Here we will make an attempt at describing the non-Standard extensions to
the library. Some of these are from SGI's STL, some of these are GNU's,
and some just seemed to appear on the doorstep.
Before you leap in and use these, be aware of two things:
Non-Standard means exactly that. The behavior, and the very
existence, of these extensions may change with little or no
warning. (Ideally, the really good ones will appear in the next
revision of C++.) Also, other platforms, other compilers, other
versions of g++ or libstdc++-v3 may not recognize these names, or
treat them differently, or...
are all here; <bvector> exposes the old bit_vector
class that was used before specialization of vector<bool> was
available (it's actually a typedef for the specialization now).
<hash_map> and <hash_set>
are discussed further below. <rope> is the SGI
specialization for large strings ("rope," "large
strings," get it? love those SGI folks).
<slist> is a singly-linked list, for when the
doubly-linked list<> is too much space overhead, and
<tree> exposes the red-black tree classes used in the
implementation of the standard maps and sets.
Okay, about those hashing classes... I'm going to foist most of the
work off onto SGI's own site.
Each of the associative containers map, multimap, set, and multiset
have a counterpart which uses a
hashing
function to do the arranging, instead of a strict weak ordering
function. The classes take as one of their template parameters a
function object that will return the hash value; by default, an
instantiation of
hash.
You should specialize this functor for your class, or define your own,
before trying to use one of the hashing classes.
The hashing classes support all the usual associative container
functions, as well as some extra constructors specifying the number
of buckets, etc.
Why would you want to use a hashing class instead of the
"normal" implementations? Matt Austern writes:
[W]ith a well chosen hash function, hash tables
generally provide much better average-case performance than binary
search trees, and much worse worst-case performance. So if your
implementation has hash_map, if you don't mind using nonstandard
components, and if you aren't scared about the possibility of
pathological cases, you'll probably get better performance from
hash_map.
(Side note: for those of you wondering, "Why wasn't a hash
table included in the Standard in the first #!$@ place?"
I'll give a quick answer: it was proposed, but too late and in too
unorganized a fashion. Some sort of hashing will undoubtedly be
included in a future Standard.)
Some of the classes in the Standard Library have additional
publicly-available members, and some classes are themselves not in
the standard. Of those, some are intended purely for the implementors,
for example, additional typedefs. Those won't be described here
(or anywhere else).
The extensions added by SGI are so numerous that they have
their own page. Since the SGI STL is no
longer actively maintained, we will try and keep this code working
ourselves.
Extensions allowing filebufs to be constructed from
stdio types are described in the
chapter 27 notes.
Currently libstdc++-v3 uses the concept checkers from the Boost
library to perform optional
compile-time checking of template instantiations of the standard
containers. They are described in the linked-to page.
Everybody's got issues. Even the C++ Standard Library.
The Library Working Group, or LWG, is the ISO subcommittee responsible
for making changes to the library. They periodically publish an
Issues List containing problems and possible solutions. As they reach
a consensus on proposed solutions, we often incorporate the solution
into libstdc++-v3.
Here are the issues which have resulted in code changes to the library.
The links are to the specific defect reports from a partial
copy of the Issues List. You can read the full version online
at the ISO C++
Committee homepage, linked to on the
GCC "Readings"
page. If
you spend a lot of time reading the issues, we recommend downloading
the ZIP file and reading them locally.
(NB: partial copy means that not all links within
the lwg-*.html pages will work.
Specifically, links to defect reports that have not been accorded full
DR status will probably break. Rather than trying to mirror the
entire issues list on our overworked web server, we recommend you go
to the LWG homepage instead.)
If a DR is not listed here, we may simply not have gotten to it yet;
feel free to submit a patch. Search the include/bits and src
directories for appearances of _GLIBCXX_RESOLVE_LIB_DEFECTS for
examples of style. Note that we usually do not make changes to the code
until an issue has reached DR status.
An instance of ios_base::failure is constructed instead.
49:
Underspecification of ios_base::sync_with_stdio
The return type is the previous state of synchronization.
50:
Copy constructor and assignment operator of ios_base
These members functions are declared private and are
thus inaccessible. Specifying the correct semantics of
"copying stream state" was deemed too complicated.
This was not a const member function. Note that the DR says to
replace the function with a const one; we have instead provided an
overloaded version with identical contents.
117:
basic_ostream uses nonexistent num_put member functions
num_put::put() was overloaded on the wrong types.
118:
basic_istream uses nonexistent num_get member functions
Same as 117, but for num_get::get().
129:
Need error indication from seekp() and seekg()
253:
valarray helper functions are almost entirely useless
Make the copy constructor and copy-assignment operator declarations
public in gslice_array, indirect_array, mask_array, slice_array; provide
definitions.