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<h1><A href="../../index.htm"><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle" width="277" height="86"
border="0"></A>scoped_ptr class template</h1>
<p>The <b>scoped_ptr</b> class template stores a pointer to a dynamically allocated
object. (Dynamically allocated objects are allocated with the C++ <b>new</b> expression.)
The object pointed to is guaranteed to be deleted, either on destruction of the <b>scoped_ptr</b>,
or via an explicit <b>reset</b>. See the <a href="#example">example</a>.</p>
<p>The <b>scoped_ptr</b> template is a simple solution for simple needs. It
supplies a basic "resource acquisition is initialization" facility, without
shared-ownership or transfer-of-ownership semantics. Both its name and
enforcement of semantics (by being <a href="../utility/utility.htm#Class_noncopyable">
noncopyable</a>) signal its intent to retain ownership solely within the
current scope. Because it is <a href="../utility/utility.htm#Class_noncopyable">noncopyable</a>,
it is safer than <b>shared_ptr</b> or <b>std::auto_ptr</b> for pointers which
should not be copied.</p>
<p>Because <b>scoped_ptr</b> is simple, in its usual implementation every operation
is as fast as for a built-in pointer and it has no more space overhead that a
built-in pointer.</p>
<p><STRONG>scoped_ptr</STRONG> cannot be used in C++ Standard Library containers.
Use <a href="shared_ptr.htm"><b>shared_ptr</b></a> if you need a smart pointer
that can.</p>
<p><STRONG>scoped_ptr</STRONG> cannot correctly hold a pointer to a dynamically
allocated array. See <a href="scoped_array.htm"><b>scoped_array</b></a> for
that usage.</p>
<p>The class template is parameterized on <b>T</b>, the type of the object pointed
to. <b>T</b> must meet the smart pointer <a href="smart_ptr.htm#common_requirements">
common requirements</a>.</p>
<h2>Synopsis</h2>
<pre>namespace boost {
template<class T> class scoped_ptr : <a href="../utility/utility.htm#Class_noncopyable">noncopyable</a> {
public:
typedef T <a href="#element_type">element_type</a>;
explicit <a href="#constructors">scoped_ptr</a>(T * p = 0); // never throws
<a href="#destructor">~scoped_ptr</a>(); // never throws
void <a href="#reset">reset</a>(T * p = 0); // never throws
T & <a href="#indirection">operator*</a>() const; // never throws
T * <a href="#indirection">operator-></a>() const; // never throws
T * <a href="#get">get</a>() const; // never throws
operator <A href="#conversions" ><i>unspecified-bool-type</i></A>() const; // never throws
void <a href="#swap">swap</a>(scoped_ptr & b); // never throws
};
template<class T> void <a href="#free-swap">swap</a>(scoped_ptr<T> & a, scoped_ptr<T> & b); // never throws
}</pre>
<h2>Members</h2>
<h3><a name="element_type">element_type</a></h3>
<pre>typedef T element_type;</pre>
<p>Provides the type of the stored pointer.</p>
<h3><a name="constructors">constructors</a></h3>
<pre>explicit scoped_ptr(T * p = 0); // never throws</pre>
<p>Constructs a <b>scoped_ptr</b>, storing a copy of <b>p</b>, which must have been
allocated via a C++ <b>new</b> expression or be 0. <b>T</b> is not required be
a complete type. See the smart pointer <a href="smart_ptr.htm#common_requirements">common
requirements</a>.</p>
<h3><a name="destructor">destructor</a></h3>
<pre>~scoped_ptr(); // never throws</pre>
<p>Destroys the object pointed to by the stored pointer, if any, as if by using <tt>delete
this->get()</tt>.</p>
<P>
The guarantee that this does not throw exceptions depends on the requirement
that the deleted object's destructor does not throw exceptions. See the smart
pointer <a href="smart_ptr.htm#common_requirements">common requirements</a>.</P>
<h3><a name="reset">reset</a></h3>
<pre>void reset(T * p = 0); // never throws</pre>
<p>
Deletes the object pointed to by the stored pointer and then stores a copy of
p, which must have been allocated via a C++ <b>new</b> expression or be 0. The
guarantee that this does not throw exceptions depends on the requirement that
the deleted object's destructor does not throw exceptions. See the smart
pointer <a href="smart_ptr.htm#common_requirements">common requirements</a>.</p>
<h3><a name="indirection">indirection</a></h3>
<pre>T & operator*() const; // never throws</pre>
<p>Returns a reference to the object pointed to by the stored pointer. Behavior is
undefined if the stored pointer is 0.</p>
<pre>T * operator->() const; // never throws</pre>
<p>Returns the stored pointer. Behavior is undefined if the stored pointer is 0.</p>
<h3><a name="get">get</a></h3>
<pre>T * get() const; // never throws</pre>
<p>Returns the stored pointer. <b>T</b> need not be a complete type. See the smart
pointer <a href="smart_ptr.htm#common_requirements">common requirements</a>.</p>
<h3><a name="conversions">conversions</a></h3>
<pre>operator <i>unspecified-bool-type</i> () const; // never throws</pre>
<p>Returns an unspecified value that, when used in boolean contexts, is equivalent
to <code>get() != 0</code>.</p>
<h3><a name="swap">swap</a></h3>
<pre>void swap(scoped_ptr & b); // never throws</pre>
<p>Exchanges the contents of the two smart pointers. <b>T</b> need not be a
complete type. See the smart pointer <a href="smart_ptr.htm#common_requirements">common
requirements</a>.</p>
<h2><a name="functions">Free Functions</a></h2>
<h3><a name="free-swap">swap</a></h3>
<pre>template<class T> void swap(scoped_ptr<T> & a, scoped_ptr<T> & b); // never throws</pre>
<p>Equivalent to <b>a.swap(b)</b>. Matches the interface of <b>std::swap</b>.
Provided as an aid to generic programming.</p>
<h2><a name="example">Example</a></h2>
<p>Here's an example that uses <b>scoped_ptr</b>.</p>
<blockquote>
<pre>#include <boost/scoped_ptr.hpp>
#include <iostream>
struct Shoe { ~Shoe() { std::cout << "Buckle my shoe\n"; } };
class MyClass {
boost::scoped_ptr<int> ptr;
public:
MyClass() : ptr(new int) { *ptr = 0; }
int add_one() { return ++*ptr; }
};
int main()
{
boost::scoped_ptr<Shoe> x(new Shoe);
MyClass my_instance;
std::cout << my_instance.add_one() << '\n';
std::cout << my_instance.add_one() << '\n';
}</pre>
</blockquote>
<p>The example program produces the beginning of a child's nursery rhyme:</p>
<blockquote>
<pre>1
2
Buckle my shoe</pre>
</blockquote>
<h2>Rationale</h2>
<p>The primary reason to use <b>scoped_ptr</b> rather than <b>auto_ptr</b> is to
let readers of your code know that you intend "resource acquisition is
initialization" to be applied only for the current scope, and have no intent to
transfer ownership.</p>
<p>A secondary reason to use <b>scoped_ptr</b> is to prevent a later maintenance
programmer from adding a function that transfers ownership by returning the <b>auto_ptr</b>,
because the maintenance programmer saw <b>auto_ptr</b>, and assumed ownership
could safely be transferred.</p>
<p>Think of <b>bool</b> vs <b>int</b>. We all know that under the covers <b>bool</b>
is usually just an <b>int</b>. Indeed, some argued against including <b>bool</b>
in the C++ standard because of that. But by coding <b>bool</b> rather than <b>int</b>,
you tell your readers what your intent is. Same with <b>scoped_ptr</b>; by
using it you are signaling intent.</p>
<p>It has been suggested that <b>scoped_ptr<T></b> is equivalent to <b>std::auto_ptr<T>
const</b>. Ed Brey pointed out, however, that <b>reset</b> will not work on
a <b>std::auto_ptr<T> const.</b></p>
<h2><a name="Handle/Body">Handle/Body</a> Idiom</h2>
<p>One common usage of <b>scoped_ptr</b> is to implement a handle/body (also called
pimpl) idiom which avoids exposing the body (implementation) in the header
file.</p>
<p>The <a href="example/scoped_ptr_example_test.cpp">scoped_ptr_example_test.cpp</a>
sample program includes a header file, <a href="example/scoped_ptr_example.hpp">scoped_ptr_example.hpp</a>,
which uses a <b>scoped_ptr<></b> to an incomplete type to hide the
implementation. The instantiation of member functions which require a complete
type occurs in the <a href="example/scoped_ptr_example.cpp">scoped_ptr_example.cpp</a>
implementation file.</p>
<h2>Frequently Asked Questions</h2>
<p><b>Q</b>. Why doesn't <b>scoped_ptr</b> have a release() member?<br>
<b>A</b>. When reading source code, it is valuable to be able to draw
conclusions about program behavior based on the types being used. If <STRONG>scoped_ptr</STRONG>
had a release() member, it would become possible to transfer ownership of the
held pointer, weakening its role as a way of limiting resource lifetime to a
given context. Use <STRONG>std::auto_ptr</STRONG> where transfer of ownership
is required. (supplied by Dave Abrahams)</p>
<hr>
<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B %Y" startspan -->
09 January 2003<!--webbot bot="Timestamp" endspan i-checksum="32310" --></p>
<p><small>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
Copyright 2002-2005 Peter Dimov. Distributed under the Boost Software License, Version
1.0. See accompanying file <A href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</A> or
copy at <A href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>.</small></p>
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