1<section xmlns="http://docbook.org/ns/docbook" version="5.0" 2 xml:id="std.util.memory.auto_ptr" xreflabel="auto_ptr"> 3<?dbhtml filename="auto_ptr.html"?> 4 5<info><title>auto_ptr</title> 6 <keywordset> 7 <keyword>ISO C++</keyword> 8 <keyword>auto_ptr</keyword> 9 </keywordset> 10</info> 11 12 13 14<section xml:id="auto_ptr.limitations"><info><title>Limitations</title></info> 15 16 17 <para>Explaining all of the fun and delicious things that can 18 happen with misuse of the <classname>auto_ptr</classname> class 19 template (called <acronym>AP</acronym> here) would take some 20 time. Suffice it to say that the use of <acronym>AP</acronym> 21 safely in the presence of copying has some subtleties. 22 </para> 23 <para> 24 The AP class is a really 25 nifty idea for a smart pointer, but it is one of the dumbest of 26 all the smart pointers -- and that's fine. 27 </para> 28 <para> 29 AP is not meant to be a supersmart solution to all resource 30 leaks everywhere. Neither is it meant to be an effective form 31 of garbage collection (although it can help, a little bit). 32 And it can <emphasis>not</emphasis>be used for arrays! 33 </para> 34 <para> 35 <acronym>AP</acronym> is meant to prevent nasty leaks in the 36 presence of exceptions. That's <emphasis>all</emphasis>. This 37 code is AP-friendly: 38 </para> 39 <programlisting> 40 // Not a recommend naming scheme, but good for web-based FAQs. 41 typedef std::auto_ptr<MyClass> APMC; 42 43 extern function_taking_MyClass_pointer (MyClass*); 44 extern some_throwable_function (); 45 46 void func (int data) 47 { 48 APMC ap (new MyClass(data)); 49 50 some_throwable_function(); // this will throw an exception 51 52 function_taking_MyClass_pointer (ap.get()); 53 } 54 </programlisting> 55 <para>When an exception gets thrown, the instance of MyClass that's 56 been created on the heap will be <function>delete</function>'d as the stack is 57 unwound past <function>func()</function>. 58 </para> 59 <para>Changing that code as follows is not <acronym>AP</acronym>-friendly: 60 </para> 61 <programlisting> 62 APMC ap (new MyClass[22]); 63 </programlisting> 64 <para>You will get the same problems as you would without the use 65 of <acronym>AP</acronym>: 66 </para> 67 <programlisting> 68 char* array = new char[10]; // array new... 69 ... 70 delete array; // ...but single-object delete 71 </programlisting> 72 <para> 73 AP cannot tell whether the pointer you've passed at creation points 74 to one or many things. If it points to many things, you are about 75 to die. AP is trivial to write, however, so you could write your 76 own <code>auto_array_ptr</code> for that situation (in fact, this has 77 been done many times; check the mailing lists, Usenet, Boost, etc). 78 </para> 79</section> 80 81<section xml:id="auto_ptr.using"><info><title>Use in Containers</title></info> 82 83 84 <para> 85 </para> 86 <para>All of the <link linkend="std.containers">containers</link> 87 described in the standard library require their contained types 88 to have, among other things, a copy constructor like this: 89 </para> 90 <programlisting> 91 struct My_Type 92 { 93 My_Type (My_Type const&); 94 }; 95 </programlisting> 96 <para> 97 Note the const keyword; the object being copied shouldn't change. 98 The template class <code>auto_ptr</code> (called AP here) does not 99 meet this requirement. Creating a new AP by copying an existing 100 one transfers ownership of the pointed-to object, which means that 101 the AP being copied must change, which in turn means that the 102 copy ctors of AP do not take const objects. 103 </para> 104 <para> 105 The resulting rule is simple: <emphasis>Never ever use a 106 container of auto_ptr objects</emphasis>. The standard says that 107 <quote>undefined</quote> behavior is the result, but it is 108 guaranteed to be messy. 109 </para> 110 <para> 111 To prevent you from doing this to yourself, the 112 <link linkend="manual.ext.compile_checks">concept checks</link> built 113 in to this implementation will issue an error if you try to 114 compile code like this: 115 </para> 116 <programlisting> 117 #include <vector> 118 #include <memory> 119 120 void f() 121 { 122 std::vector< std::auto_ptr<int> > vec_ap_int; 123 } 124 </programlisting> 125 <para> 126Should you try this with the checks enabled, you will see an error. 127 </para> 128</section> 129 130</section> 131