%\pagebreak
\begin{cppspecific}[4ex]
\section{Lambda Expressions}
\label{sec:lambda_expressions}

\index{lambda expressions}


The following example illustrates the usage of lambda expressions and their
corresponding closure objects within a \kcode{target} region.

In Case 1, a lambda expression is defined inside a \kcode{target} construct
that implicitly maps the structure \ucode{s}. Inside the construct, the
lambda captures (by reference) the corresponding \ucode{s}, and the resulting
closure object is assigned to \ucode{lambda1}.  When the call operator is
invoked on \ucode{lambda1}, the captured reference to \ucode{s} is used in
the call.  The modified \ucode{s} is then copied back to the host device on
exit from the \kcode{target} construct.

In Case 2, a lambda expression is instead defined before the \kcode{target}
construct and captures (by copy) the pointer \ucode{sp}. A
\kcode{target data} construct is used to first map the structure, and
then the \kcode{target} construct implicitly maps the closure object
referenced by \ucode{lambda2}, a zero-length array section based on the
structure pointer \ucode{sp}, and a zero-length array section based on the
captured pointer in the closure object. The implicit maps result in attached
pointers to the corresponding structure. The call for \ucode{lambda2} inside
the \kcode{target} construct will access \ucode{sp->a} and \ucode{sp->b}
from the corresponding structure.

Case 3 is similar to Case 2, except \ucode{s} is instead captured by
reference by the lambda expression. As for Case 2, the structure is first
mapped by an enclosing \kcode{target data} construct, and then the
\kcode{target} construct implicitly maps \ucode{s} and the closure object
referenced by \ucode{lambda3}. The effect of the map is to make the
the call for \ucode{lambda3} refer to the corresponding \ucode{s} inside the
\kcode{target} construct rather than the original \ucode{s}.

In Case 4, the program defines a static variable \ucode{ss} of the same
structure type as \ucode{s}. While the body of the lambda expression refers
to \ucode{ss}, it is not captured. In order for \ucode{lambda4} to be
callable in the \kcode{target} region, the reference to \ucode{ss} should be
to a device copy of \ucode{ss} that also has static storage. This is achieved
with the use of the \kcode{declare target} directive.  Inside the
\kcode{target} construct, all references to \ucode{ss}, including in the
\ucode{lambda4} call, will refer to the corresponding \ucode{ss} that
results from the \kcode{declare target} directive. The \kcode{always}
modifier is used on the \kcode{map} clause to transfer the updated values for
the structure back to the host device.

\topmarker{C++}
\cppnexample[5.0]{lambda_expressions}{1}
\end{cppspecific}