# Extensions

Extensions
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:

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++ may not recognize these names, or treat them differently, or...

2. 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 Extensions

### Backwards Compatibility

A 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`.

## Algorithms

25.1.6 (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?

## Numerics

26.4, 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.

## Iterators

24.3.2 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_iterator

- output_iterator

- forward_iterator

- bidirectional_iterator

- random_access_iterator

24.3.4 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 `filebuf`s to be constructed from "C" types like FILE*s and file descriptors.

### Derived filebufs

The v2 library included non-standard extensions to construct `std::filebuf`s 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 `filebuf`s 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, `filebuf`s 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.)