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53 lines
2.7 KiB
TeX
53 lines
2.7 KiB
TeX
\pagebreak
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\section{Lambda Expressions}
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\label{sec:lambda_expressions}
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\index{lambda expressions}
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\cppspecificstart
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The following example illustrates the usage of lambda expressions and their
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corresponding closure objects within a \scode{target} region.
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In CASE 1, a lambda expression is defined inside a \scode{target} construct
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that implicitly maps the structure \textit{s}. Inside the construct, the
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lambda captures (by reference) the corresponding \textit{s}, and the resulting
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closure object is assigned to \textit{lambda1}. When the call operator is
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invoked on \textit{lambda1}, the captured reference to \textit{s} is used in
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the call. The modified \textit{s} is then copied back to the host device on
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exit from the \scode{target} construct.
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In CASE 2, a lambda expression is instead defined before the \scode{target}
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construct and captures (by copy) the pointer \textit{sp}. A
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\scode{target}~\scode{data} construct is used to first map the structure, and
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then the \scode{target} construct implicitly maps the closure object
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referenced by \textit{lambda2}, a zero-length array section based on the
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structure pointer \textit{sp}, and a zero-length array section based on the
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captured pointer in the closure object. The implicit maps result in attached
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pointers to the corresponding structure. The call for \textit{lambda2} inside
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the \scode{target} construct will access \textit{sp->a} and \textit{sp->b}
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from the corresponding structure.
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CASE 3 is similar to CASE 2, except \textit{s} is instead captured by
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reference by the lambda expression. As for CASE 2, the structure is first
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mapped by an enclosing \scode{target}~\scode{data} construct, and then the
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\scode{target} construct implicitly maps \textit{s} and the closure object
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referenced by \textit{lambda3}. The effect of the map is to make the
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the call for \textit{lambda3} refer to the corresponding \textit{s} inside the
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\scode{target} construct rather than the original \textit{s}.
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In CASE 4, the program defines a static variable \textit{ss} of the same
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structure type as \textit{s}. While the body of the lambda expression refers
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to \textit{ss}, it is not captured. In order for \textit{lambda4} to be
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callable in the \scode{target} region, the reference to \textit{ss} should be
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to a device copy of \textit{ss} that also has static storage. This is achieved
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with the use of the \scode{declare}~\scode{target} directive. Inside the
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\scode{target} construct, all references to \textit{ss}, including in the
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\textit{lambda4()} call, will refer to the corresponding \textit{ss} that
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results from the \scode{declare}~\scode{target} directive. The \scode{always}
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modifier is used on the \scode{map} clause to transfer the updated values for
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the structure back to the host device.
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\cppnexample[5.0]{lambda_expressions}{1}
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\cppspecificend
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