1 <?xml version="1.0" encoding="UTF-8" standalone="no"?> 2 <!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>Chapter4. 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="PartII. Standard Contents" /><link rel="prev" href="std_contents.html" title="PartII. 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">Chapter4. 3 Support 4 5 </th></tr><tr><td width="20%" align="left"><a accesskey="p" href="std_contents.html">Prev</a></td><th width="60%" align="center">PartII. 6 Standard Contents 7 </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>Chapter4. 8 Support 9 <a id="id-1.3.4.2.1.1.1" class="indexterm"></a> 10 </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> 11 This part deals with the functions called and objects created 12 automatically during the course of a program's existence. 13 </p><p> 14 While we can't reproduce the contents of the Standard here (you 15 need to get your own copy from your nation's member body; see our 16 homepage for help), we can mention a couple of changes in what 17 kind of support a C++ program gets from the Standard Library. 18 </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> 19 C++ has the following builtin types: 20 </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> 21 char 22 </p></li><li class="listitem"><p> 23 signed char 24 </p></li><li class="listitem"><p> 25 unsigned char 26 </p></li><li class="listitem"><p> 27 signed short 28 </p></li><li class="listitem"><p> 29 signed int 30 </p></li><li class="listitem"><p> 31 signed long 32 </p></li><li class="listitem"><p> 33 unsigned short 34 </p></li><li class="listitem"><p> 35 unsigned int 36 </p></li><li class="listitem"><p> 37 unsigned long 38 </p></li><li class="listitem"><p> 39 bool 40 </p></li><li class="listitem"><p> 41 wchar_t 42 </p></li><li class="listitem"><p> 43 float 44 </p></li><li class="listitem"><p> 45 double 46 </p></li><li class="listitem"><p> 47 long double 48 </p></li></ul></div><p> 49 These fundamental types are always available, without having to 50 include a header file. These types are exactly the same in 51 either C++ or in C. 52 </p><p> 53 Specializing parts of the library on these types is prohibited: 54 instead, use a POD. 55 </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> 56 The header <code class="filename"><limits></code> defines 57 traits classes to give access to various implementation 58 defined-aspects of the fundamental types. The traits classes -- 59 fourteen in total -- are all specializations of the class template 60 <code class="classname">numeric_limits</code> 61 and defined as follows: 62 </p><pre class="programlisting"> 63 template<typename T> 64 struct class 65 { 66 static const bool is_specialized; 67 static T max() throw(); 68 static T min() throw(); 69 70 static const int digits; 71 static const int digits10; 72 static const bool is_signed; 73 static const bool is_integer; 74 static const bool is_exact; 75 static const int radix; 76 static T epsilon() throw(); 77 static T round_error() throw(); 78 79 static const int min_exponent; 80 static const int min_exponent10; 81 static const int max_exponent; 82 static const int max_exponent10; 83 84 static const bool has_infinity; 85 static const bool has_quiet_NaN; 86 static const bool has_signaling_NaN; 87 static const float_denorm_style has_denorm; 88 static const bool has_denorm_loss; 89 static T infinity() throw(); 90 static T quiet_NaN() throw(); 91 static T denorm_min() throw(); 92 93 static const bool is_iec559; 94 static const bool is_bounded; 95 static const bool is_modulo; 96 97 static const bool traps; 98 static const bool tinyness_before; 99 static const float_round_style round_style; 100 }; 101 </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> 102 The only change that might affect people is the type of 103 <code class="constant">NULL</code>: while it is required to be a macro, 104 the definition of that macro is <span class="emphasis"><em>not</em></span> allowed 105 to be an expression with pointer type such as 106 <code class="constant">(void*)0</code>, which is often used in C. 107 </p><p> 108 For <span class="command"><strong>g++</strong></span>, <code class="constant">NULL</code> is 109 <code class="code">#define</code>'d to be 110 <code class="constant">__null</code>, a magic keyword extension of 111 <span class="command"><strong>g++</strong></span> that is slightly safer than a plain integer. 112 </p><p> 113 The biggest problem of #defining <code class="constant">NULL</code> to be 114 something like <span class="quote"><span class="quote">0L</span></span> is that the compiler will view 115 that as a long integer before it views it as a pointer, so 116 overloading won't do what you expect. It might not even have the 117 same size as a pointer, so passing <code class="constant">NULL</code> to a 118 varargs function where a pointer is expected might not even work 119 correctly if <code class="code">sizeof(NULL) < sizeof(void*)</code>. 120 The G++ <code class="constant">__null</code> extension is defined so that 121 <code class="code">sizeof(__null) == sizeof(void*)</code> to avoid this problem. 122 </p><p> 123 Scott Meyers explains this in more detail in his book 124 <a class="link" href="https://www.aristeia.com/books.html" target="_top"><span class="emphasis"><em>Effective 125 Modern C++</em></span></a> and as a guideline to solve this problem 126 recommends to not overload on pointer-vs-integer types to begin with. 127 </p><p> 128 The C++ 2011 standard added the <code class="constant">nullptr</code> keyword, 129 which is a null pointer constant of a special type, 130 <code class="classname">std::nullptr_t</code>. Values of this type can be 131 implicitly converted to <span class="emphasis"><em>any</em></span> pointer type, 132 and cannot convert to integer types or be deduced as an integer type. 133 Unless you need to be compatible with C++98/C++03 or C you should prefer 134 to use <code class="constant">nullptr</code> instead of <code class="constant">NULL</code>. 135 </p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="std_contents.html">Prev</a></td><td width="20%" align="center"><a accesskey="u" href="std_contents.html">Up</a></td><td width="40%" align="right"><a accesskey="n" href="dynamic_memory.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">PartII. 136 Standard Contents 137 </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top">Dynamic Memory</td></tr></table></div></body></html>
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