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| 4 | <title>Smart Pointer Changes</title> |
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| 8 | <h1><A href="../../index.htm"><img src="../../boost.png" alt="boost.png (6897 bytes)" align="middle" width="277" height="86" |
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| 9 | border="0"></A>Smart Pointer Changes</h1> |
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| 10 | <p>The February 2002 change to the Boost smart pointers introduced a number of |
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| 11 | changes. Since the previous version of the smart pointers was in use for a long |
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| 12 | time, it's useful to have a detailed list of what changed from a library user's |
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| 13 | point of view.</p> |
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| 14 | <p>Note that for compilers that don't support member templates well enough, a |
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| 15 | separate implementation is used that lacks many of the new features and is more |
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| 16 | like the old version.</p> |
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| 17 | <h2>Features Requiring Code Changes to Take Advantage</h2> |
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| 18 | <ul> |
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| 19 | <li> |
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| 20 | The smart pointer class templates now each have their own header file. For |
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| 21 | compatibility, the <a href="../../boost/smart_ptr.hpp"><boost/smart_ptr.hpp></a> |
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| 22 | header now includes the headers for the four classic smart pointer class |
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| 23 | templates. |
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| 24 | <li> |
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| 25 | The <b>weak_ptr</b> |
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| 26 | template was added. |
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| 27 | <li> |
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| 28 | The new <b>shared_ptr</b> and <b>shared_array</b> relax the requirement that |
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| 29 | the pointed-to object's destructor must be visible when instantiating the <b>shared_ptr</b> |
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| 30 | destructor. This makes it easier to have shared_ptr members in classes without |
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| 31 | explicit destructors. |
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| 32 | <li> |
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| 33 | A custom deallocator can be passed in when creating a <b>shared_ptr</b> or <b>shared_array</b>. |
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| 34 | <li> |
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| 35 | <b>shared_static_cast</b> and <b>shared_dynamic_cast</b> function templates are |
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| 36 | provided which work for <b>shared_ptr</b> and <b>weak_ptr</b> as <b>static_cast</b> |
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| 37 | and <b>dynamic_cast</b> |
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| 38 | do for pointers. |
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| 39 | <li> |
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| 40 | The self-assignment misfeature has been removed from <b>shared_ptr::reset</b>, |
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| 41 | although it is still present in <b>scoped_ptr</b>, and in <b>std::auto_ptr</b>. |
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| 42 | Calling <b>reset</b> with a pointer to the object that's already owned by the <b>shared_ptr</b> |
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| 43 | results in undefined behavior (an assertion, or eventually a double-delete if |
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| 44 | assertions are off). |
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| 45 | <li> |
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| 46 | The <b>BOOST_SMART_PTR_CONVERSION</b> |
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| 47 | feature has been removed. |
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| 48 | <li> |
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| 49 | <b>shared_ptr<void></b> is now allowed.</li> |
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| 50 | </ul> |
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| 51 | <h2>Features That Improve Robustness</h2> |
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| 52 | <ul> |
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| 53 | <li> |
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| 54 | The manipulation of use counts is now <a name="threadsafe">thread safe</a> on |
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| 55 | Windows, Linux, and platforms that support pthreads. See the <a href="../../boost/detail/atomic_count.hpp"> |
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| 56 | <boost/detail/atomic_count.hpp></a> |
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| 57 | file for details |
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| 58 | <li> |
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| 59 | The new shared_ptr will always delete the object using the pointer it was |
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| 60 | originally constructed with. This prevents subtle problems that could happen if |
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| 61 | the last <b>shared_ptr</b> was a pointer to a sub-object of a class that did |
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| 62 | not have a virtual destructor.</li> |
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| 63 | </ul> |
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| 64 | <h2>Implementation Details</h2> |
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| 65 | <ul> |
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| 66 | <li> |
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| 67 | Some bugs in the assignment operator implementations and in <b>reset</b> |
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| 68 | have been fixed by using the "copy and swap" idiom. |
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| 69 | <li> |
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| 70 | Assertions have been added to check preconditions of various functions; |
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| 71 | however, since these use the new <a href="../../boost/assert.hpp"><boost/assert.hpp></a> |
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| 72 | header, the assertions are disabled by default. |
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| 73 | <li> |
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| 74 | The partial specialization of <b>std::less</b> has been replaced by <b>operator<</b> |
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| 75 | overloads which accomplish the same thing without relying on undefined |
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| 76 | behavior. |
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| 77 | <li> |
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| 78 | The incorrect overload of <b>std::swap</b> has been replaced by <b>boost::swap</b>, |
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| 79 | which has many of the same advantages for generic programming but does not |
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| 80 | violate the C++ standard.</li> |
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| 81 | </ul> |
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| 82 | <hr> |
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| 83 | <p>Revised 1 February 2002</p> |
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| 84 | <p>Copyright 2002 Darin Adler. Permission to copy, use, modify, sell and distribute |
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| 85 | this document is granted provided this copyright notice appears in all copies. |
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| 86 | This document is provided "as is" without express or implied warranty, and with |
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| 87 | no claim as to its suitability for any purpose.</p> |
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| 88 | </body> |
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| 89 | </html> |
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