forked from boostorg/smart_ptr
shared_ptr techniques initial commit
[SVN r16978]
This commit is contained in:
Peter Dimov
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sp_techniques.html
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sp_techniques.html
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<title>Smart pointer programming techniques</title>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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</head>
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<body text="#000000" bgColor="#ffffff">
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<h1><IMG height="86" alt="c++boost.gif (8819 bytes)" src="../../c++boost.gif" width="277" align="middle">Smart
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pointer programming techniques</h1>
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<p>
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<A href="#incomplete">Using incomplete classes for implementation hiding</A><br>
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<A href="#pimpl">The "Pimpl" idiom</A><br>
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<A href="#abstract">Using abstract classes for implementation hiding</A><br>
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||||
<A href="#preventing_delete">Preventing <code>delete px.get()</code></A><br>
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||||
<A href="#array">Using a <code>shared_ptr</code> to hold a pointer to an array</A><br>
|
||||
<A href="#encapsulation">Encapsulating allocation details, wrapping factory
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||||
functions</A><br>
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||||
<A href="#static">Using a <code>shared_ptr</code> to hold a pointer to a statically allocated
|
||||
object</A><br>
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||||
<A href="#com">Using a <code>shared_ptr</code> to hold a pointer to a COM object</A><br>
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<A href="#intrusive">Using a <code>shared_ptr</code> to hold a pointer to an object with an
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||||
embedded reference count</A><br>
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<A href="#another_sp">Using a <code>shared_ptr</code> to hold another shared ownership smart
|
||||
pointer</A><br>
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||||
<A href="#from_raw">Obtaining a <code>shared_ptr</code> from a raw pointer</A><br>
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<A href="#in_constructor">Obtaining a <code>shared_ptr</code> (<code>weak_ptr</code>) to <code>this</code> in a
|
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constructor</A><br>
|
||||
<A href="#from_this">Obtaining a <code>shared_ptr</code> to <code>this</code></A><br>
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<A href="#handle">Using <code>shared_ptr</code> as a smart counted handle</A><br>
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<A href="#on_block_exit">Using <code>shared_ptr</code> to execute code on block exit</A><br>
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<A href="#pvoid">Using <code>shared_ptr<void></code> to hold an arbitrary object</A><br>
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<A href="#extra_data">Associating arbitrary data with heterogeneous <code>shared_ptr</code>
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instances</A><br>
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<A href="#pre_destructor">Post-constructors and pre-destructors</A><br>
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<A href="#as_lock">Using <code>shared_ptr</code> as a CopyConstructible mutex lock</A><br>
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<A href="#wrapper">Using <code>shared_ptr</code> to wrap member function calls</A><br>
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<A href="#delayed">Delayed deallocation</A><br>
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<A href="#weak_without_shared">Weak pointers to objects not managed by a <code>shared_ptr</code></A><br>
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</p>
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<h2><A name="incomplete">Using incomplete classes for implementation hiding</A></h2>
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<p>[Old, proven technique; can be used in C]</p>
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<pre>
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class FILE;
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FILE * fopen(char const * name, char const * mode);
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void fread(FILE * f, void * data, size_t size);
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void fclose(FILE * f);
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</pre>
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<p>[Compare with]</p>
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<pre>
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class FILE;
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shared_ptr<FILE> fopen(char const * name, char const * mode);
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void fread(shared_ptr<FILE> f, void * data, size_t size);
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</pre>
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<p>Note that there is no <code>fclose</code> function; <code>shared_ptr</code>'s ability to execute a custom deleter makes it unnecessary.</p>
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<p>[<code>shared_ptr<X></code> can be copied and destroyed when <code>X</code> is incomplete.]</p>
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<h2><A name="pimpl">The "Pimpl" idiom</A></h2>
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<p>[...]</p>
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<pre>
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// file.hpp:
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class file
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{
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private:
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class impl;
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shared_ptr<impl> pimpl_;
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public:
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file(char const * name, char const * mode);
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// compiler generated members are fine and useful
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void read(void * data, size_t size);
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};
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</pre>
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<pre>
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// file.cpp:
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#include "file.hpp"
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class file::impl
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{
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private:
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impl(impl const &);
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impl & operator=(impl const &);
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// private data
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|
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public:
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|
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impl(char const * name, char const * mode) { ... }
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~impl() { ... }
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void read(void * data, size_t size) { ... }
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};
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file::file(char const * name, char const * mode): pimpl_(new impl(name, mode))
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{
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}
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void file::read(void * data, size_t size)
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{
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pimpl_->read(data, size);
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}
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</pre>
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<p>[<code>file</code> is CopyConstructible and Assignable.]</p>
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<h2><A name="abstract">Using abstract classes for implementation hiding</A></h2>
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<p>[Interface based programming]</p>
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<pre>
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// X.hpp:
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class X
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{
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public:
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|
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virtual void f() = 0;
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virtual void g() = 0;
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|
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protected:
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~X() {}
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};
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shared_ptr<X> createX();
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</pre>
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<pre>
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-- X.cpp:
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class X_impl: public X
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{
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private:
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X_impl(X_impl const &);
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X_impl & operator=(X_impl const &);
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|
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public:
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virtual void f()
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{
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// ...
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}
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virtual void g()
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{
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// ...
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}
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};
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shared_ptr<X> createX()
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{
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shared_ptr<X> px(new X_impl);
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return px;
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}
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</pre>
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<p>[Note protected and nonvirtual destructor; client cannot delete <code>X</code>; <code>shared_ptr</code> correctly calls <code>~X_impl</code> even when nonvirtual.]</p>
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<h2><A name="preventing_delete">Preventing <code>delete px.get()</code></A></h2>
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<p>[Alternative 1, use the above.]</p>
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<p>[Alternative 2, use a private deleter:]</p>
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<pre>
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class X
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{
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private:
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~X();
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class deleter;
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friend class deleter;
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class deleter
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||||
{
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public:
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void operator()(X * p) { delete p; }
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};
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public:
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static shared_ptr<X> create()
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{
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shared_ptr<X> px(new X, X::deleter());
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return px;
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}
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};
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</pre>
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<h2><A name="array">Using a <code>shared_ptr</code> to hold a pointer to an array</A></h2>
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<p>[...]</p>
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<pre>
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shared_ptr<X> px(new X[1], checked_array_deleter<X>());
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</pre>
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<p>[<code>shared_array</code> is preferable, has a better interface; <code>shared_ptr</code> has *, ->, derived to base conversions.]</p>
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<h2><A name="encapsulation">Encapsulating allocation details, wrapping factory
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functions</A></h2>
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<p>[Existing interface, possibly allocates <code>X</code> from its own heap, <code>~X</code> is private, or <code>X</code> is incomplete.]</p>
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<pre>
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X * CreateX();
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void DestroyX(X *);
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</pre>
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<p>[Wrapper:]</p>
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||||
<pre>
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||||
shared_ptr<X> createX()
|
||||
{
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||||
shared_ptr<X> px(CreateX(), DestroyX);
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}
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</pre>
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<p>[Client remains blissfully oblivious of allocation details; doesn't need to remember to call <code>destroyX</code>.]</p>
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<h2><A name="static">Using a <code>shared_ptr</code> to hold a pointer to a statically allocated
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object</A></h2>
|
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<p>[...]</p>
|
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<pre>
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shared_ptr<X> createX();
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</pre>
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<p>[Sometimes needs to return a pointer to a statically allocated <code>X</code> instance.]</p>
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<pre>
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struct null_deleter
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{
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void operator()(void const *) const
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{
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}
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};
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static X x;
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shared_ptr<X> createX()
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{
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shared_ptr<X> px(&x, null_deleter());
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return px;
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}
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</pre>
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<p>[The same technique works for any object known to outlive the pointer.]</p>
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<h2><A name="com">Using a <code>shared_ptr</code> to hold a pointer to a COM Object</A></h2>
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<p>[COM objects have an embedded reference count, <code>AddRef()</code> and <code>Release()</code>, <code>Release()</code> self-destroys when reference count drops to zero.]</p>
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<pre>
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shared_ptr<IWhatever> make_shared_from_COM(IWhatever * p)
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{
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p->AddRef();
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shared_ptr<IWhatever> pw(p, mem_fn(&IWhatever::Release));
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return pw;
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}
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</pre>
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<p>[All pw copies will share a single reference.]</p>
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<h2><A name="intrusive">Using a <code>shared_ptr</code> to hold a pointer to an object with an
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embedded reference count</A></h2>
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<p>[A generalization of the above. Example assumes <code>intrusive_ptr</code>-compatible object.]</p>
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<pre>
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template<class T> struct intrusive_deleter
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{
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void operator()(T * p)
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{
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if(p) intrusive_ptr_release(p);
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}
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};
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shared_ptr<X> make_shared_from_intrusive(X * p)
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{
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if(p) intrusive_ptr_add_ref(p);
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shared_ptr<X> px(p, intrusive_deleter<X>());
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return px;
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}
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</pre>
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<h2><A name="another_sp">Using a <code>shared_ptr</code> to hold another shared ownership smart
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pointer</A></h2>
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<p>[...]</p>
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<pre>
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template<class P> class smart_pointer_deleter
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{
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private:
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P p_;
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public:
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smart_pointer_deleter(P const & p): p_(p)
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{
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}
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void operator()(void const *)
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{
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p_.reset();
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}
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};
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shared_ptr<X> make_shared_from_another(another_ptr<X> qx)
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{
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shared_ptr<X> px(qx.get(), smart_pointer_deleter< another_ptr<X> >(qx));
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return px;
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}
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</pre>
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<p>[If <code>p_.reset()</code> can throw - wrap in <code>try {} catch(...) {}</code> block, will release <code>p_</code> when all weak pointers are eliminated.]</p>
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<h2><A name="from_raw">Obtaining a <code>shared_ptr</code> from a raw pointer</A></h2>
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<p>[...]</p>
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<pre>
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void f(X * p)
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{
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shared_ptr<X> px(<i>???</i>);
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}
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</pre>
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<p>[Not possible in general, either switch to]</p>
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<pre>
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void f(shared_ptr<X> px);
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</pre>
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<p>[This transformation can be used for nonvirtual member functions, too; before:]</p>
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<pre>
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void X::f(int m);
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</pre>
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<p>[after]</p>
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<pre>
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void f(shared_ptr<X> this_, int m);
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</pre>
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<p>[If <code>f</code> cannot be changed, use knowledge about <code>p</code>'s lifetime and allocation details and apply one of the above.]</p>
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<h2><A name="in_constructor">Obtaining a <code>shared_ptr</code> (<code>weak_ptr</code>) to <code>this</code> in a
|
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constructor</A></h2>
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<p>[...]</p>
|
||||
<pre>
|
||||
class X
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||||
{
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||||
public:
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||||
|
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X()
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||||
{
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shared_ptr<X> this_(<i>???</i>);
|
||||
}
|
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};
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</pre>
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||||
<p>[Not possible in general. If <code>X</code> can have automatic or static storage, and <code>this_</code> doesn't need to keep the object alive,
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use a <code>null_deleter</code>. If <code>X</code> is supposed to always live on the heap, and be managed by a <code>shared_ptr</code>, use:]</p>
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<pre>
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class X
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||||
{
|
||||
private:
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||||
|
||||
X() { ... }
|
||||
|
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public:
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|
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static shared_ptr<X> create()
|
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{
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shared_ptr<X> px(new X);
|
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// use px as 'this_'
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return px;
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||||
}
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||||
};
|
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</pre>
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<h2><A name="from_this">Obtaining a <code>shared_ptr</code> to <code>this</code></A></h2>
|
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<p>[Sometimes it is needed to obtain a shared_ptr from this in a virtual member function.]</p>
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<p>[The transformations from above cannot be applied.]</p>
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||||
<pre>
|
||||
class X
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||||
{
|
||||
public:
|
||||
|
||||
virtual void f() = 0;
|
||||
|
||||
protected:
|
||||
|
||||
~X() {}
|
||||
};
|
||||
|
||||
class Y
|
||||
{
|
||||
public:
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||||
|
||||
virtual shared_ptr<X> getX() = 0;
|
||||
|
||||
protected:
|
||||
|
||||
~Y() {}
|
||||
};
|
||||
|
||||
// --
|
||||
|
||||
class impl: public X, public Y
|
||||
{
|
||||
public:
|
||||
|
||||
impl() { ... }
|
||||
|
||||
virtual void f() { ... }
|
||||
|
||||
virtual shared_ptr<X> getX()
|
||||
{
|
||||
shared_ptr<X> px(<i>???</i>);
|
||||
return px;
|
||||
}
|
||||
};
|
||||
</pre>
|
||||
<p>[Solution:]</p>
|
||||
<pre>
|
||||
class impl: public X, public Y
|
||||
{
|
||||
private:
|
||||
|
||||
weak_ptr<impl> weak_this;
|
||||
|
||||
impl(impl const &);
|
||||
impl & operator=(impl const &);
|
||||
|
||||
impl() { ... }
|
||||
|
||||
public:
|
||||
|
||||
static shared_ptr<impl> create()
|
||||
{
|
||||
shared_ptr<impl> pi(new impl);
|
||||
pi->weak_this = pi;
|
||||
return pi;
|
||||
}
|
||||
|
||||
virtual void f() { ... }
|
||||
|
||||
virtual shared_ptr<X> getX()
|
||||
{
|
||||
shared_ptr<X> px = make_shared(weak_this);
|
||||
return px;
|
||||
}
|
||||
};
|
||||
</pre>
|
||||
<p>[Future support planned, <code>impl: public enable_shared_from_this<impl></code>.]</p>
|
||||
<h2><A name="handle">Using <code>shared_ptr</code> as a smart counted handle</A></h2>
|
||||
<p>[Win32 API allusion]</p>
|
||||
<pre>
|
||||
typedef void * HANDLE;
|
||||
HANDLE CreateProcess();
|
||||
void CloseHandle(HANDLE);
|
||||
</pre>
|
||||
<p>[Quick wrapper]</p>
|
||||
<pre>
|
||||
typedef shared_ptr<void> handle;
|
||||
|
||||
handle createProcess()
|
||||
{
|
||||
shared_ptr<void> pv(CreateProcess(), CloseHandle);
|
||||
return pv;
|
||||
}
|
||||
</pre>
|
||||
<p>[Better, typesafe:]</p>
|
||||
<pre>
|
||||
class handle
|
||||
{
|
||||
private:
|
||||
|
||||
shared_ptr<void> pv_;
|
||||
|
||||
public:
|
||||
|
||||
explicit handle(HANDLE h): pv_(h, CloseHandle) {}
|
||||
HANDLE get() { return pv_.get(); }
|
||||
};
|
||||
</pre>
|
||||
<h2><A name="on_block_exit">Using <code>shared_ptr</code> to execute code on block exit</A></h2>
|
||||
<p>[1. Executing <code>f(p)</code>, where <code>p</code> is a pointer:]</p>
|
||||
<pre>
|
||||
shared_ptr<void> guard(p, f);
|
||||
</pre>
|
||||
<p>[2. Executing arbitrary code: <code>f(x, y)</code>:]</p>
|
||||
<pre>
|
||||
shared_ptr<void> guard(static_cast<void*>(0), bind(f, x, y));
|
||||
</pre>
|
||||
<h2><A name="pvoid">Using <code>shared_ptr<void></code> to hold an arbitrary object</A></h2>
|
||||
<p>[...]</p>
|
||||
<pre>
|
||||
shared_ptr<void> pv(new X);
|
||||
</pre>
|
||||
<p>[Will correctly call <code>~X</code>.]</p>
|
||||
<p>[Can be used to strip type information: <code>shared_ptr<X></code> -> <code>(shared_ptr<void>, typeid(X))</code>]</p>
|
||||
<h2><A name="extra_data">Associating arbitrary data with heterogeneous <code>shared_ptr</code>
|
||||
instances</A></h2>
|
||||
<p>[...]</p>
|
||||
<pre>
|
||||
typedef int Data;
|
||||
|
||||
std::map< shared_ptr<void>, Data > userData;
|
||||
// or std::map< weak_ptr<void>, Data > userData; to not affect the lifetime
|
||||
|
||||
shared_ptr<X> px(new X);
|
||||
shared_ptr<int> pi(new int(3));
|
||||
|
||||
userData[px] = 42;
|
||||
userData[pi] = 91;
|
||||
</pre>
|
||||
<h2><A name="pre_destructor">Post-constructors and pre-destructors</A></h2>
|
||||
<p>[...]</p>
|
||||
<pre>
|
||||
class X
|
||||
{
|
||||
public:
|
||||
|
||||
X();
|
||||
virtual void postconstructor();
|
||||
virtual void predestructor() throw();
|
||||
~X() throw();
|
||||
|
||||
struct deleter
|
||||
{
|
||||
void operator()(X * p)
|
||||
{
|
||||
p->predestructor();
|
||||
delete p;
|
||||
}
|
||||
}
|
||||
|
||||
static shared_ptr<X> create()
|
||||
{
|
||||
shared_ptr<X> px(new X, X::deleter());
|
||||
px->postconstructor(); // can throw
|
||||
return px;
|
||||
}
|
||||
};
|
||||
</pre>
|
||||
<h2><A name="as_lock">Using <code>shared_ptr</code> as a CopyConstructible mutex lock</A></h2>
|
||||
<p>[Sometimes it's necessary to return a mutex lock from a function. A noncopyable lock cannot be used.]</p>
|
||||
<pre>
|
||||
class mutex
|
||||
{
|
||||
public:
|
||||
|
||||
void lock();
|
||||
void unlock();
|
||||
};
|
||||
|
||||
shared_ptr<mutex> lock(mutex & m)
|
||||
{
|
||||
m.lock();
|
||||
return shared_ptr<mutex>(&m, mem_fn(&mutex::unlock));
|
||||
}
|
||||
</pre>
|
||||
<p>[Or to encapsulate it in a dedicated class:]</p>
|
||||
<pre>
|
||||
class shared_lock
|
||||
{
|
||||
private:
|
||||
|
||||
shared_ptr<void> pv;
|
||||
|
||||
public:
|
||||
|
||||
template<class Mutex> explicit shared_lock(Mutex & m): pv((m.lock(), &m), mem_fn(&Mutex::unlock)) {}
|
||||
};
|
||||
</pre>
|
||||
<p>[Usage:]</p>
|
||||
<pre>
|
||||
shared_lock lock(m);
|
||||
</pre>
|
||||
<p>[Note that <code>shared_lock</code> is not templated on the mutex type, thanks to <code>shared_ptr<void></code>'s ability to hide type information.]</p>
|
||||
<h2><A name="wrapper">Using <code>shared_ptr</code> to wrap member function calls</A></h2>
|
||||
<p>[http://www.research.att.com/~bs/wrapper.pdf]</p>
|
||||
<pre>
|
||||
template<class T> class pointer
|
||||
{
|
||||
private:
|
||||
|
||||
T * p_;
|
||||
|
||||
public:
|
||||
|
||||
explicit pointer(T * p): p_(p)
|
||||
{
|
||||
}
|
||||
|
||||
shared_ptr<T> operator->() const
|
||||
{
|
||||
p_->prefix();
|
||||
return shared_ptr<T>(p_, mem_fn(&T::suffix));
|
||||
}
|
||||
};
|
||||
|
||||
class X
|
||||
{
|
||||
private:
|
||||
|
||||
void prefix();
|
||||
void suffix();
|
||||
friend class pointer<X>;
|
||||
|
||||
public:
|
||||
|
||||
void f();
|
||||
void g();
|
||||
};
|
||||
|
||||
int main()
|
||||
{
|
||||
X x;
|
||||
|
||||
pointer<X> px(&x);
|
||||
|
||||
px->f();
|
||||
px->g();
|
||||
}
|
||||
</pre>
|
||||
<h2><A name="delayed">Delayed deallocation</A></h2>
|
||||
<p>[In some situations, a single <code>px.reset()</code> can trigger an expensive deallocation in a performance-critical region.]</p>
|
||||
<pre>
|
||||
class X; // ~X is expensive
|
||||
|
||||
class Y
|
||||
{
|
||||
shared_ptr<X> px;
|
||||
|
||||
public:
|
||||
|
||||
void f()
|
||||
{
|
||||
px.reset();
|
||||
}
|
||||
};
|
||||
</pre>
|
||||
<p>[Solution 1]</p>
|
||||
<pre>
|
||||
vector< shared_ptr<void> > free_list;
|
||||
|
||||
class Y
|
||||
{
|
||||
shared_ptr<X> px;
|
||||
|
||||
public:
|
||||
|
||||
void f()
|
||||
{
|
||||
free_list.push_back(px);
|
||||
px.reset();
|
||||
}
|
||||
};
|
||||
|
||||
// periodically invoke free_list.clear() when convenient
|
||||
</pre>
|
||||
<p>[Solution 2, as above, but use a delayed deleter]</p>
|
||||
<pre>
|
||||
struct delayed_deleter
|
||||
{
|
||||
template<class T> void operator()(T * p)
|
||||
{
|
||||
try
|
||||
{
|
||||
shared_ptr<void> pv(p);
|
||||
free_list.push_back(pv);
|
||||
}
|
||||
catch(...)
|
||||
{
|
||||
}
|
||||
}
|
||||
};
|
||||
</pre>
|
||||
<h2><A name="weak_without_shared">Weak pointers to objects not managed by a <code>shared_ptr</code></A></h2>
|
||||
<p>Make the object hold a <code>shared_ptr</code> to itself, using a <code>null_deleter</code>:</p>
|
||||
<pre>
|
||||
class X
|
||||
{
|
||||
private:
|
||||
|
||||
shared_ptr<X> this_;
|
||||
int i_;
|
||||
|
||||
public:
|
||||
|
||||
explicit X(int i): this_(this, null_deleter()), i_(i)
|
||||
{
|
||||
}
|
||||
|
||||
// repeat in all constructors (including the copy constructor!)
|
||||
|
||||
X(X const & rhs): this_(this, null_deleter()), i_(rhs.i_)
|
||||
{
|
||||
}
|
||||
|
||||
// do not forget to not assign this_ in the copy assignment
|
||||
|
||||
X & operator=(X const & rhs)
|
||||
{
|
||||
i_ = rhs.i_;
|
||||
}
|
||||
|
||||
weak_ptr<X> get_weak_ptr() const { return this_; }
|
||||
};
|
||||
</pre>
|
||||
<p>When the object's lifetime ends, <code>X::this_</code> will be destroyed, and all weak pointers will automatically expire.</p>
|
||||
<hr>
|
||||
<p>
|
||||
$Date$</p>
|
||||
<p>
|
||||
Copyright © 2003 Peter Dimov. Permission to copy, use, modify, sell and
|
||||
distribute this document is granted provided this copyright notice appears in
|
||||
all copies. This document is provided "as is" without express or implied
|
||||
warranty, and with no claim as to its suitability for any purpose.</p>
|
||||
</body>
|
||||
</html>
|
||||
Reference in New Issue
Block a user