weitzel](http://tbfe.de)
customer:  [für MicroConsult Training & Consulting GmbH](http://microconsult.de)

<!--
  *****************************************************************************
  Template used for for pages NOT referring to any Info-Graphic
  *****************************************************************************
  The following attributes are mandatory FOR THE TEMPLATE PAGE and should
  simply be left empty if not meaningful.

copyright: will be reproduced in each page footer first
  branding: will reproduced in each page footer next
  customer: will be reproduced in each page footer last

As the above attributes are part of several page templates a global replace
  should be used for consistent changes.

On pages USING THIS TEMPLATE the following attributes must be set:

header: ## and header text (i.e. including the markdown formatting indicator)

-->

.pagefooter[
{{copyright}}: {{branding}} {{customer}} .microconsult-logo[]
]

---
layout: true
name: linkinfo
copyright: (CC) BY-SA
branding:  [Dipl.-Ing. Martin Weitzel](http://tbfe.de)
customer:  [für MicroConsult Training & Consulting GmbH](http://microconsult.de)

<!--
  *****************************************************************************
  Template used for for pages INTRODUCING to a new Info-Graphic
  *****************************************************************************
  On this kind of pages a size-reduced version of the whole info graphic will
  be reproduced and occupies approximately 2/3 of the page width. So only add
  little information, preferably links to later pages dealing with single
  sections of the info graphic.

On pages USING THIS TEMPLATE the following attributes must be set:

graphic: file path to of the infographic EXCLUDING the suffix.
  header: ## and header text (i.e. including the markdown formatting indicator)

-->

.infographic[
[![Info-Grafik](InfoGraphics/{{graphic}}.png)](InfoGraphics/{{graphic}}.png
                "Click to open – add [CTRL+] SHIFT for new [tabbed] window")
]

.pagefooter[
{{copyright}}: {{branding}} {{customer}} .microconsult-logo[]
]

---
layout: true
name: withinfo
copyright: (CC) BY-SA
branding:  [Dipl.-Ing. Martin Weitzel](http://tbfe.de)
customer:  [für MicroConsult Training & Consulting GmbH](http://microconsult.de)

<!--
  *****************************************************************************
  Template used for pages dealing with a SPECIFIC SECTION of an Info-Graphic
  *****************************************************************************
  On such pages a link to the info graphic is reproduced in the top-right
  corner (or maybe elsewhere depending on the style sheet), so there are no
  restrictions with respect to the space available for the content of the page.

On pages USING THIS TEMPLATE the following attributes must be set:

graphic: file path to of the info graphic EXCLUDING the suffix and
  section: specific section in the info graphic this page refers to
  header: ## and header text (i.e. including the markdown formatting indicator)
-->

.infolink.right[
[Click here for Info-Graphic  
{{graphic}}](InfoGraphics/{{graphic}}.png "add [CTRL+] SHIFT for own [tabbed] window")  
{{section}}
]

.pagefooter[
{{copyright}}: {{branding}} {{customer}} .microconsult-logo[]
]

---
layout: false
template: blank

[C++ STL]: 00_topics.html#agenda

# [C++ STL] – Fundamental Architecture
.center[(Day 1, Part 1)]

-------------------------------------------------------------------------------

* [Architectural Overview		](#architectural_overview)
* [Abstract Specification 		](#abstract_specification)
* [Common Data Structures 		](#common_data_structures)
* [Understanding Iterators		](#understanding_iterators)
* ["Big-O" Algorithmic Complexity 	](#big_o_algorithmic_complexity)

-------------------------------------------------------------------------------

.center[**NOTE**]

This part mainly intends to give an overview – the *Big Picture* – with
many of the details covered in more depth in subsequent parts.

---
template: linkinfo
graphic: STL-IteratorUsages
name: architectural_overview
header: ## Architectural Overview

* [Container Dimension](#container_dimension)

* [Algorithm Dimension](#algorithm_dimension)

* [The Role of Iterators](#role_of_iterators)

---
template: withinfo
graphic: STL-IteratorUsages
section: Container Dimension
name: container_dimension
header: ### The Container Dimension

[`<array>`]: http://en.cppreference.com/w/cpp/container/array
[`<deque>`]: http://en.cppreference.com/w/cpp/container/deque
[`<forward_list>`]: http://en.cppreference.com/w/cpp/container/forward_list
[`<list>`]: http://en.cppreference.com/w/cpp/container/list
[`<map>`]: http://en.cppreference.com/w/cpp/container/map
[`<priority_queue>`]: http://en.cppreference.com/w/cpp/container/unordered_set
[`<queue>`]: http://en.cppreference.com/w/cpp/container/unordered_set
[`<set>`]: http://en.cppreference.com/w/cpp/container/set
[`<stack>`]: http://en.cppreference.com/w/cpp/container/unordered_set
[`<string>`]: http://en.cppreference.com/w/cpp/container/array
[`<unordered_map>`]: http://en.cppreference.com/w/cpp/container/unordered_map
[`<unordered_set>`]: http://en.cppreference.com/w/cpp/container/unordered_set

Along the Container Dimension there are._[]

* 10 Standard Container Classes, of which

* (only) 9 are considered part of the STL in the narrower sense, but

* as the standard string class provided all the typical member functions of
    containers (with sequential and random access) it may be counted here too;

* furthermore 3 [Container Adaptors] (creating containers with special features or
  restrictions based on the above).

.I[
For more information on STL container classes see:
http://en.cppreference.com/w/cpp/container
http://en.cppreference.com/w/cpp/string
]

[Container Adaptors]: http://en.cppreference.com/w/cpp/container#Container_adaptors

.F[:
The classic containers are defined in the header files [`<array>`], [`<deque>`],
[`<forward_list>`], [`<list>`], [`<map>`], [`<set>`], [`<unordered_map>`], and
[`<unordered_set>`];
container adaptors are defined in header files [`<queue>`], [`<stack>`], and
[`<priority_queue>`]; the standard string class is defined in header file
[`<string>`].
]

---
template: withinfo
graphic: STL-IteratorUsages
section: Algorithm Dimension
name: algorithm_dimension
header: ### The Algorithm Dimension

[`<alorithm>`]: http://en.cppreference.com/w/cpp/container/algorithm
[`<numeric>`]: http://en.cppreference.com/w/cpp/container/numeric

At the Algorithm Dimension there are (counting what is provided by the
header file `<algorithm>)`

* (about) 80+ entries, from

* (about) 30 related groups (or families._[]),

which may be considered augmented by the header `<numeric>` contributing

* another 5 algorithms.

.I[
For more information on STL algorithms see:
http://en.cppreference.com/w/cpp/algorithm and subsection
[Generic Numeric Operations] in
http://en.cppreference.com/w/cpp/numeric
]

[Generic Numeric Operations]: http://en.cppreference.com/w/cpp/numeric#Generic_numeric_operations

.F[:
There are no strict rules about grouping algorithms to families. It could be done
by name prefix – e.g. by putting `copy` in the same group as `copy_if` and `unique`
in the same group as `unique_copy`, or by similarities in purpose – e.g. by putting
`all_of`, `none_of`, and `any_of` in the same group, or everything that has to do
with permuting elements, everything that has to do with sorting …
]

---
template: withinfo
graphic: STL-IteratorUsages
section: Iterators as "Glue"
name: role_of_iterators
header: ### Role of Iterators

Viewed from the big perspective,

* STL Iterators just "provide the glue" that

* brings together Containers and Algorithms,

* without having the one to know details of the other.._[]

The latter is a big advantage for making the architecture extensible.

.I[
For more information on STL iterators see
http://en.cppreference.com/w/cpp/iterator
]

.F[:
At the time the STL was designed by [Alexander Stepanov] and [David Musser]
this was a rather uncommon and maybe even unique approach. Contemporary C++
class libraries either imitated the style used in other OO-languages, that
had a common base class for all kinds of objects, or – after templates came
into more widespread use – implemented a more or less closed set of standard
containers all complete with member function implementations for the expected
use cases.
]

[Alexander Stepanov]: http://en.wikipedia.org/wiki/Alexander_Stepanov
[David Musser]: http://en.wikipedia.org/wiki/David_Musser

---
template: plain
name: abstract_specification
header: ## Abstract Specification

One of the most distinguishing marks of the STL its strong preference for
abstract specifications.

Originally and formally until today, the STL documentation disclosed few about
its implementation,._[] nor with respect to data structures, neither for details
on the algorithms used.

Instead:

* most often just performance guarantees are defined (which of course give clues
  or even certainty with respect to the implementation);

* for iterators a number of categories are defined, from which the interplay of
  containers with algorithms – including the restrictions – can be derived.

.F[:
Emphasis is to put on "documentation" here, as the STL, being to nearly 100%
a "header file only" library, could effectively hide about its implementation.
]

---
template: plain
header: ### Consequences of Abstract Specifications

Sticking to abstract specifications has an important impact on writing correct
code:

.N.center[
**A substantial number of software developers tends to save the pains of
studying the documentation for libraries they use.**
]

Instead, a "trial and error" approach is adopted:

* A piece of code is considered to be "correct"
  * if it compiles without an error message._[], and
  * passes some simple – far from exhaustive – testing.

.W[
Completely ignoring the written documentation of the STL can lead to (compile
time) error message hard to decipher or even undefined behaviour (at run time).
]

.F[:
Furthermore and increasingly, there is the expectation the compiler should give
a concise description of what is wrong (as if it could read the intentions of
the developer) and ideally describe the appropriate fix in the message.
]

---
template: plain
name: breaches_of_contract
header: ### Breaches of Contract

With respect to the STL there are two substantially different ways to breach
the contract and use the components in unsupported ways:

1. [At compile time](#compile_time_errors):
   * Currently only sometimes a concise error description is the result.
   * More often it causes myriads of messages, difficult to decipher.._[]

2. [At run time](#run_time_errors):
   * In this case – at least principally – "all bets are off", but
   * quality implementation sometimes add checking code during *Debug Builds* or
     may allow to enable selected STL safety checks for *Release Builds*.

.W[
Using the STL with implementation dependent additional checks enabled may incur
both: (Much) **slower execution** and (much) **larger memory footprint.**
]

.F[:
At least for the "un-enlightened" … (though the situation may improve once
[Concepts Lite] are part of the language, which is expected for C++1z).
]

[Concepts Lite]: https://groups.google.com/a/isocpp.org/group/concepts/attach/df9a57cb574ae9f9/concepts-lite.pdf?part=4&authuser=0

---
template: plain
name: compile_time_errors
header: #### Compile Time Errors with STL Components

Compile time errors usually result from an insufficient understanding which set
of operations a class must be provide to be acceptable for the instantiation of
some template.._[]

* The typical result are compiler error messages with a whole bunch of
  information

* how the situation was reached, often including,
  * every alternative considered to resolve the problem, and
  * why finally everything failed.

* with the often only important piece buried somewhere inside.

.N.center[
**Which line of source code triggered the whole drama?**._[]
]

.F[:
Besides reading the documentation, until sufficient experience is acquired with
respect to sorting out the irrelevant from the relevant pieces in STL error
messages, the usual advice is to add lines to your code in baby steps, i.e.
intersperse your coding with frequent compilation (syntax checks usually
suffice, no need for a complete build) so that it is immediately obvious which
change you made triggered the problem.
]

---
template: plain
name: run_time_errors
header: #### Undefined Behaviour of STL Components

To allow – **not to mandate(!)** – highly efficient implementations, the C++
standard often imposes little obligations on run time error checking.

.W[
Breaking documented pre-conditions that cannot be checked at compile time
may – **but not necessarily needs to(!)** – lead to catastrophic failure at
run time.
]

Before complaining over an inflationary use of the term [Undefined Behaviour]
in the C++ standard, be sure to understand the following:

* C++ has a very broad user base with widely varying demands.

* **Undefined behavior provides the leeway to allow any implementation to meet
  the specific needs of its user base**.

* Therefore any C++ implementation is at liberty to replace undefined behaviour
  with defined behaviour.._[]

[Undefined Behaviour]: http://en.cppreference.com/book/undefined_behavior

.F[:
Obviously this assumes such needs are sufficiently common and do not cause
too many users to migrate away, who for which the price is not acceptable
(typically some loss of performance or an enlarged memory footprint).
]

---
template: plain
name: concepts_and_models
header: ### Concepts and Models

[*Concept* and *Model*]: http://en.wikipedia.org/wiki/Concepts_%28C%2B%2B%29

The two key terms in the STL documentation are [*Concept* and *Model*].._[]

A concept defines a (usually templated) type by

* use patterns that need to be valid, e.g. required
  * embedded type definitions,
  * member function, or
  * acceptance by overloads.

A model states for some type that it

* actually fulfills a concept._[], hence making that type acceptable 
  * to instantiate a templated type,
  * which requires that concept.

For a concept *C* and a model *M* the typical wording is:

* *M* **is a model of** *C*.

.F[:
Concepts and models are related to what is provided by an *interface* and its
*implementations* at run time, but apply at compile time, much in the way of
[Duck Typing]. For a video lecture by Bjarne Stroustrup how concept and models
may be made visible to the compiler (and hence get out of their "documentation
only" role), see:
http://channel9.msdn.com/Events/GoingNative/GoingNative-2012/A-Concept-Design-for-C-
]

[Duck Typing]: http://en.wikipedia.org/wiki/Duck_typing

---
template: plain
name: common_data_structures
header: ## Common Data Structures

The term data structure usually means to somehow combine several pieces of
information so that they can be handled as a unit.

Despite the many variations there are only two main principles:

* To get from one piece to another piece
  * either requires some address calculation with respect to the first
    (or the data structure in its entirety),
  * or following a pointer being part of the first (or used to identify
    the whole unit).

* Hence a data structure may be stored
  * in closely adjacent memory locations, or
  * more or less spread-out over all memory
  * (or a mix of both).

---
template: plain
header: ### Adjacent Memory

Typical for adjacent memory is to store related pieces of information in

* a `struct` (C or C++)
* a `class`._[] (C++)
* a native array (C and C++),
* an `std::array` wrapper (C++),

Furthermore (after the first element is located) also

* `std::vector` and
* `std::basic_string`

store adjacent elements (chars) in adjacent memory, and

* `std::deque`

splits the whole data in smaller chunks, each of which is stored in
adjacent memory.

.F[:
In C++ a `class` and a `struct` are basically the same, only that the first
starts with private and the latter with public members (data and functions).
Once the first `private:` or `public:` label has been explicitly inserted, all
difference is gone.
]

---
template: plain
header: ### Linked Lists

Typical for spreading out data structures over memory is to store related pieces
of information in

* `std::list` and
* `std::forward_list`.

Each element (i.e. piece of information) stores the memory address of its
neighbour(s).

Therefore the per-element overhead is one (`std::forward_list`) or two
(`std::list`) pointers (in addition to possible padding).

.N[
Searching through lists requires to visit any element, one after the other,
until the element of interest is found.
]

---
template: plain
header: ### Binary Trees

Binary tree structures also spread out data structures over memory.._[]

All the associative containers from C++98, i.e.

* `std::set`,
* `std::multi_set`
* `std::map`, and
* `std::multi_map`

are stored in some form of binary tree, called [Red-Black-Tree].

The per-element overhead is two pointers, as – besides the element it
holds – each "node" points to two sub-trees, usually called "left" and "right".

.N[
The left sub-tree consists of nodes only with elements._[] comparing
*less-than* to itself, and the right sub-tree holds the others, searching
through a binary tree is considerably faster as searching through a linked
list.
]

[Red-Black-Tree]: http://en.wikipedia.org/wiki/Red%E2%80%93black_tree

.F[:
Only for sets the element value is compared, for maps it is the key value.
]

---
template: plain
header: ### Hash-Tables

[Hash-Table]: http://en.wikipedia.org/wiki/Hash_table

The unordered counterparts to sets and maps, added in C++11, i.e.

* `std::unordered_set`,
* `std::unordered_multiset`,
* `std::unordered_map`, and
* `std::unordered_multimap`

are based on [Hash-Table]s and therefore are a mix of

* *adjacent memory* (for key-lookup based on their hash-value)

* and a tendency to *spread-out data* (in case of hash-duplicates).

Essential for efficient key-based ("O(1)") access is a good hash function.

.N[
As typical STL-implementations store the per-element data in separate linked
lists per hash bucket, poor cache performance may result on modern hardware
architectures and sequential access may be slowed down, especially in
comparison an `std::array`.
]

---
template: plain
header: #### Hash-Tables Explained (1)

The idea of hash tables starts with having a [Hash-Function] that calculates a
numeric index from an element's value (for sets) or key (for maps).

E.g. for a classic `char` string this may be done as follows:
```
std::unsigned str_hash(const char *s) {
    constexpr int p[] = { 3, 5, 7, 11, 13 };
    constexpr int N = sizeof N / sizeof N[0];
    std::size_t r = 0, i = 0;
    while (*s) r += (0xFF & *s++) * p[i++ % N];
    return r;
}
```

For the actual storage adjacent memory is chosen, usually 10 … 30% oversized,
and the result of the hash function is then folded into that range of values.._[]

.F[:
This could be done in many ways, with a modulo operation being the simplest, but
– depending on the hash function – not necessarily the best for an equal spreading
over the whole range, with the same probability for all outcomes.
]

[Hash-Function]: http://en.wikipedia.org/wiki/Hash_function

---
template: plain
header: #### Hash-Tables Explained (2)

In that way, i.e. assuming
```
constexpr HASH_TABLE_SIZE = 1200; // expecting ~1000 elements
const char *data[HASH_TABLE_SIZE];
const char* element;
…
```
the index where an element is to be stored can be quickly determined
```
data[str_hash(element) % HASH_TABLE_SIZE] = element;
```
or the existence of an element may be looked-up:
```
if (data[str_hash(element) % HASH_TABLE_SIZE] != nullptr) …
```

---
template: plain
header: #### Hash-Tables Explained (3)

What is still left is to solve is the problem of over-runners, i.e.

* different elements hashing to same value or maybe
* hashing differently but being folded to the same index.

There are two classic strategies:._[]

1. **Elements are not directly stored in the hash table.**  
   Instead, each entry – also called hash-bucket – is the start of a linked
   list, holding all elements assigned to that entry.

2. **Neighbouring free cells are used.**  
   This solution is good under the aspect of proximity (see next page), but has
   implications for search times of all entries,i.e. the non-overrunning too.

.F[:
With only some few degenerated hash function results, giving the same result for
some groups of elements, with the *linked-list solution* the performance
degradation is a local problem only (affects members of these groups only). With
the *use neighbouring cells* solution it might contaminate large sections of the
table and can get especially troublesome if not only new elements are inserted
into a hash-table but existing ones need to be removed.
]

---
template: plain
header: ### Caching Considerations

Improving efficiency through cached access to main memory has become
increasingly important with modern CPU designs.

.N[
Adjacent storage is crucial for achieving good hit-rates in the several cache
levels usually preceding slow main memory.._[]
]

The overall effects of caching are often are not (yet) reflected in older
literature and more theoretical performance considerations.

* This does not mean that everything written ten or more years ago is not
  applicable any more today (or even plainly wrong).

* It only means that the enormous coverage the performance of data structures
  has received in information technology should be read with the proverbially
  "grain of salt".

**Usually it will be the best to test and evaluate the performance of the actual
code, based on realistic use patterns**.

.F[:
For a video lecture by *Chandler Carruth* expanding on this see (from
[minute 34:47]) in: https://www.youtube.com/watch?v=fHNmRkzxHWs
]

[minute 34:47]: https://www.youtube.com/watch?feature=player_detailpage&v=fHNmRkzxHWs#t=2086

---
template: plain
header: ### Container Sub-Categories

Technically the STL groups its containers into the sub-categories of

* [Sequence Containers], 
* [Associative Containers], and
* [Unordered Associative Containers].

The difference between the first and the other two is:

* Sequence containers have a *front* and a *back*, i.e. two distinct "sides"
  to which insertions may refer elements,._[]
* while both flavours of associative containers decide on their own where new
  elements are placed.

For sequential access to all elements the order *begin* (front) to *end* (back)

* depends on the insertion order for the first sub-category.
* is in ascending sort order of the elements value or key for the second,
* in an unspecified order for the third.

.F[:
Note that nevertheless an `std::vector` restricts adding (and removing) elements
efficiently to its *back*-side (no `push_front` or `pop_front`) while an
`std::forward_list` restricts this to its *front*-side (no `push_back` or
`pop_back`).
]

[Sequence Containers]: http://en.cppreference.com/w/cpp/container#Sequence_containers 
[Associative Containers]: http://en.cppreference.com/w/cpp/container#Associative_containers 
[Unordered Associative Containers]: http://en.cppreference.com/w/cpp/container#Unordered_associative_containers

---
template: plain
header: #### Sequential Access

Sequential access is supported by all containers.

As of C++98 the typical sequential access looked like follows, with `Container`
replaced by a concrete instantiation of any STL container, or a type alias:._[]
```
Container c;
for (Container::iterator it = c.begin(); it != c.end() ++it) …
```

To ensure non-modifying access in case of a modifiable container, the loop
iterator must be of type `…::const_iterator`.

.F[:
With C++11 some variants became possible.

```
   // with auto-typed iterator:
for (auto it = c.begin(); it != c.end() ++it) …
for (auto it = c.cbegin(); it != c.cend() ++it) … // non-modifying
for (auto it = std::begin(c); it != std::end(c) ++it) …
for (auto it = std::cbegin(c); it != std::cend(c) ++it) … // non-modifying, C++14 only

// with range-based loops and auto-typed placeholder:
for (auto e : c) … // by-value access (modifications to `e` affect a copy only)
for (auto &e : c) … // modifications to `e` affect container elements
for (const auto &e : c) … // modifications to `e` prevented at compile-time
```
]

---
template: plain
header: #### Random Access by Index (Offset)

Another frequent type of access is with a numerical index, which is supported
by `std::array`, `std::vector`, and `std::deque`.._[] The index is basically an
integral value,

* enumerating the range of elements,
* in good C-tradition with 0-origin.

The following examples return a reference to the element at position `i`:
```
Container c;
std::size_t i;
…           // requires 0 <= i and i < c.size() ...
… c[i] …    // ... may cause undefined behavior otherwise
… c.at(i) … // ... will throw std::out_of_range otherwise
```

.F[:
Efficiency is directly coupled with the simplicity of the required address
arithmetic, which is (slightly) more complex for `std::deque` as for both
of the others.
]

---
template: plain
header: #### Random Access by Key

The operations shown on the previous page are implemented for `std::map` and
`std::unordered_map`, where the key is to be used as index.

Assuming a map-container with keys of type `std::string`, the following examples
return a reference to the element associated with key `k`:._[]
```
Container c;
std::string k;
…
… c[k] …    // returns existing OR NEWLY CREATED entry for key k
… c.at(k) … // will throw std::out_of_range for non-existing keys
```

.F[:
Another way to access an element for a given key is to obtain an iterator to it.
The non-existence then can be tested by comparison with the end-iterator of the
given container.
]

---
template: plain
name: understanding_iterators
header: ## Understanding Iterators

A client may expect a certain set of operations, which an iterator has to
support, e.g.:

* copy construction and assignment,
* comparisons (at least for equal / unequal),
* increment (at least, sometimes decrement too), and
* dereferencing.._[]

.N[
These operations deliberately are provided in the syntax of (equivalent)
pointer operations.
]

As a later chapter covers iterators in much more detail, the following pages
are only meant as quick overview.

.F[:
The operations possible after dereferencing may be limited too *value access* or
*assigning a new value*. Technically the former works by `const`-qualifying the
result, the latter uses a proxy-type as result, for which (only) assignment is
implemented.
]

---
template: plain
header: ### Iterator Classes

.pull-left[
```
class SomeContainer {
    …
public:
    typedef … value_type;
    class iterator;
    …
};
```
]
.pull-right[
Iterators are usually implemented as nested classes of the container to which they
belong.

With **many** necessary details omitted, for some container as sketched on the left …
]

… the iterator class may (roughly) look as sketched below:

```
class SomeContainer::iterator {
    …
public:
    iterator(const iterator &);
    iterator& operator=(const iterator &);
    value_type &operator*() const;
    friend
    bool operator==(const iterator &rhs, const iterator &rhs) {
        return …; // rhs and lhs denote the same container element
    }
    …
};
```

---
template: plain
header: ### Pointers as Iterators

Especially for containers storing their elements in contiguous memory, plain
pointers may suffice as iterators:._[]
```
class SimpleContainer {
    …
public:
    typedef … value_type;
    typedef value_type *iterator;
    typedef const value_type *const_iterator;
    iterator begin();
    const_iterator begin() const;
    const_iterator cbegin() const;
    iterator end();
    const_iterator end() const;
    const_iterator cend() const;
    …
};
```

.F[:
Note that the examples is only meant to demonstrate the principle. No STL
container will actually use this, as the standard has more requirements on
their iterators, which could not be fulfilled that way.
]

---
template: plain
header: ### Iterators for Native Arrays

The main advantage of using pointer operations for iterators is that it made
native (C-style) arrays compatible with STL algorithms.

For an array `arr` with a size of `N`

* a begin iterator may be obtained by taking the address of the first element,
  i.e. `arr` or `&arr[0]`;._[]

* an end iterator may be obtained by taking the address beyond the last element,
  i.e. `arr+N` or `&arr[N]`.

.W[
Taking the **address of** `arr[N]` is always possible (and it must compare
greater as all addresses from elements of `arr`), but **accessing** that
location in memory has undefined behaviour.
]

.F[:
Note that `arr` and `&arr` denote different types and the latter is **not
suitable** for iterating over all elements of `arr`.
]

---
template: plain
header: ### Restrictions on Iterators

As the standard does not intend to out-rule implementations combining **high
runtime performance** with **low memory footprint**, some requirements on
iterators are deliberately chosen very loose.

.W[
Programmers must be aware of this or otherwise risk undefined behaviour of the
resulting code.._[]
]

Usually it is possible to implement iterators for STL containers

* with the same memory footprint as native pointers,

* their operation dereferencing with the same runtime performance as native
  pointers, and

* their increment operation comparable to a native pointer increment or
  assignment.

.F[:
One of the most important drawbacks of this is that bad effects may not show
in test scenarios or depend on data constellation difficult to reproduce.
]

---
template: plain
header: #### Valid Iterator Positions

Iterators may refer to

* any element in an STL container

* **plus one more valid position**.

The latter is usually understood as "one beyond the last", but implementations 
are only required to behave "as if" in this respect._[]

.W[
Dereferencing an iterator has undefined behavior if it does not point to an
existing element of a container.
]

.F[:
Especially, while "one beyond the last" is easily defined for contiguous memory,
it is much mire difficult to come ob with a definition for linked list or stream
iterators.
]

---
template: plain
header: #### Comparing Iterators (Reachability)

For most containers iterators can only be compare for equal (`==`) or unequal
(`!=`).

.N[
In general there is no easy way to decide whether some iterator incremented
a number of times will compare equal to another iterator before it reach the
end of the container before.._[]
]

Therefore, a typical requirement of algorithms is that an **iterator is
reachable** by some other iterator, in the following meaning:

* When applying increment operations, the latter will stay valid
* until it finally compares equal to the former.

.W[
Programmers need to be aware of this and strictly ensure the reachability
requirement. Otherwise iterators may assume invalid position and access
memory outside the container.
]

.F[:
As always with behavior that is undefined per C++ standard, implementations may
chose to catch errors. Especially for iterators that provide a relational
comparison a "not-reachable" error might be caught internally.
]

---
template: plain
header: #### Invalidating Iterators

For most – **but not all** – containers their iterators stay valid (i.e. always
denote the same element)

* once the iterator has been set to an existing element and

* as long as that is not removed from the container.

.N[
Notable exceptions from this are containers storing their elements in contiguous
memory, when the allocated storage is exhausted and must be extended.
]

As heap storage often cannot be extended in place, its content may need to be
moved in memory, thus invalidating iterators.

.W[
Programmers need to be aware of this and must either re-calculated saved
iterator positions or rather used indices.
]

---
template: plain
header: ### Iterators vs. Indices

Technically indices might have a slight performance penalty compared to pointers
and iterators, as an addition (base address plus offset) is required at
run-time.

.N[
On modern hardware performance effects are negligible or invisible,._[] as
these additions are usually carried out in special hardware units, while
decoding the relevant machine operations.
]

Using indices instead of iterators to permanently denote the position of *some
container elements of special interest* should be strongly considered

* for `std::vector` (and `std::basic_string` including its type aliases)

* if the content changes frequently and may trigger growth beyond the size
  the container had when the positions were determined.

.F[:
Though there may be indirect effects as register usage tends to grow, hence
reducing the number of registers available for temporaries.
]

---
template: plain
header: ### Iterators vs. Range-For

.pull-left[
```
container data;
for (auto e : data) // 1
    … 
for (auto &e : data) // 2
    …
for (const auto &e : data) // 3
    …
```
]
.pull-right[
**Iterators are neither superior nor inferior to range-`for` loops.**

The code shown left (with `container` replaced by any STL container
instantiated for a concrete type) …._[]
]

… is equivalent to the code shown below:
```
for (auto it = std::begin(data); it != std::end(data); ++it) { // 1
    auto e = *it; …
}
for (auto it = std::begin(data); it != std::end(data); ++it) { // 2
    auto &e = *it; …
}
for (auto it = std::begin(data); it != std::end(data); ++it) { // 3
    const auto &e = *it; …
}
```

.F[:
Actually this is true for any (also non-STL) container and the global overloads
are automatically mapped to member functions of the same name (but without
arguments).
]

---
template: linkinfo
graphic: AlgorithmicComplexity
name: big_o_algorithmic_complexity
header: ## "Big-O" Algorithmic Complexity

Please follow the verbal explanations given by trainer, based on the adjoining
Info-Graphic …

… and feel free to ask any question you want to ask.

---
template: plain
header: ## Summary

This part's goal was to give an overview to the STL, starting with

* The [Architecture](#architectural_overview), centering on
  * [Containers](#container_dimension)
    – to store and organizer data items) and
  * [Algorithms](#algorithm_dimension)
    – to process such data collections

* emphasizing the Role of [Iterators](#understanding_iterators)
  * providing the necessary "glue" between the former
  * a generalising mechanism for access and traversal

* On the way some key terms were introduced, like
  * [Concepts and Models](#concepts_and_models)
  * the ["Big-O"-Notation](#big_o_algorithmic_complexity)

-------------------------------------------------------------------------------

* What to do next?
  * [Go back the Agenda](00_topics.html#agenda) …
  * …  or [continue with next part](02_day1.html).