| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> |
| <html> |
| <head> |
| <title>shared_ptr</title> |
| <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> |
| </head> |
| <body text="#000000" bgcolor="#ffffff" link="#0000ff" vlink="#0000ff"> |
| <h1><img height="86" alt="boost.png (6897 bytes)" src="../../boost.png" |
| width="277" align="middle" border="0">shared_ptr class template</h1> |
| <p><a href="#Introduction">Introduction</a><br> |
| <a href="#BestPractices">Best Practices</a><br> |
| <a href="#Synopsis">Synopsis</a><br> |
| <a href="#Members">Members</a><br> |
| <a href="#functions">Free Functions</a><br> |
| <a href="#example">Example</a><br> |
| <a href="#HandleBody">Handle/Body Idiom</a><br> |
| <a href="#ThreadSafety">Thread Safety</a><br> |
| <a href="#FAQ">Frequently Asked Questions</a><br> |
| <a href="smarttests.htm">Smart Pointer Timings</a><br> |
| <a href="sp_techniques.html">Programming Techniques</a></p> |
| <h2 id="Introduction">Introduction</h2> |
| <p>The <code>shared_ptr</code> class template stores a pointer to a dynamically allocated |
| object, typically with a C++ <em>new-expression</em>. The object pointed to is |
| guaranteed to be deleted when the last <code>shared_ptr</code> pointing to it is |
| destroyed or reset.</p> |
| <blockquote><em>Example:</em><br><pre>shared_ptr<X> p1( new X ); |
| shared_ptr<void> p2( new int(5) ); |
| </pre></blockquote> |
| |
| <p><code>shared_ptr</code> deletes the exact pointer that has been passed at construction time, |
| complete with its original type, regardless of the template parameter. In the second example above, |
| when <code>p2</code> is destroyed or reset, it will call <code>delete</code> on the original <code>int*</code> |
| that has been passed to the constructor, even though <code>p2</code> itself is of type |
| <code>shared_ptr<void></code> and stores a pointer of type <code>void*</code>.</p> |
| |
| <p>Every <code>shared_ptr</code> meets the <code>CopyConstructible</code>, <code>MoveConstructible</code>, |
| <code>CopyAssignable</code> and <code>MoveAssignable</code> |
| requirements of the C++ Standard Library, and can be used in standard |
| library containers. Comparison operators are supplied so that <code>shared_ptr</code> |
| works with the standard library's associative containers.</p> |
| <p>Because the implementation uses reference counting, cycles of <code>shared_ptr</code> instances |
| will not be reclaimed. For example, if <code>main()</code> holds a <code>shared_ptr</code> to |
| <code>A</code>, which directly or indirectly holds a <code>shared_ptr</code> back to <code>A</code>, |
| <code>A</code>'s use count will be 2. Destruction of the original <code>shared_ptr</code> will |
| leave <code>A</code> dangling with a use count of 1. Use <a href="weak_ptr.htm">weak_ptr</a> |
| to "break cycles."</p> |
| <p>The class template is parameterized on <code>T</code>, the type of the object pointed |
| to. <code>shared_ptr</code> and most of its member functions place no |
| requirements on <code>T</code>; it is allowed to be an incomplete type, or |
| <code>void</code>. Member functions that do place additional requirements |
| (<a href="#pointer_constructor">constructors</a>, <a href="#reset">reset</a>) are explicitly |
| documented below.</p> |
| <p><code>shared_ptr<T></code> can be implicitly converted to <code>shared_ptr<U></code> |
| whenever <code>T*</code> can be implicitly converted to <code>U*</code>. |
| In particular, <code>shared_ptr<T></code> is implicitly convertible |
| to <code>shared_ptr<T const></code>, to <code>shared_ptr<U></code> |
| where <code>U</code> is an accessible base of <code>T</code>, and to <code> |
| shared_ptr<void></code>.</p> |
| <p><code>shared_ptr</code> is now part of the C++11 Standard, as <code>std::shared_ptr</code>.</p> |
| <p>Starting with Boost release 1.53, <code>shared_ptr</code> can be used to hold a pointer to a dynamically |
| allocated array. This is accomplished by using an array type (<code>T[]</code> or <code>T[N]</code>) as |
| the template parameter. There is almost no difference between using an unsized array, <code>T[]</code>, |
| and a sized array, <code>T[N]</code>; the latter just enables <code>operator[]</code> to perform a range check |
| on the index.</p> |
| <blockquote><em>Example:</em><br><pre>shared_ptr<double[1024]> p1( new double[1024] ); |
| shared_ptr<double[]> p2( new double[n] ); |
| </pre></blockquote> |
| |
| <h2 id="BestPractices">Best Practices</h2> |
| <p>A simple guideline that nearly eliminates the possibility of memory leaks is: |
| always use a named smart pointer variable to hold the result of <code>new</code>. |
| Every occurence of the <code>new</code> keyword in the code should have the |
| form:</p> |
| <pre>shared_ptr<T> p(new Y);</pre> |
| <p>It is, of course, acceptable to use another smart pointer in place of <code>shared_ptr</code> |
| above; having <code>T</code> and <code>Y</code> be the same type, or |
| passing arguments to <code>Y</code>'s constructor is also OK.</p> |
| <p>If you observe this guideline, it naturally follows that you will have no |
| explicit <code>delete</code> statements; <code>try/catch</code> constructs will |
| be rare.</p> |
| <p>Avoid using unnamed <code>shared_ptr</code> temporaries to save typing; to |
| see why this is dangerous, consider this example:</p> |
| <pre>void f(shared_ptr<int>, int); |
| int g(); |
| |
| void ok() |
| { |
| shared_ptr<int> p( new int(2) ); |
| f( p, g() ); |
| } |
| |
| void bad() |
| { |
| f( shared_ptr<int>( new int(2) ), g() ); |
| } |
| </pre> |
| <p>The function <code>ok</code> follows the guideline to the letter, whereas |
| <code>bad</code> constructs the temporary <code>shared_ptr</code> in place, |
| admitting the possibility of a memory leak. Since function arguments are |
| evaluated in unspecified order, it is possible for <code>new int(2)</code> to |
| be evaluated first, <code>g()</code> second, and we may never get to the |
| <code>shared_ptr</code>constructor if <code>g</code> throws an exception. |
| See <a href="http://www.gotw.ca/gotw/056.htm">Herb Sutter's treatment</a> (also <a href="http://www.cuj.com/reference/articles/2002/0212/0212_sutter.htm"> |
| here</a>) of the issue for more information.</p> |
| <p>The exception safety problem described above may also be eliminated by using |
| the <a href="make_shared.html"><code>make_shared</code></a> |
| or <a href="make_shared.html"><code>allocate_shared</code></a> |
| factory functions defined in <code>boost/make_shared.hpp</code>. |
| These factory functions also provide an efficiency benefit by consolidating allocations.</p> |
| <h2 id="Synopsis">Synopsis</h2> |
| <pre>namespace boost { |
| |
| class bad_weak_ptr: public std::exception; |
| |
| template<class T> class <a href="weak_ptr.htm" >weak_ptr</a>; |
| |
| template<class T> class shared_ptr { |
| |
| public: |
| |
| typedef <em>see below</em> <a href="#element_type" >element_type</a>; |
| |
| <a href="#default_constructor" >shared_ptr</a>(); // never throws |
| <a href="#default_constructor" >shared_ptr</a>(std::nullptr_t); // never throws |
| |
| template<class Y> explicit <a href="#pointer_constructor" >shared_ptr</a>(Y * p); |
| template<class Y, class D> <a href="#deleter_constructor" >shared_ptr</a>(Y * p, D d); |
| template<class Y, class D, class A> <a href="#deleter_constructor" >shared_ptr</a>(Y * p, D d, A a); |
| template<class D> <a href="#deleter_constructor" >shared_ptr</a>(std::nullptr_t p, D d); |
| template<class D, class A> <a href="#deleter_constructor" >shared_ptr</a>(std::nullptr_t p, D d, A a); |
| |
| <a href="#destructor" >~shared_ptr</a>(); // never throws |
| |
| <a href="#copy_constructor" >shared_ptr</a>(shared_ptr const & r); // never throws |
| template<class Y> <a href="#copy_constructor" >shared_ptr</a>(shared_ptr<Y> const & r); // never throws |
| |
| <a href="#move_constructor" >shared_ptr</a>(shared_ptr && r); // never throws |
| template<class Y> <a href="#move_constructor" >shared_ptr</a>(shared_ptr<Y> && r); // never throws |
| |
| template<class Y> <a href="#aliasing_constructor" >shared_ptr</a>(shared_ptr<Y> const & r, element_type * p); // never throws |
| |
| template<class Y> explicit <a href="#weak_ptr_constructor" >shared_ptr</a>(<a href="weak_ptr.htm" >weak_ptr</a><Y> const & r); |
| |
| template<class Y> explicit <a href="#auto_ptr_constructor" >shared_ptr</a>(std::auto_ptr<Y> & r); |
| template<class Y> <a href="#auto_ptr_constructor" >shared_ptr</a>(std::auto_ptr<Y> && r); |
| |
| template<class Y, class D> <a href="#unique_ptr_constructor" >shared_ptr</a>(std::unique_ptr<Y, D> && r); |
| |
| shared_ptr & <a href="#assignment" >operator=</a>(shared_ptr const & r); // never throws |
| template<class Y> shared_ptr & <a href="#assignment" >operator=</a>(shared_ptr<Y> const & r); // never throws |
| |
| shared_ptr & <a href="#assignment" >operator=</a>(shared_ptr const && r); // never throws |
| template<class Y> shared_ptr & <a href="#assignment" >operator=</a>(shared_ptr<Y> const && r); // never throws |
| |
| template<class Y> shared_ptr & <a href="#assignment" >operator=</a>(std::auto_ptr<Y> & r); |
| template<class Y> shared_ptr & <a href="#assignment" >operator=</a>(std::auto_ptr<Y> && r); |
| |
| template<class Y, class D> shared_ptr & <a href="#assignment" >operator=</a>(std::unique_ptr<Y, D> && r); |
| |
| shared_ptr & <a href="#assignment" >operator=</a>(std::nullptr_t); // never throws |
| |
| void <a href="#reset" >reset</a>(); // never throws |
| |
| template<class Y> void <a href="#reset" >reset</a>(Y * p); |
| template<class Y, class D> void <a href="#reset" >reset</a>(Y * p, D d); |
| template<class Y, class D, class A> void <a href="#reset" >reset</a>(Y * p, D d, A a); |
| |
| template<class Y> void <a href="#reset" >reset</a>(shared_ptr<Y> const & r, element_type * p); // never throws |
| |
| T & <a href="#indirection" >operator*</a>() const; // never throws; only valid when T is not an array type |
| T * <a href="#indirection" >operator-></a>() const; // never throws; only valid when T is not an array type |
| |
| element_type & <a href="#indirection" >operator[]</a>(std::ptrdiff_t i) const; // never throws; only valid when T is an array type |
| |
| element_type * <a href="#get" >get</a>() const; // never throws |
| |
| bool <a href="#unique" >unique</a>() const; // never throws |
| long <a href="#use_count" >use_count</a>() const; // never throws |
| |
| explicit <a href="#conversions" >operator bool</a>() const; // never throws |
| |
| void <a href="#swap" >swap</a>(shared_ptr & b); // never throws |
| |
| template<class Y> bool <a href="#owner_before" >owner_before</a>(shared_ptr<Y> const & rhs) const; // never throws |
| template<class Y> bool <a href="#owner_before" >owner_before</a>(weak_ptr<Y> const & rhs) const; // never throws |
| }; |
| |
| template<class T, class U> |
| bool <a href="#comparison" >operator==</a>(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws |
| |
| template<class T, class U> |
| bool <a href="#comparison" >operator!=</a>(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws |
| |
| template<class T, class U> |
| bool <a href="#comparison" >operator<</a>(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws |
| |
| template<class T> |
| bool <a href="#comparison" >operator==</a>(shared_ptr<T> const & p, std::nullptr_t); // never throws |
| |
| template<class T> |
| bool <a href="#comparison" >operator==</a>(std::nullptr_t, shared_ptr<T> const & p); // never throws |
| |
| template<class T> |
| bool <a href="#comparison" >operator!=</a>(shared_ptr<T> const & p, std::nullptr_t); // never throws |
| |
| template<class T> |
| bool <a href="#comparison" >operator!=</a>(std::nullptr_t, shared_ptr<T> const & p); // never throws |
| |
| template<class T> void <a href="#free-swap" >swap</a>(shared_ptr<T> & a, shared_ptr<T> & b); // never throws |
| |
| template<class T> typename shared_ptr<T>::element_type * <a href="#get_pointer" >get_pointer</a>(shared_ptr<T> const & p); // never throws |
| |
| template<class T, class U> |
| shared_ptr<T> <a href="#static_pointer_cast" >static_pointer_cast</a>(shared_ptr<U> const & r); // never throws |
| |
| template<class T, class U> |
| shared_ptr<T> <a href="#const_pointer_cast" >const_pointer_cast</a>(shared_ptr<U> const & r); // never throws |
| |
| template<class T, class U> |
| shared_ptr<T> <a href="#dynamic_pointer_cast" >dynamic_pointer_cast</a>(shared_ptr<U> const & r); // never throws |
| |
| template<class T, class U> |
| shared_ptr<T> <a href="#reinterpret_pointer_cast" >reinterpet_pointer_cast</a>(shared_ptr<U> const & r); // never throws |
| |
| template<class E, class T, class Y> |
| std::basic_ostream<E, T> & <a href="#insertion-operator" >operator<<</a> (std::basic_ostream<E, T> & os, shared_ptr<Y> const & p); |
| |
| template<class D, class T> |
| D * <a href="#get_deleter">get_deleter</a>(shared_ptr<T> const & p); |
| }</pre> |
| <h2 id="Members">Members</h2> |
| <h3 id="element_type">element_type</h3> |
| <pre>typedef <em>...</em> element_type;</pre> |
| <blockquote> |
| <p><code>element_type</code> is <code>T</code> when <code>T</code> is not an array type, |
| and <code>U</code> when <code>T</code> is <code>U[]</code> or <code>U[N]</code>.</p> |
| </blockquote> |
| <h3 id="default_constructor">default constructor</h3> |
| <pre>shared_ptr(); // never throws |
| shared_ptr(std::nullptr_t); // never throws</pre> |
| <blockquote> |
| <p><b>Effects:</b> Constructs an <em>empty</em> <code>shared_ptr</code>.</p> |
| <p><b>Postconditions:</b> <code>use_count() == 0 && get() == 0</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <p><em>[The nothrow guarantee is important, since <code>reset()</code> is specified |
| in terms of the default constructor; this implies that the constructor must not |
| allocate memory.]</em></p> |
| <h3 id="pointer_constructor">pointer constructor</h3> |
| <pre>template<class Y> explicit shared_ptr(Y * p);</pre> |
| <blockquote> |
| <p><b>Requirements:</b> |
| <code>Y</code> must be a complete type. |
| The expression <code>delete[] p</code>, when <code>T</code> is an array type, or <code>delete p</code>, |
| when <code>T</code> is not an array type, |
| must be well-formed, must not invoke undefined behavior, and must not throw exceptions. |
| When <code>T</code> is <code>U[N]</code>, <code>Y (*) [N]</code> must be convertible to <code>T*</code>; |
| when <code>T</code> is <code>U[]</code>, <code>Y (*) []</code> must be convertible to <code>T*</code>; |
| otherwise, <code>Y*</code> must be convertible to <code>T*</code>. |
| </p> |
| <p><b>Effects:</b> |
| When <code>T</code> is not an array type, constructs a <code>shared_ptr</code> that <em>owns</em> |
| the pointer <code>p</code>. |
| Otherwise, constructs a <code>shared_ptr</code> that <em>owns</em> |
| <code>p</code> and a deleter of an unspecified type that calls <code>delete[] p</code>.</p> |
| <p><b>Postconditions:</b> <code>use_count() == 1 && get() == p</code>. |
| If <code>T</code> is not an array type and <code>p</code> is unambiguously convertible to <code> |
| <a href="enable_shared_from_this.html">enable_shared_from_this</a><V>*</code> |
| for some <code>V</code>, <code>p->shared_from_this()</code> returns a copy of |
| <code>*this</code>.</p> |
| <p><b>Throws:</b> <code>std::bad_alloc</code>, or an implementation-defined |
| exception when a resource other than memory could not be obtained.</p> |
| <p><b>Exception safety:</b> If an exception is thrown, the constructor calls |
| <code>delete[] p</code>, when <code>T</code> is an array type, |
| or <code>delete p</code>, when <code>T</code> is not an array type.</p> |
| <p><b>Notes:</b> <code>p</code> must be a pointer to an object that was |
| allocated via a C++ <code>new</code> expression or be 0. The postcondition that <a href="#use_count"> |
| use count</a> is 1 holds even if <code>p</code> is 0; invoking <code>delete</code> |
| on a pointer that has a value of 0 is harmless.</p> |
| </blockquote> |
| <p><em>[This constructor is a template in order to remember the actual |
| pointer type passed. The destructor will call <code>delete</code> with the |
| same pointer, complete with its original type, even when <code>T</code> does |
| not have a virtual destructor, or is <code>void</code>.]</em></p> |
| <h3 id="deleter_constructor">constructors taking a deleter</h3> |
| <pre>template<class Y, class D> shared_ptr(Y * p, D d); |
| template<class Y, class D, class A> shared_ptr(Y * p, D d, A a); |
| template<class D> shared_ptr(std::nullptr_t p, D d); |
| template<class D, class A> shared_ptr(std::nullptr_t p, D d, A a);</pre> |
| <blockquote> |
| <p><b>Requirements:</b> |
| <code>D</code> must be <code>CopyConstructible</code>. The copy constructor and destructor |
| of <code>D</code> must not throw. The expression <code>d(p)</code> must be |
| well-formed, must not invoke undefined behavior, and must not throw exceptions. |
| <code>A</code> must be an <em>Allocator</em>, as described in section 20.1.5 |
| (<code>Allocator requirements</code>) of the C++ Standard. |
| When <code>T</code> is <code>U[N]</code>, <code>Y (*) [N]</code> must be convertible to <code>T*</code>; |
| when <code>T</code> is <code>U[]</code>, <code>Y (*) []</code> must be convertible to <code>T*</code>; |
| otherwise, <code>Y*</code> must be convertible to <code>T*</code>. |
| </p> |
| <p><b>Effects:</b> Constructs a <code>shared_ptr</code> that <em>owns</em> the pointer <code> |
| p</code> and the deleter <code>d</code>. The constructors taking an allocator <code>a</code> |
| allocate memory using a copy of <code>a</code>.</p> |
| <p><b>Postconditions:</b> <code>use_count() == 1 && get() == p</code>. |
| If <code>T</code> is not an array type and <code>p</code> is unambiguously convertible to <code> |
| <a href="enable_shared_from_this.html">enable_shared_from_this</a><V>*</code> |
| for some <code>V</code>, <code>p->shared_from_this()</code> returns a copy of |
| <code>*this</code>.</p> |
| <p><b>Throws:</b> <code>std::bad_alloc</code>, or an implementation-defined |
| exception when a resource other than memory could not be obtained.</p> |
| <p><b>Exception safety:</b> If an exception is thrown, <code>d(p)</code> is called.</p> |
| <p><b>Notes:</b> When the the time comes to delete the object pointed to by <code>p</code>, |
| the stored copy of <code>d</code> is invoked with the stored copy of <code>p</code> |
| as an argument.</p> |
| </blockquote> |
| <p><em>[Custom deallocators allow a factory function returning a <code>shared_ptr</code> |
| to insulate the user from its memory allocation strategy. Since the deallocator |
| is not part of the type, changing the allocation strategy does not break source |
| or binary compatibility, and does not require a client recompilation. For |
| example, a "no-op" deallocator is useful when returning a <code>shared_ptr</code> |
| to a statically allocated object, and other variations allow a <code>shared_ptr</code> |
| to be used as a wrapper for another smart pointer, easing interoperability.</em></p> |
| <p><em>The support for custom deallocators does not impose significant overhead. Other <code> |
| shared_ptr</code> features still require a deallocator to be kept.</em></p> |
| <p><em>The requirement that the copy constructor of <code>D</code> does not throw comes from |
| the pass by value. If the copy constructor throws, the pointer would leak.]</em></p> |
| <h3 id="copy_constructor">copy and converting constructors</h3> |
| <pre>shared_ptr(shared_ptr const & r); // never throws |
| template<class Y> shared_ptr(shared_ptr<Y> const & r); // never throws</pre> |
| <blockquote> |
| <p><b>Requires:</b> <code>Y*</code> should be convertible to <code>T*</code>.</p> |
| <p><b>Effects:</b> If <code>r</code> is <em>empty</em>, constructs an <em>empty</em> <code>shared_ptr</code>; |
| otherwise, constructs a <code>shared_ptr</code> that <em>shares ownership</em> with <code>r</code>.</p> |
| <p><b>Postconditions:</b> <code>get() == r.get() && use_count() == |
| r.use_count()</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h3 id="move_constructor">move constructors</h3> |
| <pre>shared_ptr(shared_ptr && r); // never throws |
| template<class Y> shared_ptr(shared_ptr<Y> && r); // never throws</pre> |
| <blockquote> |
| <p><b>Requires:</b> <code>Y*</code> should be convertible to <code>T*</code>.</p> |
| <p><b>Effects:</b> Move-constructs a <code>shared_ptr</code> from <code>r</code>.</p> |
| <p><b>Postconditions:</b> <code>*this</code> contains the old value of <code>r</code>. <code>r</code> is <em>empty</em> and <code>r.get() == 0</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h3 id="aliasing_constructor">aliasing constructor</h3> |
| <pre>template<class Y> shared_ptr(shared_ptr<Y> const & r, element_type * p); // never throws</pre> |
| <blockquote> |
| <p><b>Effects:</b> constructs a <code>shared_ptr</code> that <em>shares ownership</em> with |
| <code>r</code> and stores <code>p</code>.</p> |
| <p><b>Postconditions:</b> <code>get() == p && use_count() == r.use_count()</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h3 id="weak_ptr_constructor">weak_ptr constructor</h3> |
| <pre>template<class Y> explicit shared_ptr(<a href="weak_ptr.htm" >weak_ptr</a><Y> const & r);</pre> |
| <blockquote> |
| <p><b>Requires:</b> <code>Y*</code> should be convertible to <code>T*</code>.</p> |
| <p><b>Effects:</b> Constructs a <code>shared_ptr</code> that <em>shares ownership</em> with |
| <code>r</code> and stores a copy of the pointer stored in <code>r</code>.</p> |
| <p><b>Postconditions:</b> <code>use_count() == r.use_count()</code>.</p> |
| <p><b>Throws:</b> <code>bad_weak_ptr</code> when <code>r.use_count() == 0</code>.</p> |
| <p><b>Exception safety:</b> If an exception is thrown, the constructor has no |
| effect.</p> |
| </blockquote> |
| <h3 id="auto_ptr_constructor">auto_ptr constructors</h3> |
| <pre>template<class Y> shared_ptr(std::auto_ptr<Y> & r); |
| template<class Y> shared_ptr(std::auto_ptr<Y> && r);</pre> |
| <blockquote> |
| <p><b>Requires:</b> <code>Y*</code> should be convertible to <code>T*</code>.</p> |
| <p><b>Effects:</b> Constructs a <code>shared_ptr</code>, as if by storing a copy of <code>r.release()</code>.</p> |
| <p><b>Postconditions:</b> <code>use_count() == 1</code>.</p> |
| <p><b>Throws:</b> <code>std::bad_alloc</code>, or an implementation-defined |
| exception when a resource other than memory could not be obtained.</p> |
| <p><b>Exception safety:</b> If an exception is thrown, the constructor has no |
| effect.</p> |
| </blockquote> |
| <h3 id="unique_ptr_constructor">unique_ptr constructor</h3> |
| <pre>template<class Y, class D> shared_ptr(std::unique_ptr<Y, D> && r);</pre> |
| <blockquote> |
| <p><b>Requires:</b> <code>Y*</code> should be convertible to <code>T*</code>.</p> |
| <p><b>Effects:</b> |
| Equivalent to <code>shared_ptr(r.release(), r.get_deleter())</code> when <code>D</code> is not a reference type. |
| Otherwise, equivalent to <code>shared_ptr(r.release(), <em>del</em>)</code>, where <em>del</em> is a deleter |
| that stores the reference <code>rd</code> returned from <code>r.get_deleter()</code> and <code>del(p)</code> calls <code>rd(p)</code>.</p> |
| <p><b>Postconditions:</b> <code>use_count() == 1</code>.</p> |
| <p><b>Throws:</b> <code>std::bad_alloc</code>, or an implementation-defined |
| exception when a resource other than memory could not be obtained.</p> |
| <p><b>Exception safety:</b> If an exception is thrown, the constructor has no |
| effect.</p> |
| </blockquote> |
| <h3 id="destructor">destructor</h3> |
| <pre>~shared_ptr(); // never throws</pre> |
| <blockquote> |
| <p><b>Effects:</b></p> |
| <ul> |
| <li> |
| If <code>*this</code> is <em>empty</em>, or <em>shares ownership</em> with |
| another <code>shared_ptr</code> instance (<code>use_count() > 1</code>), |
| there are no side effects.</li> |
| <li> |
| Otherwise, if <code>*this</code> <em>owns</em> a pointer <code>p</code> |
| and a deleter <code>d</code>, <code>d(p)</code> |
| is called.</li> |
| <li> |
| Otherwise, <code>*this</code> <em>owns</em> a pointer <code>p</code>, |
| and <code>delete p</code> is called.</li> |
| </ul> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h3 id="assignment">assignment</h3> |
| <pre>shared_ptr & operator=(shared_ptr const & r); // never throws |
| template<class Y> shared_ptr & operator=(shared_ptr<Y> const & r); // never throws |
| template<class Y> shared_ptr & operator=(std::auto_ptr<Y> & r);</pre> |
| <blockquote> |
| <p><b>Effects:</b> Equivalent to <code>shared_ptr(r).swap(*this)</code>.</p> |
| <p><b>Returns:</b> <code>*this</code>.</p> |
| <p><b>Notes:</b> The use count updates caused by the temporary object construction |
| and destruction are not considered observable side effects, and the |
| implementation is free to meet the effects (and the implied guarantees) via |
| different means, without creating a temporary. In particular, in the example:</p> |
| <pre>shared_ptr<int> p(new int); |
| shared_ptr<void> q(p); |
| p = p; |
| q = p; |
| </pre> |
| <p>both assignments may be no-ops.</p> |
| </blockquote> |
| <pre>shared_ptr & operator=(shared_ptr && r); // never throws |
| template<class Y> shared_ptr & operator=(shared_ptr<Y> && r); // never throws |
| template<class Y> shared_ptr & operator=(std::auto_ptr<Y> && r); |
| template<class Y, class D> shared_ptr & operator=(std::unique_ptr<Y, D> && r);</pre> |
| <blockquote> |
| <p><b>Effects:</b> Equivalent to <code>shared_ptr(std::move(r)).swap(*this)</code>.</p> |
| <p><b>Returns:</b> <code>*this</code>.</p> |
| </blockquote> |
| <pre>shared_ptr & operator=(std::nullptr_t); // never throws</pre> |
| <blockquote> |
| <p><b>Effects:</b> Equivalent to <code>shared_ptr().swap(*this)</code>.</p> |
| <p><b>Returns:</b> <code>*this</code>.</p> |
| </blockquote> |
| <h3 id="reset">reset</h3> |
| <pre>void reset(); // never throws</pre> |
| <blockquote> |
| <p><b>Effects:</b> Equivalent to <code>shared_ptr().swap(*this)</code>.</p> |
| </blockquote> |
| <pre>template<class Y> void reset(Y * p);</pre> |
| <blockquote> |
| <p><b>Effects:</b> Equivalent to <code>shared_ptr(p).swap(*this)</code>.</p> |
| </blockquote> |
| <pre>template<class Y, class D> void reset(Y * p, D d);</pre> |
| <blockquote> |
| <p><b>Effects:</b> Equivalent to <code>shared_ptr(p, d).swap(*this)</code>.</p> |
| </blockquote> |
| <pre>template<class Y, class D, class A> void reset(Y * p, D d, A a);</pre> |
| <blockquote> |
| <p><b>Effects:</b> Equivalent to <code>shared_ptr(p, d, a).swap(*this)</code>.</p> |
| </blockquote> |
| <pre>template<class Y> void reset(shared_ptr<Y> const & r, element_type * p); // never throws</pre> |
| <blockquote> |
| <p><b>Effects:</b> Equivalent to <code>shared_ptr(r, p).swap(*this)</code>.</p> |
| </blockquote> |
| <h3 id="indirection">indirection</h3> |
| <pre>T & operator*() const; // never throws</pre> |
| <blockquote> |
| <p><b>Requirements:</b> <code>T</code> should not be an array type. The stored pointer must not be 0.</p> |
| <p><b>Returns:</b> a reference to the object pointed to by the stored pointer.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <pre>T * operator->() const; // never throws</pre> |
| <blockquote> |
| <p><b>Requirements:</b> <code>T</code> should not be an array type. The stored pointer must not be 0.</p> |
| <p><b>Returns:</b> the stored pointer.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <pre>element_type & operator[](std::ptrdiff_t i) const; // never throws</pre> |
| <blockquote> |
| <p><b>Requirements:</b> <code>T</code> should be an array type. The stored pointer must not be 0. |
| <code>i >= 0</code>. If <code>T</code> is <code>U[N]</code>, <code>i < N</code>.</p> |
| <p><b>Returns:</b> <code>get()[i]</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h3 id="get">get</h3> |
| <pre>element_type * get() const; // never throws</pre> |
| <blockquote> |
| <p><b>Returns:</b> the stored pointer.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h3 id="unique">unique</h3> |
| <pre>bool unique() const; // never throws</pre> |
| <blockquote> |
| <p><b>Returns:</b> <code>use_count() == 1</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| <p><b>Notes:</b> <code>unique()</code> may be faster than <code>use_count()</code>. |
| If you are using <code>unique()</code> to implement copy on write, do not rely |
| on a specific value when the stored pointer is zero.</p> |
| </blockquote> |
| <h3 id="use_count">use_count</h3> |
| <pre>long use_count() const; // never throws</pre> |
| <blockquote> |
| <p><b>Returns:</b> the number of <code>shared_ptr</code> objects, <code>*this</code> included, |
| that <i>share ownership</i> with <code>*this</code>, or 0 when <code>*this</code> |
| is <em>empty</em>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| <p><b>Notes:</b> <code>use_count()</code> is not necessarily efficient. Use only |
| for debugging and testing purposes, not for production code.</p> |
| </blockquote> |
| <h3 id="conversions">conversions</h3> |
| <pre>explicit operator bool() const; // never throws</pre> |
| <blockquote> |
| <p><b>Returns:</b> <code>get() != 0</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| <p><b>Notes:</b> This conversion operator allows <code>shared_ptr</code> objects to be |
| used in boolean contexts, like <code>if(p && p->valid()) {}</code>.</p> |
| </blockquote> |
| <p><em>[The conversion to bool is not merely syntactic sugar. It allows <code>shared_ptr</code>s |
| to be declared in conditions when using <a href="#dynamic_pointer_cast">dynamic_pointer_cast</a> |
| or <a href="weak_ptr.htm#lock">weak_ptr::lock</a>.]</em></p> |
| <h3 id="swap">swap</h3> |
| <pre>void swap(shared_ptr & b); // never throws</pre> |
| <blockquote> |
| <p><b>Effects:</b> Exchanges the contents of the two smart pointers.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h3 id="owner_before">swap</h3> |
| <pre>template<class Y> bool owner_before(shared_ptr<Y> const & rhs) const; // never throws |
| template<class Y> bool owner_before(weak_ptr<Y> const & rhs) const; // never throws</pre> |
| <blockquote> |
| <p><b>Effects:</b> See the description of <a href="#comparison"><code>operator<</code></a>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h2 id="functions">Free Functions</h2> |
| <h3 id="comparison">comparison</h3> |
| <pre>template<class T, class U> |
| bool operator==(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws</pre> |
| <blockquote> |
| <p><b>Returns:</b> <code>a.get() == b.get()</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <pre>template<class T, class U> |
| bool operator!=(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws</pre> |
| <blockquote> |
| <p><b>Returns:</b> <code>a.get() != b.get()</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <pre>template<class T> |
| bool operator==(shared_ptr<T> const & p, std::nullptr_t); // never throws |
| template<class T> |
| bool operator==(std::nullptr_t, shared_ptr<T> const & p); // never throws</pre> |
| <blockquote> |
| <p><b>Returns:</b> <code>p.get() == 0</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <pre>template<class T> |
| bool operator!=(shared_ptr<T> const & p, std::nullptr_t); // never throws |
| template<class T> |
| bool operator!=(std::nullptr_t, shared_ptr<T> const & p); // never throws</pre> |
| <blockquote> |
| <p><b>Returns:</b> <code>p.get() != 0</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <pre>template<class T, class U> |
| bool operator<(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws</pre> |
| <blockquote> |
| <p><b>Returns:</b> an unspecified value such that</p> |
| <ul> |
| <li> |
| <code>operator<</code> is a strict weak ordering as described in section 25.3 <code>[lib.alg.sorting]</code> |
| of the C++ standard;</li> |
| <li> |
| under the equivalence relation defined by <code>operator<</code>, <code>!(a |
| < b) && !(b < a)</code>, two <code>shared_ptr</code> instances |
| are equivalent if and only if they <em>share ownership</em> or are both <em>empty</em>.</li></ul> |
| <p><b>Throws:</b> nothing.</p> |
| <p><b>Notes:</b> Allows <code>shared_ptr</code> objects to be used as keys in |
| associative containers.</p> |
| </blockquote> |
| <p><em>[<code>Operator<</code> has been preferred over a <code>std::less</code> |
| specialization for consistency and legality reasons, as <code>std::less</code> |
| is required to return the results of <code>operator<</code>, and many |
| standard algorithms use <code>operator<</code> instead of <code>std::less</code> |
| for comparisons when a predicate is not supplied. Composite objects, like <code>std::pair</code>, |
| also implement their <code>operator<</code> in terms of their contained |
| subobjects' <code>operator<</code>.</em></p> |
| <p><em>The rest of the comparison operators are omitted by design.]</em></p> |
| <h3 id="free-swap">swap</h3> |
| <pre>template<class T> |
| void swap(shared_ptr<T> & a, shared_ptr<T> & b); // never throws</pre> |
| <blockquote> |
| <p><b>Effects:</b> Equivalent to <code>a.swap(b)</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| <p><b>Notes:</b> Matches the interface of <code>std::swap</code>. Provided as an aid to |
| generic programming.</p> |
| </blockquote> |
| <p><em>[<code>swap</code> is defined in the same namespace as <code>shared_ptr</code> |
| as this is currently the only legal way to supply a <code>swap</code> function |
| that has a chance to be used by the standard library.]</em></p> |
| <h3 id="get_pointer">get_pointer</h3> |
| <pre>template<class T> |
| typename shared_ptr<T>::element_type * get_pointer(shared_ptr<T> const & p); // never throws</pre> |
| <blockquote> |
| <p><b>Returns:</b> <code>p.get()</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| <p><b>Notes:</b> Provided as an aid to generic programming. Used by <a href="../bind/mem_fn.html"> |
| mem_fn</a>.</p> |
| </blockquote> |
| <h3 id="static_pointer_cast">static_pointer_cast</h3> |
| <pre>template<class T, class U> |
| shared_ptr<T> static_pointer_cast(shared_ptr<U> const & r); // never throws</pre> |
| <blockquote> |
| <p><b>Requires:</b> The expression <code>static_cast<T*>( (U*)0 )</code> |
| must be well-formed.</p> |
| <p><b>Returns:</b> <code>shared_ptr<T>( r, static_cast<typename shared_ptr<T>::element_type*>(r.get()) )</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| <p><b>Notes:</b> the seemingly equivalent expression |
| <code>shared_ptr<T>(static_cast<T*>(r.get()))</code> |
| will eventually result in undefined behavior, attempting to delete the same |
| object twice.</p> |
| </blockquote> |
| <h3 id="const_pointer_cast">const_pointer_cast</h3> |
| <pre>template<class T, class U> |
| shared_ptr<T> const_pointer_cast(shared_ptr<U> const & r); // never throws</pre> |
| <blockquote> |
| <p><b>Requires:</b> The expression <code>const_cast<T*>( (U*)0 )</code> |
| must be well-formed.</p> |
| <p><b>Returns:</b> <code>shared_ptr<T>( r, const_cast<typename shared_ptr<T>::element_type*>(r.get()) )</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h3 id="dynamic_pointer_cast">dynamic_pointer_cast</h3> |
| <pre>template<class T, class U> |
| shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const & r);</pre> |
| <blockquote> |
| <p><b>Requires:</b> The expression <code>dynamic_cast<T*>( (U*)0 )</code> |
| must be well-formed.</p> |
| <p><b>Returns:</b></p> |
| <ul> |
| <li> |
| When <code>dynamic_cast<typename shared_ptr<T>::element_type*>(r.get())</code> returns a nonzero value <code>p</code>, |
| <code>shared_ptr<T>(r, p)</code>;</li> |
| <li> |
| Otherwise, <code>shared_ptr<T>()</code>.</li></ul> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h3 id="reinterpret_pointer_cast">reinterpret_pointer_cast</h3> |
| <pre>template<class T, class U> |
| shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U> const & r); // never throws</pre> |
| <blockquote> |
| <p><b>Requires:</b> The expression <code>reinterpret_cast<T*>( (U*)0 )</code> |
| must be well-formed.</p> |
| <p><b>Returns:</b> <code>shared_ptr<T>( r, reinterpret_cast<typename shared_ptr<T>::element_type*>(r.get()) )</code>.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h3 id="insertion-operator">operator<<</h3> |
| <pre>template<class E, class T, class Y> |
| std::basic_ostream<E, T> & operator<< (std::basic_ostream<E, T> & os, shared_ptr<Y> const & p);</pre> |
| <blockquote> |
| <p><b>Effects:</b> <code>os << p.get();</code>.</p> |
| <p><b>Returns:</b> <code>os</code>.</p> |
| </blockquote> |
| <h3 id="get_deleter">get_deleter</h3> |
| <pre>template<class D, class T> |
| D * get_deleter(shared_ptr<T> const & p);</pre> |
| <blockquote> |
| <p><b>Returns:</b> If <code>*this</code> <em>owns</em> a deleter <code>d</code> |
| of type (cv-unqualified) <code>D</code>, returns <code>&d</code>; |
| otherwise returns 0.</p> |
| <p><b>Throws:</b> nothing.</p> |
| </blockquote> |
| <h2 id="example">Example</h2> |
| <p>See <a href="example/shared_ptr_example.cpp">shared_ptr_example.cpp</a> for a |
| complete example program. The program builds a <code>std::vector</code> and <code>std::set</code> |
| of <code>shared_ptr</code> objects.</p> |
| <p>Note that after the containers have been populated, some of the <code>shared_ptr</code> |
| objects will have a use count of 1 rather than a use count of 2, since the set |
| is a <code>std::set</code> rather than a <code>std::multiset</code>, and thus does not |
| contain duplicate entries. Furthermore, the use count may be even higher at |
| various times while <code>push_back</code> and <code>insert</code> container operations are |
| performed. More complicated yet, the container operations may throw exceptions |
| under a variety of circumstances. Getting the memory management and exception |
| handling in this example right without a smart pointer would be a nightmare.</p> |
| <h2 id="HandleBody">Handle/Body Idiom</h2> |
| <p>One common usage of <code>shared_ptr</code> 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/shared_ptr_example2_test.cpp">shared_ptr_example2_test.cpp</a> |
| sample program includes a header file, <a href="example/shared_ptr_example2.hpp">shared_ptr_example2.hpp</a>, |
| which uses a <code>shared_ptr</code> to an incomplete type to hide the |
| implementation. The instantiation of member functions which require a complete |
| type occurs in the <a href="example/shared_ptr_example2.cpp">shared_ptr_example2.cpp</a> |
| implementation file. Note that there is no need for an explicit destructor. |
| Unlike <code>~scoped_ptr</code>, <code>~shared_ptr</code> does not require that <code>T</code> be a complete |
| type.</p> |
| <h2 id="ThreadSafety">Thread Safety</h2> |
| <p><code>shared_ptr</code> objects offer the same level of thread safety as |
| built-in types. A <code>shared_ptr</code> instance can be "read" (accessed |
| using only const operations) simultaneously by multiple threads. Different <code>shared_ptr</code> |
| instances can be "written to" (accessed using mutable operations such as <code>operator= |
| </code>or <code>reset</code>) simultaneously by multiple threads (even |
| when these instances are copies, and share the same reference count |
| underneath.)</p> |
| <p>Any other simultaneous accesses result in undefined behavior.</p> |
| <p>Examples:</p> |
| <pre>shared_ptr<int> p(new int(42)); |
| |
| //--- Example 1 --- |
| |
| // thread A |
| shared_ptr<int> p2(p); // reads p |
| |
| // thread B |
| shared_ptr<int> p3(p); // OK, multiple reads are safe |
| |
| //--- Example 2 --- |
| |
| // thread A |
| p.reset(new int(1912)); // writes p |
| |
| // thread B |
| p2.reset(); // OK, writes p2 |
| |
| //--- Example 3 --- |
| |
| // thread A |
| p = p3; // reads p3, writes p |
| |
| // thread B |
| p3.reset(); // writes p3; undefined, simultaneous read/write |
| |
| //--- Example 4 --- |
| |
| // thread A |
| p3 = p2; // reads p2, writes p3 |
| |
| // thread B |
| // p2 goes out of scope: undefined, the destructor is considered a "write access" |
| |
| //--- Example 5 --- |
| |
| // thread A |
| p3.reset(new int(1)); |
| |
| // thread B |
| p3.reset(new int(2)); // undefined, multiple writes |
| </pre> |
| <p> </p> |
| <p>Starting with Boost release 1.33.0, <code>shared_ptr</code> uses a lock-free |
| implementation on most common platforms.</p> |
| <p>If your program is single-threaded and does not link to any libraries that might |
| have used <code>shared_ptr</code> in its default configuration, you can <code> |
| #define</code> the macro <code>BOOST_SP_DISABLE_THREADS</code> on a |
| project-wide basis to switch to ordinary non-atomic reference count updates.</p> |
| <p>(Defining <code>BOOST_SP_DISABLE_THREADS</code> in some, but not all, |
| translation units is technically a violation of the One Definition Rule and |
| undefined behavior. Nevertheless, the implementation attempts to do its best to |
| accommodate the request to use non-atomic updates in those translation units. |
| No guarantees, though.)</p> |
| <p>You can define the macro <code>BOOST_SP_USE_PTHREADS</code> to turn off the |
| lock-free platform-specific implementation and fall back to the generic |
| <code>pthread_mutex_t</code>-based code.</p> |
| <h2 id="FAQ">Frequently Asked Questions</h2> |
| <p><b>Q.</b> There are several variations of shared pointers, with different |
| tradeoffs; why does the smart pointer library supply only a single |
| implementation? It would be useful to be able to experiment with each type so |
| as to find the most suitable for the job at hand?</p> |
| <p> |
| <b>A.</b> An important goal of <code>shared_ptr</code> is to provide a |
| standard shared-ownership pointer. Having a single pointer type is important |
| for stable library interfaces, since different shared pointers typically cannot |
| interoperate, i.e. a reference counted pointer (used by library A) cannot share |
| ownership with a linked pointer (used by library B.) |
| </p> |
| <p><b>Q.</b> Why doesn't <code>shared_ptr</code> have template parameters supplying |
| traits or policies to allow extensive user customization?</p> |
| <p> |
| <b>A.</b> Parameterization discourages users. The <code>shared_ptr</code> template is |
| carefully crafted to meet common needs without extensive parameterization. Some |
| day a highly configurable smart pointer may be invented that is also very easy |
| to use and very hard to misuse. Until then, <code>shared_ptr</code> is the smart |
| pointer of choice for a wide range of applications. (Those interested in policy |
| based smart pointers should read <a href="http://www.awprofessional.com/bookstore/product.asp?isbn=0201704315&rl=1"> |
| Modern C++ Design</a> by Andrei Alexandrescu.) |
| </p> |
| <p><b>Q.</b> I am not convinced. Default parameters can be used where appropriate |
| to hide the complexity. Again, why not policies?</p> |
| <p> |
| <b>A.</b> Template parameters affect the type. See the answer to the first |
| question above. |
| </p> |
| <p><b>Q.</b> Why doesn't <code>shared_ptr</code> use a linked list implementation?</p> |
| <p> |
| <b>A.</b> A linked list implementation does not offer enough advantages to |
| offset the added cost of an extra pointer. See <a href="smarttests.htm">timings</a> |
| page. In addition, it is expensive to make a linked list implementation thread |
| safe. |
| </p> |
| <p><b>Q.</b> Why doesn't <code>shared_ptr</code> (or any of the other Boost smart |
| pointers) supply an automatic conversion to <code>T*</code>?</p> |
| <p> |
| <b>A.</b> Automatic conversion is believed to be too error prone. |
| </p> |
| <p><b>Q.</b> Why does <code>shared_ptr</code> supply <code>use_count()</code>?</p> |
| <p> |
| <b>A.</b> As an aid to writing test cases and debugging displays. One of the |
| progenitors had <code>use_count()</code>, and it was useful in tracking down bugs in a |
| complex project that turned out to have cyclic-dependencies. |
| </p> |
| <p><b>Q.</b> Why doesn't <code>shared_ptr</code> specify complexity requirements?</p> |
| <p> |
| <b>A.</b> Because complexity requirements limit implementors and complicate the |
| specification without apparent benefit to <code>shared_ptr</code> users. For example, |
| error-checking implementations might become non-conforming if they had to meet |
| stringent complexity requirements. |
| </p> |
| <p><b>Q.</b> Why doesn't <code>shared_ptr</code> provide a <code>release()</code> function?</p> |
| <p> |
| <b>A.</b> <code>shared_ptr</code> cannot give away ownership unless it's <code>unique()</code> |
| because the other copy will still destroy the object.</p> |
| <p>Consider:</p> |
| <blockquote><pre>shared_ptr<int> a(new int); |
| shared_ptr<int> b(a); // a.use_count() == b.use_count() == 2 |
| |
| int * p = a.release(); |
| |
| // Who owns p now? b will still call delete on it in its destructor.</pre> |
| </blockquote> |
| <p>Furthermore, the pointer returned by <code>release()</code> would be difficult |
| to deallocate reliably, as the source <code>shared_ptr</code> could have been created |
| with a custom deleter. |
| </p> |
| <p><b>Q.</b> Why is <code>operator->()</code> const, but its return value is a |
| non-const pointer to the element type?</p> |
| <p> |
| <b>A.</b> Shallow copy pointers, including raw pointers, typically don't |
| propagate constness. It makes little sense for them to do so, as you can always |
| obtain a non-const pointer from a const one and then proceed to modify the |
| object through it. <code>shared_ptr</code> is "as close to raw pointers as possible |
| but no closer". |
| </p> |
| <hr> |
| <p>$Date$</p> |
| <p><small>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler. |
| Copyright 2002-2005, 2012, 2013 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> |
| </body> |
| </html> |