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<HTML><HEAD><TITLE>19.2 Declaration and Initialization of Auto Pointers</TITLE></HEAD><BODY><A HREF="ug1.htm"><IMG SRC="images/banner.gif"></A><BR><A HREF="ove_3674.htm"><IMG SRC="images/prev.gif"></A><A HREF="booktoc1.htm"><IMG SRC="images/toc.gif"></A><A HREF="tindex1.htm"><IMG SRC="images/tindex.gif"></A><A HREF="exa_5786.htm"><IMG SRC="images/next.gif"></A><BR><STRONG>Click on the banner to return to the user guide home page.</STRONG><H2>19.2 Declaration and Initialization of Auto Pointers</H2><P>You attach an <A HREF="../stdlibcr/aut_3512.htm"><B><I>auto_ptr</I></B></A> 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 <B><I>auto_ptr</I></B>'s copy constructor or assignment operator transfers ownership from one <B><I>auto_ptr</I></B> object to another. For instance, suppose we create <B><I>auto_ptr</I></B> <SAMP>a</SAMP> like this:</P><PRE>auto_ptr<string> a(new string);</PRE><P>The <A HREF="../stdlibcr/aut_3512.htm"><B><I>auto_ptr</I></B></A> 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 will be deleted. But, if we assign <SAMP>a</SAMP> to <SAMP>b</SAMP>, using the assignment operator:</P><PRE>auto_ptr<string> b = a;</PRE><P><SAMP>b</SAMP> now owns the pointer. Use of the assignment operator causes <SAMP>a</SAMP> to release ownership of the pointer. Now if <SAMP>a</SAMP> goes out of scope the pointer will not be affected. However, the pointer <I>will</I> be deleted when <SAMP>b</SAMP> goes out of scope.</P><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. But in this case, the <A HREF="../stdlibcr/aut_3512.htm"><B><I>auto_ptr</I></B></A>'s sole responsibility is to manage the deletion. This syntax is actually preferable since it prevents us from accidentally deleting the pointer ourselves. </P><P>Use <SAMP>operator*</SAMP>, <SAMP>operator-></SAMP>, or the member function<SAMP> get()</SAMP> to access the pointer held by an <A HREF="../stdlibcr/aut_3512.htm"><B><I>auto_ptr</I></B></A>. For instance, we can use any of the three following statements to assign "What's up Doc" to the string now pointed to by the <B><I>auto_ptr</I></B> <SAMP>b</SAMP>.</P><PRE>*b = "What's up Doc";*(b.get()) = "What's up Doc"; b->assign("What's up Doc");</PRE><P><A HREF="../stdlibcr/aut_3512.htm"><B><I>auto_ptr </I></B></A>also provides a release member function that releases ownership of a pointer. Any <A HREF="../stdlibcr/aut_3512.htm"><B><I>auto_ptr</I></B></A> that does not own a specific pointer is assumed to point to the NULL pointer, so calling release on an <B><I>auto_ptr </I></B>will set it to the NULL pointer. In the example above, when <SAMP>a</SAMP> is assigned to <SAMP>b</SAMP>, the pointer held by <SAMP>a</SAMP> is released and <SAMP>a</SAMP> is set to the NULL pointer.</P><HR><A HREF="ove_3674.htm"><IMG SRC="images/prev.gif"></A> <A HREF="booktoc1.htm"><IMG SRC="images/toc.gif"></A><A HREF="tindex1.htm"><IMG SRC="images/tindex.gif"></A><A HREF="exa_5786.htm"><IMG SRC="images/next.gif"></A><P>©Copyright 1996, Rogue Wave Software, Inc.</P></BODY></HTML>?? 快捷鍵說明
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