Extensions
<indexterm><primary>Extensions</primary></indexterm>
ISO C++library
Here we will make an attempt at describing the non-Standard
extensions to the library. Some of these are from older versions of
standard library components, namely SGI's STL, and some of these are
GNU's.
Before you leap in and use any of these
extensions, 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++ may not recognize these names, or
treat them differently, or...
You should know how to access these headers properly.
Compile Time Checks
Also known as concept checking.
In 1999, SGI added concept checkers to their implementation
of the STL: code which checked the template parameters of
instantiated pieces of the STL, in order to insure that the parameters
being used met the requirements of the standard. For example,
the Standard requires that types passed as template parameters to
vector be Assignable (which means what you think
it means). The checking was done during compilation, and none of
the code was executed at runtime.
Unfortunately, the size of the compiler files grew significantly
as a result. The checking code itself was cumbersome. And bugs
were found in it on more than one occasion.
The primary author of the checking code, Jeremy Siek, had already
started work on a replacement implementation. The new code has been
formally reviewed and accepted into
the
Boost libraries, and we are pleased to incorporate it into the
GNU C++ library.
The new version imposes a much smaller space overhead on the generated
object file. The checks are also cleaner and easier to read and
understand.
They are off by default for all GCC 3.0 and all later versions.
They can be enabled at configure time with
--enable-concept-checks.
You can enable them on a per-translation-unit basis with
#define
_GLIBCXX_CONCEPT_CHECKS for GCC 3.4 and higher
(or with #define _GLIBCPP_CONCEPT_CHECKS for versions
3.1, 3.2 and 3.3).
Please note that the concept checks only validate the requirements
of the old C++03 standard and reject some valid code that meets the relaxed
requirements of C++11 and later standards.
C++11 was expected to have first-class support for template parameter
constraints based on concepts in the core language.
This would have obviated the need for the library-simulated concept checking
described above, but was not part of C++11.
C++20 adds a different model of concepts, which is now used to constrain
some new parts of the C++20 library, e.g. the
<ranges> header and the new overloads in the
<algorithm> header for working with ranges.
The old library-simulated concept checks might be removed at a future date.
HP/SGI ExtensionsBackwards CompatibilityA few extensions and nods to backwards-compatibility have
been made with containers. Those dealing with older SGI-style
allocators are dealt with elsewhere. The remaining ones all deal
with bits:
The old pre-standard bit_vector class is
present for backwards compatibility. It is simply a typedef for
the vector<bool> specialization.
The bitset class has a number of extensions, described in the
rest of this item. First, we'll mention that this implementation of
bitset<N> is specialized for cases where N number of
bits will fit into a single word of storage. If your choice of N is
within that range (<=32 on i686-pc-linux-gnu, for example), then all
of the operations will be faster.
There are
versions of single-bit test, set, reset, and flip member functions which
do no range-checking. If we call them member functions of an instantiation
of bitset<N>, then their names and signatures are:
bitset<N>& _Unchecked_set (size_t pos);
bitset<N>& _Unchecked_set (size_t pos, int val);
bitset<N>& _Unchecked_reset (size_t pos);
bitset<N>& _Unchecked_flip (size_t pos);
bool _Unchecked_test (size_t pos);
Note that these may in fact be removed in the future, although we have
no present plans to do so (and there doesn't seem to be any immediate
reason to).
The member function operator[] on a const bitset returns
a bool, and for a non-const bitset returns a reference (a
nested type). No range-checking is done on the index argument, in keeping
with other containers' operator[] requirements.
Finally, two additional searching functions have been added. They return
the index of the first "on" bit, and the index of the first
"on" bit that is after prev, respectively:
size_t _Find_first() const;
size_t _Find_next (size_t prev) const;The same caveat given for the _Unchecked_* functions applies here also.
Deprecated
The SGI hashing classes hash_set and
hash_set have been deprecated by the
unordered_set, unordered_multiset, unordered_map,
unordered_multimap containers in TR1 and C++11, and
may be removed in future releases.
The SGI headers
<hash_map>
<hash_set>
<rope>
<slist>
<rb_tree>
are all here;
<backwards/hash_map> and
<backwards/hash_set>
are deprecated but available as backwards-compatible extensions,
as discussed further below.
<ext/rope> is the SGI
specialization for large strings ("rope," "large strings," get it? Love
that geeky humor.)
<ext/slist> (superseded in
C++11 by <forward_list>)
is a singly-linked list, for when the doubly-linked list<>
is too much space overhead, and
<ext/rb_tree> exposes the
red-black tree classes used in the implementation of the standard maps
and sets.
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
normalimplementations? 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.
The deprecated hash tables are superseded by the standard unordered
associative containers defined in the ISO C++ 2011 standard in the
headers <unordered_map>
and <unordered_set>.
Utilities
The <functional> header
contains many additional functors
and helper functions, extending section 20.3. They are
implemented in the file stl_function.h:
identity_element for addition and multiplication.
The functor identity, whose operator()
returns the argument unchanged.
Composition functors unary_function and
binary_function, and their helpers compose1
and compose2.
select1st and select2nd, to strip pairs.
project1st and project2nd. A set of functors/functions which always return the same result. They
are constant_void_fun, constant_binary_fun,
constant_unary_fun, constant0,
constant1, and constant2. The class subtractive_rng. mem_fun adaptor helpers mem_fun1 and
mem_fun1_ref are provided for backwards compatibility.
20.4.1 can use several different allocators; they are described on the
main extensions page.
20.4.3 is extended with a special version of
get_temporary_buffer taking a second argument. The
argument is a pointer, which is ignored, but can be used to specify
the template type (instead of using explicit function template
arguments like the standard version does). That is, in addition to
get_temporary_buffer<int>(5);
you can also use
get_temporary_buffer(5, (int*)0);
A class temporary_buffer is given in stl_tempbuf.h.
The specialized algorithms of section 20.4.4 are extended with
uninitialized_copy_n.
Algorithms25.1.6 (count, count_if) is extended with two more versions of count
and count_if. The standard versions return their results. The
additional signatures return void, but take a final parameter by
reference to which they assign their results, e.g.,
void count (first, last, value, n);25.2 (mutating algorithms) is extended with two families of signatures,
random_sample and random_sample_n.
25.2.1 (copy) is extended with
copy_n (_InputIter first, _Size count, _OutputIter result);which copies the first 'count' elements at 'first' into 'result'.
25.3 (sorting 'n' heaps 'n' stuff) is extended with some helper
predicates. Look in the doxygen-generated pages for notes on these.
is_heap tests whether or not a range is a heap.is_sorted tests whether or not a range is sorted in
nondescending order.25.3.8 (lexicographical_compare) is extended with
lexicographical_compare_3way(_InputIter1 first1, _InputIter1 last1,
_InputIter2 first2, _InputIter2 last2)which does... what?
Numerics26.4, the generalized numeric operations such as accumulate,
are extended with the following functions:
power (x, n);
power (x, n, monoid_operation);Returns, in FORTRAN syntax, "x ** n" where
n >= 0. In the
case of n == 0, returns the identity element for the
monoid operation. The two-argument signature uses multiplication (for
a true "power" implementation), but addition is supported as well.
The operation functor must be associative.
The iota function wins the award for Extension With the
Coolest Name (the name comes from Ken Iverson's APL language.) As
described in the SGI
documentation, it "assigns sequentially increasing values to a range.
That is, it assigns value to *first,
value + 1 to *(first + 1) and so on."
void iota(_ForwardIter first, _ForwardIter last, _Tp value);The iota function is included in the ISO C++ 2011 standard.
Iterators24.3.2 describes struct iterator, which didn't exist in the
original HP STL implementation (the language wasn't rich enough at the
time). For backwards compatibility, base classes are provided which
declare the same nested typedefs:
input_iteratoroutput_iteratorforward_iteratorbidirectional_iteratorrandom_access_iterator24.3.4 describes iterator operation distance, which takes
two iterators and returns a result. It is extended by another signature
which takes two iterators and a reference to a result. The result is
modified, and the function returns nothing.
Input and Output
Extensions allowing filebufs to be constructed from
"C" types like FILE*s and file descriptors.
Derived filebufsThe v2 library included non-standard extensions to construct
std::filebufs from C stdio types such as
FILE*s and POSIX file descriptors.
Today the recommended way to use stdio types with libstdc++
IOStreams is via the stdio_filebuf class (see below),
but earlier releases provided slightly different mechanisms.
3.0.x filebufs have another ctor with this signature:
basic_filebuf(__c_file_type*, ios_base::openmode, int_type);
This comes in very handy in a number of places, such as
attaching Unix sockets, pipes, and anything else which uses file
descriptors, into the IOStream buffering classes. The three
arguments are as follows:
__c_file_type* F
// the __c_file_type typedef usually boils down to stdio's FILE
ios_base::openmode M
// same as all the other uses of openmode
int_type B
// buffer size, defaults to BUFSIZ if not specified
For those wanting to use file descriptors instead of FILE*'s, I
invite you to contemplate the mysteries of C's fdopen().
In library snapshot 3.0.95 and later, filebufs bring
back an old extension: the fd() member function. The
integer returned from this function can be used for whatever file
descriptors can be used for on your platform. Naturally, the
library cannot track what you do on your own with a file descriptor,
so if you perform any I/O directly, don't expect the library to be
aware of it.
Beginning with 3.1, the extra
basic_filebuf constructor and
the fd() function were removed from the standard
filebuf. Instead,
<ext/stdio_filebuf.h>
contains a derived class template called
__gnu_cxx::stdio_filebuf.
This class can be constructed from a C FILE* or a file
descriptor, and provides the fd() function.
Demangling
Transforming C++ ABI identifiers (like RTTI symbols) into the
original C++ source identifiers is called
demangling.
If you have read the source
documentation for namespace abi then you are
aware of the cross-vendor C++ ABI in use by GCC. One of the
exposed functions is used for demangling,
abi::__cxa_demangle.
In programs like c++filt, the linker, and other tools
have the ability to decode C++ ABI names, and now so can you.
(The function itself might use different demanglers, but that's the
whole point of abstract interfaces. If we change the implementation,
you won't notice.)
Probably the only time you'll be interested in demangling at runtime
is when you're seeing typeid strings in RTTI. For example:
#include <iostream>
#include <cstdlib>
#include <cxxabi.h>
struct empty { };
template <typename T, int N>
struct bar { };
int main()
{
int status;
char *realname;
// typeid
bar<empty,17> u;
const std::type_info &ti = typeid(u);
realname = abi::__cxa_demangle(ti.name(), NULL, NULL, &status);
std::cout << ti.name() << "\t=> " << realname << "\t: " << status << '\n';
std::free(realname);
}
This prints
3barI5emptyLi17EE => bar<empty, 17> : 0
The demangler interface is described in the source documentation
linked to above. It is actually written in C, so you don't need to
be writing C++ in order to demangle C++. (That also means we have to
use crummy memory management facilities, so don't forget to
free() the returned char array.)