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<p>"Resource Acquisition Is Initialization" or RAII, is a C++ programming technique<sup id="cite_ref-1" class="reference"><a href="raii.html#cite_note-1">[1]</a></sup> which binds the life cycle of a resource (allocated memory, open socket, open file, mutex, database connection - anything that exists in limited supply) to the <a href="lifetime.html" title="cpp/language/lifetime">lifetime</a> of an object with automatic <a href="storage_duration.html" title="cpp/language/storage duration">storage duration</a>.
</p><p>RAII guarantees that the resource is available to any function that may access the object (resource availability is a <a href="http://en.wikipedia.com/wiki/Class_invariant" class="extiw" title="enwiki:Class invariant">class invariant</a>). It also guarantees that all resources are released when their controlling objects go out of <a href="scope.html" title="cpp/language/scope">scope</a>, in reverse order of acquisition. Likewise, if resource acquisition fails (the constructor exits with an exception), all resources acquired by every fully-constructed member and base subobject are released in reverse order of initialization. This leverages the core language features (<a href="lifetime.html" title="cpp/language/lifetime">object lifetime</a>, <a href="statements.html" title="cpp/language/statements">scope exit</a>, <a href="constructor.html#Initialization_order" title="cpp/language/initializer list">order of initialization</a> and <a href="throw.html#Stack_unwinding" title="cpp/language/throw">stack unwinding</a>) to eliminate resource leaks and guarantee exception safety. Another name for this technique is "Scope-bound resource management" (SBRM).
</p><p>RAII can be summarized as follows:
</p>
<ul><li> encapsulate each resource into a class, where
</li></ul>
<dl><dd><ul><li> the constructor acquires the resource and establishes all class invariants or throws an exception if that cannot be done
</li><li> the destructor releases the resource and never throws exceptions
</li></ul>
</dd></dl>
<ul><li> always use the resource via an instance of a RAII-class that either
</li></ul>
<dl><dd><ul><li> has automatic storage duration
</li><li> is a non-static member of a class whose instance has automatic storage duration
</li></ul>
</dd></dl>
<p>Classes with open()/close(), lock()/unlock(), or init()/copyFrom()/destroy() member functions are typical examples of non-RAII classes:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><a href="../thread/mutex.html"><span class="kw1832">std::<span class="me2">mutex</span></span></a> m<span class="sy4">;</span>
<span class="kw4">void</span> bad<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
m.<span class="me1">lock</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// acquire the mutex</span>
f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// if f() throws an exception, the mutex is never released</span>
<span class="kw1">if</span><span class="br0">(</span><span class="sy3">!</span>everything_ok<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="kw1">return</span><span class="sy4">;</span> <span class="co1">// early return, the mutex is never released</span>
m.<span class="me1">unlock</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// only if execution reaches this statement, the mutex is released</span>
<span class="br0">}</span>
<span class="kw4">void</span> good<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../thread/lock_guard.html"><span class="kw1844">std::<span class="me2">lock_guard</span></span></a><span class="sy1"><</span><a href="../thread/mutex.html"><span class="kw1832">std::<span class="me2">mutex</span></span></a><span class="sy1">></span> lk<span class="br0">(</span>m<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// RAII class: mutex acquisition is initialization</span>
f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// if f() throws an exception, the mutex is released</span>
<span class="kw1">if</span><span class="br0">(</span><span class="sy3">!</span>everything_ok<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="kw1">return</span><span class="sy4">;</span> <span class="co1">// early return, the mutex is released</span>
<span class="br0">}</span> <span class="co1">// if the function returns normally, the mutex is released</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/raii&action=edit&section=1" title="Edit section: The standard library">edit</a>]</span> <span class="mw-headline" id="The_standard_library">The standard library</span></h3>
<p>The C++ library classes that manage their own resources follow RAII: <span class="t-lc">std::string</span>, <span class="t-lc">std::vector</span>, <span class="t-lc">std::thread</span>, and many others allocate their resources in constructors (which throw exceptions on errors), release them in their destructors (which never throw), and don't require explicit cleanup.
</p><p>In addition, the standard library offers several RAII wrappers to manage user-provided resources:
</p>
<ul><li> <span class="t-lc">std::unique_ptr</span> and <span class="t-lc">std::shared_ptr</span> to manage dynamically-allocated memory or, with a user-provided deleter, any resource represented by a plain pointer
</li><li> <span class="t-lc">std::lock_guard</span>, <span class="t-lc">std::unique_lock</span>, <span class="t-lc">std::shared_lock</span> to manage mutexes
</li></ul>
<h2><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/raii&action=edit&section=2" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h2>
<ol class="references">
<li id="cite_note-1"><span class="mw-cite-backlink"><a href="raii.html#cite_ref-1">↑</a></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://www.stroustrup.com/bs_faq2.html#finally">RAII in Stroustrup's C++ FAQ</a></span>
</li>
</ol>
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