include/c++/ext/rc_string_base.h File Reference

#include <ext/atomicity.h>
#include <bits/stl_iterator_base_funcs.h>
#include <stdexcept>

Include dependency graph for rc_string_base.h:

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Functions
namespace __gnu_cxx	_GLIBCXX_VISIBILITY (default)

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Detailed Description

This is an internal header file, included by other library headers. Do not attempt to use it directly. {ext/vstring.h}

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Function Documentation

namespace __gnu_cxx _GLIBCXX_VISIBILITY

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Documentation? What's that? Nathan Myers <ncm@cantrip.org>.

A string looks like this:

                                        [_Rep]
                                        _M_length
   [__rc_string_base<char_type>]        _M_capacity
   _M_dataplus                          _M_refcount
   _M_p ---------------->               unnamed array of char_type

Where the _M_p points to the first character in the string, and you cast it to a pointer-to-_Rep and subtract 1 to get a pointer to the header.

This approach has the enormous advantage that a string object requires only one allocation. All the ugliness is confined within a single pair of inline functions, which each compile to a single add instruction: _Rep::_M_refdata(), and __rc_string_base::_M_rep(); and the allocation function which gets a block of raw bytes and with room enough and constructs a _Rep object at the front.

The reason you want _M_data pointing to the character array and not the _Rep is so that the debugger can see the string contents. (Probably we should add a non-inline member to get the _Rep for the debugger to use, so users can check the actual string length.)

Note that the _Rep object is a POD so that you can have a static empty string _Rep object already constructed before static constructors have run. The reference-count encoding is chosen so that a 0 indicates one reference, so you never try to destroy the empty-string _Rep object.

All but the last paragraph is considered pretty conventional for a C++ string implementation.