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<h1 class="topictitle1">concurrent_vector</h1>
<div>
<p><samp class="codeph">A concurrent_vector<<var>T</var>></samp> is a
dynamically growable array of
<var>T</var>. It is safe to grow a
<samp class="codeph">concurrent_vector</samp> while other threads are also operating
on elements of it, or even growing it themselves. For safe concurrent growing,
<samp class="codeph">concurrent_vector</samp> has three methods that support common
uses of dynamic arrays:
<samp class="codeph">push_back</samp>,
<samp class="codeph">grow_by</samp>, and
<samp class="codeph">grow_to_at_least</samp>.
</p>
<p>Method
<samp class="codeph">push_back(<var>x</var>)</samp> safely appends x to the
array. Method
<samp class="codeph">grow_by(<var>n</var>)</samp> safely appends
<samp class="codeph"><var>n</var></samp> consecutive elements initialized
with
<samp class="codeph"><var>T</var>()</samp>. Both methods return an iterator
pointing to the first appended element. Each element is initialized with
<samp class="codeph"><var>T</var>()</samp>. So for example, the following
routine safely appends a C string to a shared vector:
</p>
<pre>void Append( concurrent_vector<char>& vector, const char* string ) {
size_t n = strlen(string)+1;
std::copy( string, string+n, vector.grow_by(n) );
}</pre>
<p>The related method
<samp class="codeph">grow_to_at_least(<var>n</var>)</samp>grows a vector to
size
<var>n</var> if it is shorter. Concurrent calls to the growth
methods do not necessarily return in the order that elements are appended to
the vector.
</p>
<p>Method
<samp class="codeph">size()</samp> returns the number of elements in the vector,
which may include elements that are still undergoing concurrent construction by
methods
<samp class="codeph">push_back</samp>,
<samp class="codeph">grow_by,</samp> or
<samp class="codeph">grow_to_at_least</samp>. The example uses
<span class="option">std::copy</span> and iterators, not
<samp class="codeph">strcpy and pointers</samp>, because elements in a
<samp class="codeph">concurrent_vector</samp> might not be at consecutive addresses.
It is safe to use the iterators while the
<samp class="codeph">concurrent_vector</samp> is being grown, as long as the iterators
never go past the current value of
<samp class="codeph">end()</samp>. However, the iterator may reference an element
undergoing concurrent construction. You must synchronize construction and
access.
</p>
<p>A
<samp class="codeph">concurrent_vector<<var>T</var>></samp> never
moves an element until the array is cleared, which can be an advantage over the
STL
<span class="option">std::vector</span> even for single-threaded code. However,
<samp class="codeph">concurrent_vector</samp> does have more overhead than
<span class="option">std::vector</span>. Use
<samp class="codeph">concurrent_vector</samp> only if you really need the ability to
dynamically resize it while other accesses are (or might be) in flight, or
require that an element never move.
</p>
<div class="Note"><h3 class="NoteTipHead">
Caution</h3>
<p>Operations on
<samp class="codeph">concurrent_vector</samp> are concurrency safe with respect to
<em>growing</em>, not for clearing or destroying a vector. Never invoke
method
<samp class="codeph">clear()</samp> if there are other operations in flight on the
<samp class="codeph">concurrent_vector</samp>.
</p>
</div>
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<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../tbb_userguide/Containers.htm">Containers</a></div>
</div>
<div>
<ul class="ullinks">
<li class="ulchildlink"><a href="../tbb_userguide/Advanced_Idiom_Waiting_on_an_Element.htm">Advanced Idiom: Waiting on an Element</a><br>
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