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:

  1. 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...
  2. You should know how to access these headers properly.


Ropes and trees and hashes, oh my!

The SGI headers


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.)

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Added members and types

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).

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Compile-time checks

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.

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LWG Issues

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.

5: string::compare specification questionable
This should be two overloaded functions rather than a single function.
17: Bad bool parsing
Apparently extracting Boolean values was messed up...
19: "Noconv" definition too vague
If codecvt::do_in returns noconv there are no changes to the values in [to, to_limit).
22: Member open vs flags
Re-opening a file stream does not clear the state flags.
25: String operator<< uses width() value wrong
Padding issues.
48: Use of non-existent exception constructor
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.
60: What is a formatted input function?
This DR made many widespread changes to basic_istream and basic_ostream all of which have been implemented.
63: Exception-handling policy for unformatted output
Make the policy consistent with that of formatted input, unformatted input, and formatted output.
68: Extractors for char* should store null at end
And they do now. An editing glitch in the last item in the list of []/7.
74: Garbled text for codecvt::do_max_length
The text of the standard was gibberish. Typos gone rampant.
75: Contradiction in codecvt::length's argument types
Change the first parameter to stateT& and implement the new effects paragraph.
83: string::npos vs. string::max_size()
Safety checks on the size of the string should test against max_size() rather than npos.
90: Incorrect description of operator>> for strings
The effect contain isspace(c,getloc()) which must be replaced by isspace(c,is.getloc()).
91: Description of operator>> and getline() for string<> might cause endless loop
They behave as a formatted input function and as an unformatted input function, respectively (except that getline is not required to set gcount).
103: set::iterator is required to be modifiable, but this allows modification of keys.
For associative containers where the value type is the same as the key type, both iterator and const_iterator are constant iterators.
109: Missing binders for non-const sequence elements
The binder1st and binder2nd didn't have an operator() taking a non-const parameter.
110: istreambuf_iterator::equal not const
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()
These functions set failbit on error now.
136: seekp, seekg setting wrong streams?
seekp should only set the output stream, and seekg should only set the input stream.
167: Improper use of traits_type::length()
op<< with a const char* was calculating an incorrect number of characters to write.
171: Strange seekpos() semantics due to joint position
Quite complex to summarize...
181: make_pair() unintended behavior
This function used to take its arguments as reference-to-const, now it copies them (pass by value).
195: Should basic_istream::sentry's constructor ever set eofbit?
Yes, it can, specifically if EOF is reached while skipping whitespace.
211: operator>>(istream&, string&) doesn't set failbit
If nothing is extracted into the string, op>> now sets failbit (which can cause an exception, etc, etc).
214: set::find() missing const overload
Both set and multiset were missing overloaded find, lower_bound, upper_bound, and equal_range functions for const instances.
231: Precision in iostream?
For conversion from a floating-point type, str.precision() is specified in the conversion specification.
235: No specification of default ctor for reverse_iterator
The declaration of reverse_iterator lists a default constructor. However, no specification is given what this constructor should do.
251: basic_stringbuf missing allocator_type
This nested typdef was originally not specified.
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.
265: std::pair::pair() effects overly restrictive
The default ctor would build its members from copies of temporaries; now it simply uses their respective default ctors.
266: bad_exception::~bad_exception() missing Effects clause
The bad_* classes no longer have destructors (they are trivial), since no description of them was ever given.
271: basic_iostream missing typedefs
The typedefs it inherits from its base classes can't be used, since (for example) basic_iostream<T>::traits_type is ambiguous.
275: Wrong type in num_get::get() overloads
Similar to 118.
292: Effects of a.copyfmt (a)
If (this == &rhs) do nothing.
300: List::merge() specification incomplete
If (this == &x) do nothing.
303: Bitset input operator underspecified
Basically, compare the input character to is.widen(0) and is.widen(1).
305: Default behavior of codecvt<wchar_t, char, mbstate_t>::length()
Do not specify what codecvt<wchar_t, char, mbstate_t>::do_length must return.
328: Bad sprintf format modifier in money_put<>::do_put()
Change the format string to "%.0Lf".
389: Const overload of valarray::operator[] returns by value
Change it to return a const T&.
402: Wrong new expression in [some_]allocator::construct
Replace "new" with "::new".
453: basic_stringbuf::seekoff need not always fail for an empty stream
Don't fail if the next pointer is null and newoff is zero.

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