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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 4. Support</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="std_contents.html" title="Part II. Standard Contents" /><link rel="prev" href="std_contents.html" title="Part II. Standard Contents" /><link rel="next" href="dynamic_memory.html" title="Dynamic Memory" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 4.
Support
</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="std_contents.html">Prev</a> </td><th width="60%" align="center">Part II.
Standard Contents
</th><td width="20%" align="right"> <a accesskey="n" href="dynamic_memory.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a id="std.support"></a>Chapter 4.
Support
<a id="id-1.3.4.2.1.1.1" class="indexterm"></a>
</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="section"><a href="support.html#std.support.types">Types</a></span></dt><dd><dl><dt><span class="section"><a href="support.html#std.support.types.fundamental">Fundamental Types</a></span></dt><dt><span class="section"><a href="support.html#std.support.types.numeric_limits">Numeric Properties</a></span></dt><dt><span class="section"><a href="support.html#std.support.types.null">NULL</a></span></dt></dl></dd><dt><span class="section"><a href="dynamic_memory.html">Dynamic Memory</a></span></dt><dd><dl><dt><span class="section"><a href="dynamic_memory.html#std.support.memory.notes">Additional Notes</a></span></dt></dl></dd><dt><span class="section"><a href="termination.html">Termination</a></span></dt><dd><dl><dt><span class="section"><a href="termination.html#support.termination.handlers">Termination Handlers</a></span></dt><dt><span class="section"><a href="termination.html#support.termination.verbose">Verbose Terminate Handler</a></span></dt></dl></dd></dl></div><p>
This part deals with the functions called and objects created
automatically during the course of a program's existence.
</p><p>
While we can't reproduce the contents of the Standard here (you
need to get your own copy from your nation's member body; see our
homepage for help), we can mention a couple of changes in what
kind of support a C++ program gets from the Standard Library.
</p><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.support.types"></a>Types</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.support.types.fundamental"></a>Fundamental Types</h3></div></div></div><p>
C++ has the following builtin types:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
char
</p></li><li class="listitem"><p>
signed char
</p></li><li class="listitem"><p>
unsigned char
</p></li><li class="listitem"><p>
signed short
</p></li><li class="listitem"><p>
signed int
</p></li><li class="listitem"><p>
signed long
</p></li><li class="listitem"><p>
unsigned short
</p></li><li class="listitem"><p>
unsigned int
</p></li><li class="listitem"><p>
unsigned long
</p></li><li class="listitem"><p>
bool
</p></li><li class="listitem"><p>
wchar_t
</p></li><li class="listitem"><p>
float
</p></li><li class="listitem"><p>
double
</p></li><li class="listitem"><p>
long double
</p></li></ul></div><p>
These fundamental types are always available, without having to
include a header file. These types are exactly the same in
either C++ or in C.
</p><p>
Specializing parts of the library on these types is prohibited:
instead, use a POD.
</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.support.types.numeric_limits"></a>Numeric Properties</h3></div></div></div><p>
The header <code class="filename"><limits></code> defines
traits classes to give access to various implementation
defined-aspects of the fundamental types. The traits classes --
fourteen in total -- are all specializations of the class template
<code class="classname">numeric_limits</code>
and defined as follows:
</p><pre class="programlisting">
template<typename T>
struct class
{
static const bool is_specialized;
static T max() throw();
static T min() throw();
static const int digits;
static const int digits10;
static const bool is_signed;
static const bool is_integer;
static const bool is_exact;
static const int radix;
static T epsilon() throw();
static T round_error() throw();
static const int min_exponent;
static const int min_exponent10;
static const int max_exponent;
static const int max_exponent10;
static const bool has_infinity;
static const bool has_quiet_NaN;
static const bool has_signaling_NaN;
static const float_denorm_style has_denorm;
static const bool has_denorm_loss;
static T infinity() throw();
static T quiet_NaN() throw();
static T denorm_min() throw();
static const bool is_iec559;
static const bool is_bounded;
static const bool is_modulo;
static const bool traps;
static const bool tinyness_before;
static const float_round_style round_style;
};
</pre></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.support.types.null"></a>NULL</h3></div></div></div><p>
The only change that might affect people is the type of
<code class="constant">NULL</code>: while it is required to be a macro,
the definition of that macro is <span class="emphasis"><em>not</em></span> allowed
to be an expression with pointer type such as
<code class="constant">(void*)0</code>, which is often used in C.
</p><p>
For <span class="command"><strong>g++</strong></span>, <code class="constant">NULL</code> is
<code class="code">#define</code>'d to be
<code class="constant">__null</code>, a magic keyword extension of
<span class="command"><strong>g++</strong></span> that is slightly safer than a plain integer.
</p><p>
The biggest problem of #defining <code class="constant">NULL</code> to be
something like <span class="quote">“<span class="quote">0L</span>”</span> is that the compiler will view
that as a long integer before it views it as a pointer, so
overloading won't do what you expect. It might not even have the
same size as a pointer, so passing <code class="constant">NULL</code> to a
varargs function where a pointer is expected might not even work
correctly if <code class="code">sizeof(NULL) < sizeof(void*)</code>.
The G++ <code class="constant">__null</code> extension is defined so that
<code class="code">sizeof(__null) == sizeof(void*)</code> to avoid this problem.
</p><p>
Scott Meyers explains this in more detail in his book
<a class="link" href="https://www.aristeia.com/books.html" target="_top"><span class="emphasis"><em>Effective
Modern C++</em></span></a> and as a guideline to solve this problem
recommends to not overload on pointer-vs-integer types to begin with.
</p><p>
The C++ 2011 standard added the <code class="constant">nullptr</code> keyword,
which is a null pointer constant of a special type,
<code class="classname">std::nullptr_t</code>. Values of this type can be
implicitly converted to <span class="emphasis"><em>any</em></span> pointer type,
and cannot convert to integer types or be deduced as an integer type.
Unless you need to be compatible with C++98/C++03 or C you should prefer
to use <code class="constant">nullptr</code> instead of <code class="constant">NULL</code>.
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