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:
The SGI headers
<bvector>
<hash_map>
<hash_set>
<rope>
<slist>
<tree>
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.)
Return to top of page or to the FAQ.
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).
filebufs to be constructed from
stdio types are described in the
chapter 27 notes.Return to top of page or to the FAQ.
Thread-safety, space efficiency, high speed, portability... this is a mess. Where to begin?
The C++ standard only gives a few directives in this area:
Allocator template parameter. This includes adding
char's to the string class, which acts as a regular STL container
in this respect.
Allocator of every container-of-T is
std::allocator<T>.
allocator<T> class is
extremely simple. It has about 20 public declarations (nested
typedefs, member functions, etc), but the two which concern us most
are:
T* allocate (size_type n, const void* hint = 0);
void deallocate (T* p, size_type n);
(This is a simplicifcation; the real signatures use nested typedefs.)
The "n" arguments in both those functions is a
count of the number of T's to allocate space for,
not their total size.
::operator new(size_t), but it is unspecified when or
how often this function is called. The use of hint
is unspecified, but intended as an aid to locality if an
implementation so desires." [20.4.1.1]/6
The easiest way of fulfilling the requirements is to call operator new each time a container needs memory, and to call operator delete each time the container releases memory. BUT this method is horribly slow.
Or we can keep old memory around, and reuse it in a pool to save time. The old libstdc++-v2 used a memory pool, and so do we. As of 3.0, it's on by default. The pool is shared among all the containers in the program: when your program's std::vector<int> gets cut in half and frees a bunch of its storage, that memory can be reused by the private std::list<WonkyWidget> brought in from a KDE library that you linked against. And we don't have to call operators new and delete to pass the memory on, either, which is a speed bonus. BUT...
What about threads? No problem: in a threadsafe environment, the memory pool is manipulated atomically, so you can grow a container in one thread and shrink it in another, etc. BUT what if threads in libstdc++-v3 aren't set up properly? That's been answered already.
BUT what if you want to use your own allocator? What if you plan on using a runtime-loadable version of malloc() which uses shared telepathic anonymous mmap'd sections serializable over a network, so that memory requests should go through malloc? And what if you need to debug it?
Well then:
First I'll describe the situation as it exists for the code which was released in GCC 3.1 and 3.2. Then I'll describe the differences for 3.0. The allocator classes also have source documentation, which is described here (you will need to retrieve the maintainer-level docs, as almost none of these entities are in the ISO standard).
As a general rule of thumb, users are not allowed to use names which begin with an underscore. This means that to be portable between compilers, none of the following may be used in your program directly. (If you decide to be unportable, then you're free do do what you want, but it's not our fault if stuff breaks.) They are presented here for information for maintainers and contributors in addition to users.
These classes are always available:
__new_alloc simply wraps ::operator new
and ::operator delete.
__malloc_alloc_template<int inst> simply wraps
malloc and free. There is also a hook
for an out-of-memory handler (for new/delete this is taken care of
elsewhere). The inst parameter is described below.
This class was called malloc_alloc in earlier versions.
allocator<T> has already been described; it is
The Standard Allocator for instances of T. It uses the internal
__alloc typedef (see below) to satisy its requests.
__simple_alloc<T,A> is a wrapper around another
allocator, A, which itself is an allocator for instances of T.
This is primarily used in an internal "allocator traits"
class which helps encapsulate the different styles of allocators.
__debug_alloc<A> is also a wrapper around an
arbitrary allocator A. It passes on slightly increased size
requests to A, and uses the extra memory to store size information.
When a pointer is passed to deallocate(), the stored
size is checked, and assert() is used to guarantee they match.
__allocator<T,A> is an adaptor. Many of these
allocator classes have a consistent yet non-standard interface.
Such classes can be changed to a conforming interface with this
wrapper: __allocator<T, __alloc> is thus the
same as allocator<T>.
Normally,
__default_alloc_template<bool thr, int inst>
is also available. This is the high-speed pool, called the default
node allocator. The reusable memory is shared among identical
instantiations of
this type. It calls through __new_alloc to obtain
new memory when its lists run out. If a client container requests a
block larger than a certain threshold size, then the pool is bypassed,
and the allocate/deallocate request is passed to
__new_alloc directly.
Its inst parameter is described below. The
thr boolean determines whether the pool should be
manipulated atomically or not. Two typedefs are provided:
__alloc is defined as this node allocator with thr=true,
and therefore is threadsafe, while __single_client_alloc
defines thr=false, and is slightly faster but unsafe for multiple
threads.
(Note that the GCC thread abstraction layer allows us to provide safe
zero-overhead stubs for the threading routines, if threads were
disabled at configuration time. In this situation,
__alloc should not be noticably slower than
__single_client_alloc.)
[Another threadsafe allocator where each thread keeps its own free list, so that no locking is needed, might be described here.]
__USE_MALLOCIf you've already read this advice but still think you remember how to use this macro from SGI STL days. We have removed it in gcc 3.3. See next section for the new way to get the same effect.
GLIBCPP_FORCE_NEWStarting with gcc 3.3, if you want to globally disable memory caching within the library for the default allocator (i.e. the one you get for all library objects when you do not specify which one to use), merely set GLIBCPP_FORCE_NEW (at this time, with any value) into your environment before running the program. You will obtain a similar effect without having to recompile your entire program and the entire library (the new operator in gcc is a light wrapper around malloc). If your program crashes with GLIBCPP_FORCE_NEW in the environment, it likely means that you linked against objects built against the older library. Code to support this extension is fully compatible with 3.2 code if GLIBCPP_FORCE_NEW is not in the environment.
Depending on your application (a specific program, a generic library, etc), allocator classes tend to be one of two styles: "SGI" or "standard". See the comments in stl_alloc.h for more information on this crucial difference.
At the bottom of that header is a helper type,
_Alloc_traits, and various specializations of it. This
allows the container classes to make possible compile-time
optimizations based on features of the allocator. You should provide
a specialization of this type for your allocator (doing so takes only
two or three statements).
You can specify different memory management schemes on a per-container
basis, by overriding the default Allocator template
parameter. For example, an easy
(but nonportable)
method of specifying that only malloc/free should be used instead of
the default node allocator is:
std::list <my_type, std::__malloc_alloc_template<0> > my_malloc_based_list;
Likewise, a debugging form of whichever allocator is currently in use:
std::deque <my_type, std::__debug_alloc<std::__alloc> > debug_deque;
instThe __malloc_alloc_template and
__default_alloc_template classes take an integer parameter,
called inst here. This number is completely unused.
The point of the number is to allow multiple instantiations of the classes without changing the semantics at all. All three of
typedef __default_alloc_template<true,0> normal;
typedef __default_alloc_template<true,1> private;
typedef __default_alloc_template<true,42> also_private;
behave exactly the same way. However, the memory pool for each type (and remember that different instantiations result in different types) remains separate.
The library uses 0 in all its instantiations. If you wish to keep separate free lists for a particular purpose, use a different number.
For 3.0.x, many of the names were incorrectly not prefixed with underscores. So symbols such as "std::single_client_alloc" are present. Be very careful to not depend on these names any more than you would depend on implementation-only names.
Certain macros like _NOTHREADS and __STL_THREADS
can affect the 3.0.x allocators. Do not use them. Those macros have
been completely removed for 3.1.
Return to top of page or to the FAQ.
Changes are coming...
Return to top of page or to the FAQ.
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.
Return to top of page or to the FAQ.
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 _GLIBCPP_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.
ios_base::failure is constructed instead.
private and are
thus inaccessible. Specifying the correct semantics of
"copying stream state" was deemed too complicated.
max_size() rather than npos.
binder1st and binder2nd didn't have an
operator() taking a non-const parameter.
num_put::put() was overloaded on the wrong types.
num_get::get().
failbit on error now.
seekp should only set the output stream, and
seekg should only set the input stream.
op<< with a const char* was
calculating an incorrect number of characters to write.
op>> now
sets failbit (which can cause an exception, etc, etc).
set and multiset were missing
overloaded find, lower_bound, upper_bound, and equal_range functions
for const instances.
bad_* classes no longer have destructors (they
are trivial), since no description of them was ever given.
Return to top of page or to the FAQ.
See license.html for copying conditions. Comments and suggestions are welcome, and may be sent to the libstdc++ mailing list.