/usr/include/dune/common/proxymemberaccess.hh is in libdune-common-dev 2.5.1-1.
This file is owned by root:root, with mode 0o644.
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#ifndef DUNE_COMMON_PROXYMEMBERACCESS_HH
#define DUNE_COMMON_PROXYMEMBERACCESS_HH
/**
* \file
* \brief infrastructure for supporting operator->() on both references and proxies
*/
#include <type_traits>
#include <utility>
namespace Dune {
namespace {
// helper struct to store a temporary / proxy
// for the duration of the member access
template<typename T>
struct member_access_proxy_holder
{
// only support moving the temporary into the holder object
member_access_proxy_holder(T&& t)
: _t(std::move(t))
{}
// The object is fundamentally a temporary, i.e. an rvalue,
//
const T* operator->() const
{
return &_t;
}
T _t;
};
} // anonymous namespace
#ifdef DOXYGEN
//! Transparent support for providing member access to both lvalues and rvalues (temporary proxies).
/**
* If an iterator facade (like entity iterators) wants to allow the embedded implementation to
* return either an (internally stored) reference or a temporary object and expose these two
* behaviors to enable performance optimizations, operator->() needs special handling: If the
* implementation returns a reference, operator->() in the facade can simply return the address
* of the referenced object, but if the returned object is a temporary, we need to capture and
* store it in a helper object to make sure it outlives the member access. This function transparently
* supports both variants. It should be used like this:
*
* \code
* class iterator
* {
* ...
*
* decltype(handle_proxy_member_access(implementation.dereference()))
* operator->() const
* {
* return handle_proxy_member_access(implementation.dereference());
* }
*
* ...
* };
* \endcode
*
* \note This function exploits the special type deduction rules for unqualified rvalue references
* to distinguish between lvalues and rvalues and thus needs to be passed the object returned
* by the implementation.
*/
template<typename T>
pointer_or_proxy_holder
handle_proxy_member_access(T&& t);
#else // DOXYGEN
// This version matches lvalues (the C++ type deduction rules state that
// the T&& signature deduces to a reference iff the argument is an lvalue).
// As the argument is an lvalue, we do not have to worry about its lifetime
// and can just return its address.
template<typename T>
inline typename std::enable_if<
std::is_lvalue_reference<T>::value,
typename std::add_pointer<
typename std::remove_reference<
T
>::type
>::type
>::type
handle_proxy_member_access(T&& target)
{
return ⌖
}
// This version matches rvalues (the C++ type deduction rules state that
// the T&& signature deduces to a non-reference iff the argument is an rvalue).
// In this case, we have to capture the rvalue in a new object to make sure it
// is kept alive for the duration of the member access. For this purpose, we move
// it into a member_access_proxy_holder instance.
template<typename T>
inline typename std::enable_if<
!std::is_lvalue_reference<T>::value,
member_access_proxy_holder<T>
>::type
handle_proxy_member_access(T&& target)
{
return {std::forward<T>(target)};
}
#endif // DOXYGEN
} // namespace Dune
#endif // DUNE_COMMON_PROXYMEMBERACCESS_HH
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