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| 4 | <title>scoped_ptr</title> |
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| 8 | <h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle" width="277" height="86"><a name="scoped_ptr">scoped_ptr</a> |
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| 9 | class template</h1> |
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| 10 | <p>The <b>scoped_ptr</b> class template stores a pointer to a dynamically allocated |
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| 11 | object. (Dynamically allocated objects are allocated with the C++ <b>new</b> expression.) |
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| 12 | The object pointed to is guaranteed to be deleted, either on destruction of the <b>scoped_ptr</b>, |
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| 13 | or via an explicit <b>reset</b>. See the <a href="#example">example</a>.</p> |
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| 14 | <p>The <b>scoped_ptr</b> template is a simple solution for simple needs. It |
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| 15 | supplies a basic "resource acquisition is initialization" facility, without |
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| 16 | shared-ownership or transfer-of-ownership semantics. Both its name and |
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| 17 | enforcement of semantics (by being <a href="../utility/utility.htm#Class_noncopyable"> |
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| 18 | noncopyable</a>) signal its intent to retain ownership solely within the |
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| 19 | current scope. Because it is <a href="../utility/utility.htm#Class_noncopyable">noncopyable</a>, |
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| 20 | it is safer than <b>shared_ptr</b> or <b>std::auto_ptr</b> for pointers which |
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| 21 | should not be copied.</p> |
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| 22 | <p>Because <b>scoped_ptr</b> is simple, in its usual implementation every operation |
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| 23 | is as fast as for a built-in pointer and it has no more space overhead that a |
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| 24 | built-in pointer.</p> |
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| 25 | <p><STRONG>scoped_ptr</STRONG> cannot be used in C++ Standard Library containers. |
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| 26 | Use <a href="shared_ptr.htm"><b>shared_ptr</b></a> if you need a smart pointer |
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| 27 | that can.</p> |
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| 28 | <p><STRONG>scoped_ptr</STRONG> cannot correctly hold a pointer to a dynamically |
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| 29 | allocated array. See <a href="scoped_array.htm"><b>scoped_array</b></a> for |
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| 30 | that usage.</p> |
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| 31 | <p>The class template is parameterized on <b>T</b>, the type of the object pointed |
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| 32 | to. <b>T</b> must meet the smart pointer <a href="smart_ptr.htm#common_requirements"> |
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| 33 | common requirements</a>.</p> |
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| 34 | <h2>Synopsis</h2> |
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| 35 | <pre>namespace boost { |
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| 36 | |
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| 37 | template<class T> class scoped_ptr : <a href="../utility/utility.htm#Class_noncopyable">noncopyable</a> { |
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| 38 | |
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| 39 | public: |
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| 40 | typedef T <a href="#element_type">element_type</a>; |
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| 41 | |
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| 42 | explicit <a href="#constructors">scoped_ptr</a>(T * p = 0); // never throws |
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| 43 | <a href="#destructor">~scoped_ptr</a>(); // never throws |
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| 44 | |
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| 45 | void <a href="#reset">reset</a>(T * p = 0); // never throws |
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| 46 | |
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| 47 | T & <a href="#indirection">operator*</a>() const; // never throws |
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| 48 | T * <a href="#indirection">operator-></a>() const; // never throws |
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| 49 | T * <a href="#get">get</a>() const; // never throws |
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| 50 | |
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| 51 | void <a href="#swap">swap</a>(scoped_ptr & b); // never throws |
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| 52 | }; |
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| 53 | |
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| 54 | template<class T> void <a href="#free-swap">swap</a>(scoped_ptr<T> & a, scoped_ptr<T> & b); // never throws |
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| 55 | |
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| 56 | }</pre> |
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| 57 | <h2>Members</h2> |
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| 58 | <h3><a name="element_type">element_type</a></h3> |
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| 59 | <pre>typedef T element_type;</pre> |
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| 60 | <p>Provides the type of the stored pointer.</p> |
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| 61 | <h3><a name="constructors">constructors</a></h3> |
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| 62 | <pre>explicit scoped_ptr(T * p = 0); // never throws</pre> |
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| 63 | <p>Constructs a <b>scoped_ptr</b>, storing a copy of <b>p</b>, which must have been |
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| 64 | allocated via a C++ <b>new</b> expression or be 0. <b>T</b> is not required be |
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| 65 | a complete type. See the smart pointer <a href="smart_ptr.htm#common_requirements">common |
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| 66 | requirements</a>.</p> |
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| 67 | <h3><a name="destructor">destructor</a></h3> |
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| 68 | <pre>~scoped_ptr(); // never throws</pre> |
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| 69 | <p>Destroys the object pointed to by the stored pointer, if any, as if by using <tt>delete |
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| 70 | this->get()</tt>.</p> |
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| 71 | <P> |
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| 72 | The guarantee that this does not throw exceptions depends on the requirement |
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| 73 | that the deleted object's destructor does not throw exceptions. See the smart |
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| 74 | pointer <a href="smart_ptr.htm#common_requirements">common requirements</a>.</P> |
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| 75 | <h3><a name="reset">reset</a></h3> |
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| 76 | <pre>void reset(T * p = 0); // never throws</pre> |
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| 77 | <p> |
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| 78 | Deletes the object pointed to by the stored pointer and then stores a copy of |
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| 79 | p, which must have been allocated via a C++ <b>new</b> expression or be 0. The |
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| 80 | guarantee that this does not throw exceptions depends on the requirement that |
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| 81 | the deleted object's destructor does not throw exceptions. See the smart |
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| 82 | pointer <a href="smart_ptr.htm#common_requirements">common requirements</a>.</p> |
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| 83 | <h3><a name="indirection">indirection</a></h3> |
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| 84 | <pre>T & operator*() const; // never throws</pre> |
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| 85 | <p>Returns a reference to the object pointed to by the stored pointer. Behavior is |
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| 86 | undefined if the stored pointer is 0.</p> |
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| 87 | <pre>T * operator->() const; // never throws</pre> |
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| 88 | <p>Returns the stored pointer. Behavior is undefined if the stored pointer is 0.</p> |
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| 89 | <h3><a name="get">get</a></h3> |
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| 90 | <pre>T * get() const; // never throws</pre> |
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| 91 | <p>Returns the stored pointer. <b>T</b> need not be a complete type. See the smart |
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| 92 | pointer <a href="smart_ptr.htm#common_requirements">common requirements</a>.</p> |
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| 93 | <h3><a name="swap">swap</a></h3> |
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| 94 | <pre>void swap(scoped_ptr & b); // never throws</pre> |
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| 95 | <p>Exchanges the contents of the two smart pointers. <b>T</b> need not be a |
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| 96 | complete type. See the smart pointer <a href="smart_ptr.htm#common_requirements">common |
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| 97 | requirements</a>.</p> |
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| 98 | <h2><a name="functions">Free Functions</a></h2> |
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| 99 | <h3><a name="free-swap">swap</a></h3> |
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| 100 | <pre>template<class T> void swap(scoped_ptr<T> & a, scoped_ptr<T> & b); // never throws</pre> |
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| 101 | <p>Equivalent to <b>a.swap(b)</b>. Matches the interface of <b>std::swap</b>. |
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| 102 | Provided as an aid to generic programming.</p> |
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| 103 | <h2><a name="example">Example</a></h2> |
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| 104 | <p>Here's an example that uses <b>scoped_ptr</b>.</p> |
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| 105 | <blockquote> |
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| 106 | <pre>#include <boost/scoped_ptr.hpp> |
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| 107 | #include <iostream> |
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| 108 | |
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| 109 | struct Shoe { ~Shoe() { std::cout << "Buckle my shoe\n"; } }; |
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| 110 | |
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| 111 | class MyClass { |
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| 112 | boost::scoped_ptr<int> ptr; |
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| 113 | public: |
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| 114 | MyClass() : ptr(new int) { *ptr = 0; } |
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| 115 | int add_one() { return ++*ptr; } |
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| 116 | }; |
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| 117 | |
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| 118 | int main() |
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| 119 | { |
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| 120 | boost::scoped_ptr<Shoe> x(new Shoe); |
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| 121 | MyClass my_instance; |
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| 122 | std::cout << my_instance.add_one() << '\n'; |
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| 123 | std::cout << my_instance.add_one() << '\n'; |
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| 124 | }</pre> |
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| 125 | </blockquote> |
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| 126 | <p>The example program produces the beginning of a child's nursery rhyme:</p> |
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| 127 | <blockquote> |
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| 128 | <pre>1 |
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| 129 | 2 |
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| 130 | Buckle my shoe</pre> |
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| 131 | </blockquote> |
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| 132 | <h2>Rationale</h2> |
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| 133 | <p>The primary reason to use <b>scoped_ptr</b> rather than <b>auto_ptr</b> is to |
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| 134 | let readers of your code know that you intend "resource acquisition is |
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| 135 | initialization" to be applied only for the current scope, and have no intent to |
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| 136 | transfer ownership.</p> |
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| 137 | <p>A secondary reason to use <b>scoped_ptr</b> is to prevent a later maintenance |
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| 138 | programmer from adding a function that transfers ownership by returning the <b>auto_ptr</b>, |
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| 139 | because the maintenance programmer saw <b>auto_ptr</b>, and assumed ownership |
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| 140 | could safely be transferred.</p> |
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| 141 | <p>Think of <b>bool</b> vs <b>int</b>. We all know that under the covers <b>bool</b> |
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| 142 | is usually just an <b>int</b>. Indeed, some argued against including <b>bool</b> |
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| 143 | in the C++ standard because of that. But by coding <b>bool</b> rather than <b>int</b>, |
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| 144 | you tell your readers what your intent is. Same with <b>scoped_ptr</b>; by |
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| 145 | using it you are signaling intent.</p> |
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| 146 | <p>It has been suggested that <b>scoped_ptr<T></b> is equivalent to <b>std::auto_ptr<T> |
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| 147 | const</b>. Ed Brey pointed out, however, that <b>reset</b> will not work on |
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| 148 | a <b>std::auto_ptr<T> const.</b></p> |
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| 149 | <h2><a name="Handle/Body">Handle/Body</a> Idiom</h2> |
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| 150 | <p>One common usage of <b>scoped_ptr</b> is to implement a handle/body (also called |
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| 151 | pimpl) idiom which avoids exposing the body (implementation) in the header |
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| 152 | file.</p> |
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| 153 | <p>The <a href="example/scoped_ptr_example_test.cpp">scoped_ptr_example_test.cpp</a> sample |
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| 154 | program includes a header file, <a href="example/scoped_ptr_example.hpp">scoped_ptr_example.hpp</a>, |
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| 155 | which uses a <b>scoped_ptr<></b> to an incomplete type to hide the |
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| 156 | implementation. The instantiation of member functions which require a complete |
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| 157 | type occurs in the <a href="example/scoped_ptr_example.cpp">scoped_ptr_example.cpp</a> |
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| 158 | implementation file.</p> |
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| 159 | <h2>Frequently Asked Questions</h2> |
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| 160 | <p><b>Q</b>. Why doesn't <b>scoped_ptr</b> have a release() member?<br> |
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| 161 | <b>A</b>. When reading source code, it is valuable to be able to draw |
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| 162 | conclusions about program behavior based on the types being used. If <STRONG>scoped_ptr</STRONG> |
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| 163 | had a release() member, it would become possible to transfer ownership of the |
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| 164 | held pointer, weakening its role as a way of limiting resource lifetime to a |
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| 165 | given context. Use <STRONG>std::auto_ptr</STRONG> where transfer of ownership |
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| 166 | is required. (supplied by Dave Abrahams)</p> |
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| 167 | <hr> |
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| 168 | <p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B %Y" startspan --> |
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| 169 | 09 January 2003<!--webbot bot="Timestamp" endspan i-checksum="32310" --></p> |
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| 170 | <p>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler. |
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| 171 | Copyright 2002 Peter Dimov. Permission to copy, use, modify, sell and |
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| 172 | distribute this document is granted provided this copyright notice appears in |
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| 173 | all copies. This document is provided "as is" without express or implied |
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| 174 | warranty, and with no claim as to its suitability for any purpose.</p> |
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| 175 | </body> |
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| 176 | </html> |
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