<chapter xmlns="http://docbook.org/ns/docbook" version="5.0"
xml:id="std.containers" xreflabel="Containers">
<?dbhtml filename="containers.html"?>
<info><title>
Containers
<indexterm><primary>Containers</primary></indexterm>
</title>
<keywordset>
<keyword>ISO C++</keyword>
<keyword>library</keyword>
</keywordset>
</info>
<!-- Sect1 01 : Sequences -->
<section xml:id="std.containers.sequences" xreflabel="Sequences"><info><title>Sequences</title></info>
<?dbhtml filename="sequences.html"?>
<section xml:id="containers.sequences.list" xreflabel="list"><info><title>list</title></info>
<?dbhtml filename="list.html"?>
<section xml:id="sequences.list.size" xreflabel="list::size() is O(n)"><info><title>list::size() is O(n)</title></info>
<para>
Yes it is, at least using the <link linkend="manual.intro.using.abi">old
ABI</link>, and that's okay. This is a decision that we preserved
when we imported SGI's STL implementation. The following is
quoted from <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/FAQ.html">their FAQ</link>:
</para>
<blockquote>
<para>
The size() member function, for list and slist, takes time
proportional to the number of elements in the list. This was a
deliberate tradeoff. The only way to get a constant-time
size() for linked lists would be to maintain an extra member
variable containing the list's size. This would require taking
extra time to update that variable (it would make splice() a
linear time operation, for example), and it would also make the
list larger. Many list algorithms don't require that extra
word (algorithms that do require it might do better with
vectors than with lists), and, when it is necessary to maintain
an explicit size count, it's something that users can do
themselves.
</para>
<para>
This choice is permitted by the C++ standard. The standard says
that size() <quote>should</quote> be constant time, and
<quote>should</quote> does not mean the same thing as
<quote>shall</quote>. This is the officially recommended ISO
wording for saying that an implementation is supposed to do
something unless there is a good reason not to.
</para>
<para>
One implication of linear time size(): you should never write
</para>
<programlisting>
if (L.size() == 0)
...
</programlisting>
<para>
Instead, you should write
</para>
<programlisting>
if (L.empty())
...
</programlisting>
</blockquote>
</section>
</section>
</section>
<!-- Sect1 02 : Associative -->
<section xml:id="std.containers.associative" xreflabel="Associative"><info><title>Associative</title></info>
<?dbhtml filename="associative.html"?>
<section xml:id="containers.associative.insert_hints" xreflabel="Insertion Hints"><info><title>Insertion Hints</title></info>
<para>
Section [23.1.2], Table 69, of the C++ standard lists this
function for all of the associative containers (map, set, etc):
</para>
<programlisting>
a.insert(p,t);
</programlisting>
<para>
where 'p' is an iterator into the container 'a', and 't' is the
item to insert. The standard says that <quote><code>t</code> is
inserted as close as possible to the position just prior to
<code>p</code>.</quote> (Library DR #233 addresses this topic,
referring to <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1780.html">N1780</link>.
Since version 4.2 GCC implements the resolution to DR 233, so
that insertions happen as close as possible to the hint. For
earlier releases the hint was only used as described below.
</para>
<para>
Here we'll describe how the hinting works in the libstdc++
implementation, and what you need to do in order to take
advantage of it. (Insertions can change from logarithmic
complexity to amortized constant time, if the hint is properly
used.) Also, since the current implementation is based on the
SGI STL one, these points may hold true for other library
implementations also, since the HP/SGI code is used in a lot of
places.
</para>
<para>
In the following text, the phrases <emphasis>greater
than</emphasis> and <emphasis>less than</emphasis> refer to the
results of the strict weak ordering imposed on the container by
its comparison object, which defaults to (basically)
<quote><</quote>. Using those phrases is semantically sloppy,
but I didn't want to get bogged down in syntax. I assume that if
you are intelligent enough to use your own comparison objects,
you are also intelligent enough to assign <quote>greater</quote>
and <quote>lesser</quote> their new meanings in the next
paragraph. *grin*
</para>
<para>
If the <code>hint</code> parameter ('p' above) is equivalent to:
</para>
<itemizedlist>
<listitem>
<para>
<code>begin()</code>, then the item being inserted should
have a key less than all the other keys in the container.
The item will be inserted at the beginning of the container,
becoming the new entry at <code>begin()</code>.
</para>
</listitem>
<listitem>
<para>
<code>end()</code>, then the item being inserted should have
a key greater than all the other keys in the container. The
item will be inserted at the end of the container, becoming
the new entry before <code>end()</code>.
</para>
</listitem>
<listitem>
<para>
neither <code>begin()</code> nor <code>end()</code>, then:
Let <code>h</code> be the entry in the container pointed to
by <code>hint</code>, that is, <code>h = *hint</code>. Then
the item being inserted should have a key less than that of
<code>h</code>, and greater than that of the item preceding
<code>h</code>. The new item will be inserted between
<code>h</code> and <code>h</code>'s predecessor.
</para>
</listitem>
</itemizedlist>
<para>
For <code>multimap</code> and <code>multiset</code>, the
restrictions are slightly looser: <quote>greater than</quote>
should be replaced by <quote>not less than</quote>and <quote>less
than</quote> should be replaced by <quote>not greater
than.</quote> (Why not replace greater with
greater-than-or-equal-to? You probably could in your head, but
the mathematicians will tell you that it isn't the same thing.)
</para>
<para>
If the conditions are not met, then the hint is not used, and the
insertion proceeds as if you had called <code> a.insert(t)
</code> instead. (<emphasis>Note </emphasis> that GCC releases
prior to 3.0.2 had a bug in the case with <code>hint ==
begin()</code> for the <code>map</code> and <code>set</code>
classes. You should not use a hint argument in those releases.)
</para>
<para>
This behavior goes well with other containers'
<code>insert()</code> functions which take an iterator: if used,
the new item will be inserted before the iterator passed as an
argument, same as the other containers.
</para>
<para>
<emphasis>Note </emphasis> also that the hint in this
implementation is a one-shot. The older insertion-with-hint
routines check the immediately surrounding entries to ensure that
the new item would in fact belong there. If the hint does not
point to the correct place, then no further local searching is
done; the search begins from scratch in logarithmic time.
</para>
</section>
<section xml:id="containers.associative.bitset" xreflabel="bitset"><info><title>bitset</title></info>
<?dbhtml filename="bitset.html"?>
<section xml:id="associative.bitset.size_variable" xreflabel="Variable"><info><title>Size Variable</title></info>
<para>
No, you cannot write code of the form
</para>
<!-- Careful, the leading spaces in PRE show up directly. -->
<programlisting>
#include <bitset>
void foo (size_t n)
{
std::bitset<n> bits;
....
}
</programlisting>
<para>
because <code>n</code> must be known at compile time. Your
compiler is correct; it is not a bug. That's the way templates
work. (Yes, it <emphasis>is</emphasis> a feature.)
</para>
<para>
There are a couple of ways to handle this kind of thing. Please
consider all of them before passing judgement. They include, in
no particular order:
</para>
<itemizedlist>
<listitem><para>A very large N in <code>bitset<N></code>.</para></listitem>
<listitem><para>A container<bool>.</para></listitem>
<listitem><para>Extremely weird solutions.</para></listitem>
</itemizedlist>
<para>
<emphasis>A very large N in
<code>bitset<N></code>. </emphasis> It has been
pointed out a few times in newsgroups that N bits only takes up
(N/8) bytes on most systems, and division by a factor of eight is
pretty impressive when speaking of memory. Half a megabyte given
over to a bitset (recall that there is zero space overhead for
housekeeping info; it is known at compile time exactly how large
the set is) will hold over four million bits. If you're using
those bits as status flags (e.g.,
<quote>changed</quote>/<quote>unchanged</quote> flags), that's a
<emphasis>lot</emphasis> of state.
</para>
<para>
You can then keep track of the <quote>maximum bit used</quote>
during some testing runs on representative data, make note of how
many of those bits really need to be there, and then reduce N to
a smaller number. Leave some extra space, of course. (If you
plan to write code like the incorrect example above, where the
bitset is a local variable, then you may have to talk your
compiler into allowing that much stack space; there may be zero
space overhead, but it's all allocated inside the object.)
</para>
<para>
<emphasis>A container<bool>. </emphasis> The
Committee made provision for the space savings possible with that
(N/8) usage previously mentioned, so that you don't have to do
wasteful things like <code>Container<char></code> or
<code>Container<short int></code>. Specifically,
<code>vector<bool></code> is required to be specialized for
that space savings.
</para>
<para>
The problem is that <code>vector<bool></code> doesn't
behave like a normal vector anymore. There have been
journal articles which discuss the problems (the ones by Herb
Sutter in the May and July/August 1999 issues of C++ Report cover
it well). Future revisions of the ISO C++ Standard will change
the requirement for <code>vector<bool></code>
specialization. In the meantime, <code>deque<bool></code>
is recommended (although its behavior is sane, you probably will
not get the space savings, but the allocation scheme is different
than that of vector).
</para>
<para>
<emphasis>Extremely weird solutions. </emphasis> If
you have access to the compiler and linker at runtime, you can do
something insane, like figuring out just how many bits you need,
then writing a temporary source code file. That file contains an
instantiation of <code>bitset</code> for the required number of
bits, inside some wrapper functions with unchanging signatures.
Have your program then call the compiler on that file using
Position Independent Code, then open the newly-created object
file and load those wrapper functions. You'll have an
instantiation of <code>bitset<N></code> for the exact
<code>N</code> that you need at the time. Don't forget to delete
the temporary files. (Yes, this <emphasis>can</emphasis> be, and
<emphasis>has been</emphasis>, done.)
</para>
<!-- I wonder if this next paragraph will get me in trouble... -->
<para>
This would be the approach of either a visionary genius or a
raving lunatic, depending on your programming and management
style. Probably the latter.
</para>
<para>
Which of the above techniques you use, if any, are up to you and
your intended application. Some time/space profiling is
indicated if it really matters (don't just guess). And, if you
manage to do anything along the lines of the third category, the
author would love to hear from you...
</para>
<para>
Also note that the implementation of bitset used in libstdc++ has
<link linkend="manual.ext.containers.sgi">some extensions</link>.
</para>
</section>
<section xml:id="associative.bitset.type_string" xreflabel="Type String"><info><title>Type String</title></info>
<para>
</para>
<para>
Bitmasks do not take char* nor const char* arguments in their
constructors. This is something of an accident, but you can read
about the problem: follow the library's <quote>Links</quote> from
the homepage, and from the C++ information <quote>defect
reflector</quote> link, select the library issues list. Issue
number 116 describes the problem.
</para>
<para>
For now you can simply make a temporary string object using the
constructor expression:
</para>
<programlisting>
std::bitset<5> b ( std::string("10110") );
</programlisting>
<para>
instead of
</para>
<programlisting>
std::bitset<5> b ( "10110" ); // invalid
</programlisting>
</section>
</section>
</section>
<!-- Sect1 03 : Unordered Associative -->
<section xml:id="std.containers.unordered" xreflabel="Unordered">
<info><title>Unordered Associative</title></info>
<?dbhtml filename="unordered_associative.html"?>
<section xml:id="containers.unordered.insert_hints" xreflabel="Insertion Hints">
<info><title>Insertion Hints</title></info>
<para>
Here is how the hinting works in the libstdc++ implementation of unordered
containers, and the rationale behind this behavior.
</para>
<para>
In the following text, the phrase <emphasis>equivalent to</emphasis> refer
to the result of the invocation of the equal predicate imposed on the
container by its <code>key_equal</code> object, which defaults to (basically)
<quote>==</quote>.
</para>
<para>
Unordered containers can be seen as a <code>std::vector</code> of
<code>std::forward_list</code>. The <code>std::vector</code> represents
the buckets and each <code>std::forward_list</code> is the list of nodes
belonging to the same bucket. When inserting an element in such a data
structure we first need to compute the element hash code to find the
bucket to insert the element to, the second step depends on the uniqueness
of elements in the container.
</para>
<para>
In the case of <code>std::unordered_set</code> and
<code>std::unordered_map</code> you need to look through all bucket's
elements for an equivalent one. If there is none the insertion can be
achieved, otherwise the insertion fails. As we always need to loop though
all bucket's elements, the hint doesn't tell us if the element is already
present, and we don't have any constraint on where the new element is to
be inserted, the hint won't be of any help and will then be ignored.
</para>
<para>
In the case of <code>std::unordered_multiset</code>
and <code>std::unordered_multimap</code> equivalent elements must be
linked together so that the <code>equal_range(const key_type&)</code>
can return the range of iterators pointing to all equivalent elements.
This is where hinting can be used to point to another equivalent element
already part of the container and so skip all non equivalent elements of
the bucket. So to be useful the hint shall point to an element equivalent
to the one being inserted. The new element will be then inserted right
after the hint. Note that because of an implementation detail inserting
after a node can require updating the bucket of the following node. To
check if the next bucket is to be modified we need to compute the
following node's hash code. So if you want your hint to be really efficient
it should be followed by another equivalent element, the implementation
will detect this equivalence and won't compute next element hash code.
</para>
<para>
It is highly advised to start using unordered containers hints only if you
have a benchmark that will demonstrate the benefit of it. If you don't then do
not use hints, it might do more harm than good.
</para>
</section>
<section xml:id="containers.unordered.hash" xreflabel="Hash">
<info><title>Hash Code</title></info>
<section xml:id="containers.unordered.cache" xreflabel="Cache">
<info><title>Hash Code Caching Policy</title></info>
<para>
The unordered containers in libstdc++ may cache the hash code for each
element alongside the element itself. In some cases not recalculating
the hash code every time it's needed can improve performance, but the
additional memory overhead can also reduce performance, so whether an
unordered associative container caches the hash code or not depends on
the properties described below.
</para>
<para>
The C++ standard requires that <code>erase</code> and <code>swap</code>
operations must not throw exceptions. Those operations might need an
element's hash code, but cannot use the hash function if it could
throw.
This means the hash codes will be cached unless the hash function
has a non-throwing exception specification such as <code>noexcept</code>
or <code>throw()</code>.
</para>
<para>
If the hash function is non-throwing then libstdc++ doesn't need to
cache the hash code for
correctness, but might still do so for performance if computing a
hash code is an expensive operation, as it may be for arbitrarily
long strings.
As an extension libstdc++ provides a trait type to describe whether
a hash function is fast. By default hash functions are assumed to be
fast unless the trait is specialized for the hash function and the
trait's value is false, in which case the hash code will always be
cached.
The trait can be specialized for user-defined hash functions like so:
</para>
<programlisting>
#include <unordered_set>
struct hasher
{
std::size_t operator()(int val) const noexcept
{
// Some very slow computation of a hash code from an int !
...
}
}
namespace std
{
template<>
struct __is_fast_hash<hasher> : std::false_type
{ };
}
</programlisting>
</section>
</section>
</section>
<!-- Sect1 04 : Interacting with C -->
<section xml:id="std.containers.c" xreflabel="Interacting with C"><info><title>Interacting with C</title></info>
<?dbhtml filename="containers_and_c.html"?>
<section xml:id="containers.c.vs_array" xreflabel="Containers vs. Arrays"><info><title>Containers vs. Arrays</title></info>
<para>
You're writing some code and can't decide whether to use builtin
arrays or some kind of container. There are compelling reasons
to use one of the container classes, but you're afraid that
you'll eventually run into difficulties, change everything back
to arrays, and then have to change all the code that uses those
data types to keep up with the change.
</para>
<para>
If your code makes use of the standard algorithms, this isn't as
scary as it sounds. The algorithms don't know, nor care, about
the kind of <quote>container</quote> on which they work, since
the algorithms are only given endpoints to work with. For the
container classes, these are iterators (usually
<code>begin()</code> and <code>end()</code>, but not always).
For builtin arrays, these are the address of the first element
and the <link linkend="iterators.predefined.end">past-the-end</link> element.
</para>
<para>
Some very simple wrapper functions can hide all of that from the
rest of the code. For example, a pair of functions called
<code>beginof</code> can be written, one that takes an array,
another that takes a vector. The first returns a pointer to the
first element, and the second returns the vector's
<code>begin()</code> iterator.
</para>
<para>
The functions should be made template functions, and should also
be declared inline. As pointed out in the comments in the code
below, this can lead to <code>beginof</code> being optimized out
of existence, so you pay absolutely nothing in terms of increased
code size or execution time.
</para>
<para>
The result is that if all your algorithm calls look like
</para>
<programlisting>
std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);
</programlisting>
<para>
then the type of foo can change from an array of ints to a vector
of ints to a deque of ints and back again, without ever changing
any client code.
</para>
<programlisting>
// beginof
template<typename T>
inline typename vector<T>::iterator
beginof(vector<T> &v)
{ return v.begin(); }
template<typename T, unsigned int sz>
inline T*
beginof(T (&array)[sz]) { return array; }
// endof
template<typename T>
inline typename vector<T>::iterator
endof(vector<T> &v)
{ return v.end(); }
template<typename T, unsigned int sz>
inline T*
endof(T (&array)[sz]) { return array + sz; }
// lengthof
template<typename T>
inline typename vector<T>::size_type
lengthof(vector<T> &v)
{ return v.size(); }
template<typename T, unsigned int sz>
inline unsigned int
lengthof(T (&)[sz]) { return sz; }
</programlisting>
<para>
Astute readers will notice two things at once: first, that the
container class is still a <code>vector<T></code> instead
of a more general <code>Container<T></code>. This would
mean that three functions for <code>deque</code> would have to be
added, another three for <code>list</code>, and so on. This is
due to problems with getting template resolution correct; I find
it easier just to give the extra three lines and avoid confusion.
</para>
<para>
Second, the line
</para>
<programlisting>
inline unsigned int lengthof (T (&)[sz]) { return sz; }
</programlisting>
<para>
looks just weird! Hint: unused parameters can be left nameless.
</para>
</section>
</section>
</chapter>