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<h1 class="topictitle1">blocked_range Template Class</h1>
<div>
<div class="section"><h2 class="sectiontitle">Summary</h2>
Template class for a recursively divisible
half-open interval.
</div>
<div class="section"><h2 class="sectiontitle">Header</h2>
<p>
<pre>#include "tbb/blocked_range.h"</pre>
</p>
</div>
<div class="section"><h2 class="sectiontitle">Syntax</h2>
<pre>template<typename Value> class blocked_range;</pre>
</div>
<div class="section"><h2 class="sectiontitle">Description</h2>
<p>A
<samp class="codeph">blocked_range<Value></samp> represents a half-open
range [<em>i</em>,<em>j</em>) that can be recursively split. The types of
<em>i</em> and
<em>j</em> must model the requirements in the following table. In the
table, type
<samp class="codeph">D</samp> is the type of the expression
<samp class="codeph">"j-i"</samp>. It can be any integral type that is convertible
to
<samp class="codeph">size_t</samp>. Examples that model the Value requirements are
integral types, pointers, and STL random-access iterators whose difference can
be implicitly converted to a
<samp class="codeph">size_t</samp>.
</p>
<p>A
<samp class="codeph">blocked_range</samp> models the Range concept.
</p>
<div class="tablenoborder"><table cellpadding="4" summary="" width="100%" frame="hsides" border="1" rules="all"><caption><span class="tablecap">Value Concept for blocked_range</span></caption>
<thead align="left">
<tr>
<th class="cellrowborder" valign="top" id="d14709e109">
<p>Pseudo-Signature
</p>
</th>
<th class="row-nocellborder" valign="top" id="d14709e115">
<p>Semantics
</p>
</th>
</tr>
</thead>
<tbody>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d14709e109 ">
<p><samp class="codeph"> Value::Value( const Value&
)</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" headers="d14709e115 ">
<p> Copy constructor.
</p>
</td>
</tr>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d14709e109 ">
<p><samp class="codeph"> Value::~Value()</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" headers="d14709e115 ">
<p> Destructor.
</p>
</td>
</tr>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d14709e109 ">
<p><samp class="codeph"> void operator=( const Value&
)</samp>
<div class="Note"><h3 class="NoteTipHead">
Note</h3>
<p> The return type
<samp class="codeph">void</samp> in the pseudo-signature denotes that
<samp class="codeph">operator=</samp> is not required to return a
value. The actual
<samp class="codeph">operator=</samp> can return a value, which will
be ignored by
<samp class="codeph">blocked_range</samp> .
</p>
</div>
</p>
</td>
<td class="row-nocellborder" valign="top" headers="d14709e115 ">
<p> Assignment
</p>
</td>
</tr>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d14709e109 ">
<p><samp class="codeph"> bool operator<( const
Value& i, const Value& j )</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" headers="d14709e115 ">
<p> Value
<em>i</em> precedes value<em> j</em>.
</p>
</td>
</tr>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d14709e109 ">
<p><samp class="codeph"> D operator-( const Value& i,
const Value& j )</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" headers="d14709e115 ">
<p> Number of values in range
<samp class="codeph">[i,j)</samp>.
</p>
</td>
</tr>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d14709e109 ">
<p><samp class="codeph"> Value operator+( const Value&
i, D k )</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" headers="d14709e115 ">
<p><em>k</em>th value after
<em>i</em>.
</p>
</td>
</tr>
</tbody>
</table>
</div>
<p>A
<samp class="codeph">blocked_range<Value></samp> specifies a
<em>grainsize</em> of type
<samp class="codeph">size_t</samp>. A
<samp class="codeph">blocked_range</samp> is splittable into two subranges if the
size of the range exceeds
<em>grain size</em>. The ideal grain size depends upon the context of the
<samp class="codeph">blocked_range<Value></samp>, which is typically as the
range argument to the loop templates
<samp class="codeph">parallel_for</samp>,
<samp class="codeph">parallel_reduce</samp>, or
<samp class="codeph">parallel_scan</samp>. A too small grainsize may cause
scheduling overhead within the loop templates to swamp speedup gained from
parallelism. A too large grainsize may unnecessarily limit parallelism. For
example, if the grain size is so large that the range can be split only once,
then the maximum possible parallelism is two.
</p>
<p>Here is a suggested procedure for choosing
<em>grainsize</em>:
</p>
<ol>
<li>Set the grainsize parameter to 10,000. This
value is high enough to amortize scheduler overhead sufficiently for
practically all loop bodies, but may be unnecessarily limit parallelism.
</li>
<li>Run your algorithm on
<em>one</em> processor.
</li>
<li>Start halving the grainsize parameter and see
how much the algorithm slows down as the value decreases.
</li>
</ol>
<p>A slowdown of about 5-10% is a good setting for
most purposes.
</p>
<div class="Note"><h3 class="NoteTipHead">
Tip</h3>
<p>For a
<samp class="codeph">blocked_range [i,j)</samp> where<samp class="codeph"> j<i</samp>,
not all methods have specified behavior. However, enough methods do have
specified behavior that
<samp class="codeph">parallel_for</samp>,
<samp class="codeph">parallel_reduce</samp>, and
<samp class="codeph">parallel_scan</samp> iterate over the same iteration space
as the serial loop
<samp class="codeph">for( Value index=i; index<j; ++index )...</samp>, even
when
<samp class="codeph">j<i</samp>. If
<samp class="codeph">TBB_USE_ASSERT</samp> is nonzero, methods with unspecified
behavior raise an assertion failure.
</p>
</div>
<p><strong>Example</strong>
</p>
<p>A
<samp class="codeph">blocked_range<Value></samp> typically appears as a
range argument to a loop template. See the examples for
<samp class="codeph">parallel_for</samp>,<samp class="codeph"> parallel_reduce</samp>, and
<samp class="codeph">parallel_scan</samp>.
</p>
</div>
<div class="section"><h2 class="sectiontitle">Members</h2>
<pre>
namespace tbb {
template<typename Value>
class blocked_range {
public:
// types
typedef size_t size_type;
typedef Value const_iterator;
// constructors
blocked_range( Value begin, Value end,
size_type grainsize=1 );
blocked_range( blocked_range& r, split );
// capacity
size_type size() const;
bool empty() const;
// access
size_type grainsize() const;
bool is_divisible() const;
// iterators
const_iterator begin() const;
const_iterator end() const;
};
}
</pre>
<div class="tablenoborder"><table cellpadding="4" summary="" frame="border" border="1" cellspacing="0" rules="all"><span class="tabledesc">The following table provides additional information on the
members of this template class.
</span><thead align="left">
<tr>
<th class="cellrowborder" valign="top" width="33.89830508474576%" id="d14709e390">Member
</th>
<th class="cellrowborder" valign="top" width="66.10169491525423%" id="d14709e393">Description
</th>
</tr>
</thead>
<tbody>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d14709e390 "><span class="keyword">size_type
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d14709e393 ">
<p>The type for measuring the size of a
<samp class="codeph">blocked_range</samp>. The type is always a
<samp class="codeph">size_t</samp>.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d14709e390 "><span class="keyword">const_iterator</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d14709e393 ">The type of a value in the range. Despite
its name, the type
<samp class="codeph">const_iterator</samp> is not necessarily an STL
iterator; it merely needs to meet the Value requirements in the table above.
However, it is convenient to call it
<samp class="codeph">const_iterator</samp> so that if it is a
const_iterator, then the<samp class="codeph"> blocked_range</samp> behaves like a
read-only STL container.
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d14709e390 "><span class="keyword">blocked_range( Value begin, Value
end, size_t grainsize=1 )
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d14709e393 "><a href="blocked_range_cls/blocked_range.htm">More Info</a>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d14709e390 "><span class="keyword">blocked_range( blocked_range&
range, split )
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d14709e393 "><a href="blocked_range_cls/blocked_range_1.htm">More Info</a>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d14709e390 "><span class="keyword">size_type size() const
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d14709e393 ">
<p><strong>Requirements</strong> :<samp class="codeph"> end()<begin()</samp> is
false.
</p>
<p><strong>Effects</strong>: Determines size of range.
</p>
<p><strong>Returns</strong>
</p>
<p><samp class="codeph"> end()-begin()</samp>
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d14709e390 "><span class="keyword">bool empty() const
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d14709e393 ">
<p><strong>Effects</strong>: Determines if range is empty.
</p>
<p><strong>Returns</strong>:
<samp class="codeph">!(begin()<end())</samp>
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d14709e390 "><span class="keyword">size_type grainsize() const
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d14709e393 ">
<p><strong>Returns</strong>: Grain size of range.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d14709e390 "><span class="keyword">bool is_divisible() const
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d14709e393 ">
<p><strong>Requirements</strong>:
<samp class="codeph"> !(end()<begin())</samp>
</p>
<p><strong>Effects</strong>: Determines if range can be split into
subranges.
</p>
<p><strong>Returns</strong>: True if
<samp class="codeph">size()>grainsize();</samp> false otherwise.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d14709e390 "><span class="keyword">const_iterator begin()
const</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d14709e393 ">
<p><strong>Returns</strong>: Inclusive lower bound on range.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d14709e390 "><span class="keyword">const_iterator end() const
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d14709e393 ">
<p><strong>Returns</strong>: Exclusive upper bound on range.
</p>
</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../../../reference/algorithms/range_concept.htm">Range Concept</a></div>
</div>
<div class="See Also">
<ul class="ullinks">
<li class="ulchildlink"><a href="../../../reference/algorithms/range_concept/blocked_range_cls/blocked_range.htm">blocked_range( Value begin, Value end, size_t grainsize=1 )</a><br>
</li>
<li class="ulchildlink"><a href="../../../reference/algorithms/range_concept/blocked_range_cls/blocked_range_1.htm">blocked_range( blocked_range& range, split )</a><br>
</li>
</ul>
<h2>See Also</h2>
<div class="linklist">
<div><a href="../parallel_for_func.htm#parallel_for_func">parallel_for Template Function
</a></div>
<div><a href="../parallel_reduce_func.htm#parallel_reduce_func">parallel_reduce Template Function
</a></div>
<div><a href="../parallel_scan_func.htm#parallel_scan_func">parallel_scan Template Function
</a></div>
<div><a href="../../environment/enabling_debugging_features.htm#enabling_debugging_features">TBB_USE_ASSERT Macro
</a></div></div>
</div>
</body>
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