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| <H2>19.2 Declaration and Initialization of Autopointers</H2> |
| <A NAME="idx431"><!></A> |
| <P>You attach an <B><I><A HREF="../stdlibref/auto-ptr.html">auto_ptr</A></I></B> object to a pointer either by using one of the constructors for <B><I>auto_ptr</I></B>, by assigning one <B><I>auto_ptr</I></B> object to another, or by using the reset member function. Only one <B><I>auto_ptr</I></B> owns a particular pointer at any one time, except for the null pointer, which all <B><I>auto_ptr</I></B>s own by default. Any use of the <B><I>auto_ptr</I></B> copy constructor or assignment operator transfers ownership from one <B><I>auto_ptr</I></B> object to another. For instance, suppose we create an <B><I>auto_ptr</I></B> <SAMP>a</SAMP> like this:</P> |
| |
| <UL><PRE> |
| std::auto_ptr<std::string> a(new std::string); |
| </PRE></UL> |
| <P>The <B><I><A HREF="../stdlibref/auto-ptr.html">auto_ptr</A></I></B> object <SAMP>a</SAMP> now owns the newly created pointer. When <SAMP>a</SAMP> is destroyed, such as when it goes out of scope, the pointer is deleted. But if we initialize <SAMP>b</SAMP> with the value of <SAMP>a</SAMP>:</P> |
| |
| <UL><PRE> |
| std::auto_ptr<std::string> b = a; |
| </PRE></UL> |
| <P><SAMP>b</SAMP> now owns the pointer. Use of the copy constructor or the assignment operator causes <SAMP>a</SAMP> to release ownership of the pointer. Now if <SAMP>a</SAMP> goes out of scope the pointer is not affected. However, the pointer <I>is</I> deleted when <SAMP>b</SAMP> goes out of scope.</P> |
| <BLOCKQUOTE><HR><B> |
| NOTE -- The standard container templates cannot be specialized on <B><I><A HREF="../stdlibref/auto-ptr.html">auto_ptr</A></I></B> objects. Copying or assigning from an <B><I>auto_ptr</I></B> object transfers ownership of the contained pointer, which changes the source object. Therefore, <B><I>auto_ptr</I></B> objects do not meet the Assignable and CopyConstructible requirements of standard containers. See <A HREF="4.html">Chapter 4</A> for container requirements. |
| </B><HR></BLOCKQUOTE> |
| <P>The use of <SAMP>new</SAMP> within the constructor for <SAMP>a</SAMP> may seem a little odd. Normally we avoid constructs like this since it puts the responsibility for deletion on a different entity than the one responsible for allocation. In this case, however, the sole responsibility of the <B><I><A HREF="../stdlibref/auto-ptr.html">auto_ptr</A></I></B> is to manage the deletion. This syntax is actually preferable since it prevents us from accidentally deleting the pointer ourselves. or initializing another <B><I>auto_ptr</I></B> object with it.</P> |
| <P>Use <SAMP>operator*()</SAMP>, <SAMP>operator->()</SAMP> or the member function <SAMP>get()</SAMP> to access the pointer held by an <B><I><A HREF="../stdlibref/auto-ptr.html">auto_ptr</A></I></B>. For instance, we can use any of the three following statements to assign <SAMP>"What's up Doc"</SAMP> to the string now pointed to by the <B><I>auto_ptr</I></B> <SAMP>b</SAMP>:</P> |
| |
| <UL><PRE> |
| *b = "What's up Doc"; |
| *(b.get()) = "What's up Doc"; |
| b->assign("What's up Doc"); |
| </PRE></UL> |
| <P>Class <B><I><A HREF="../stdlibref/auto-ptr.html">auto_ptr</A></I></B> also provides a release member function that releases ownership of a pointer. Any <B><I>auto_ptr</I></B> that does not own a specific pointer is assumed to contain the null pointer, so calling release on an <B><I>auto_ptr</I></B> will set the contained pointer to zero. In the example above, when <SAMP>a</SAMP> is copied to <SAMP>b</SAMP>, the pointer held by <SAMP>a</SAMP> is released and set to the null pointer.</P> |
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