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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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