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iterator/zip: avoid std::forward_as_tuple for zip value_type

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This tends to result in references to temporaries higher up the stack
being returned. decltype on the iterator dereference is a more reliable
method to determine the return type.
This commit is contained in:
Danny Robson 2020-02-26 12:00:29 +11:00
parent f1aa7fa775
commit ec1f354d16
1 changed files with 26 additions and 4 deletions
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30
iterator/zip.hpp
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@ -44,11 +44,13 @@ namespace cruft::iterator {
// pressing need for this functionality. // pressing need for this functionality.
using iterator_category = std::input_iterator_tag; using iterator_category = std::input_iterator_tag;
using difference_type = std::ptrdiff_t; using difference_type = std::ptrdiff_t;
using value_type = std::tuple< using value_type = std::tuple<
typename std::iterator_traits< typename std::iterator_traits<
std::tuple_element_t<IndexV, IteratorT> std::tuple_element_t<IndexV, IteratorT>
>::reference... >::reference...
>; >;
using reference = value_type; using reference = value_type;
using pointer = value_type*; using pointer = value_type*;
@ -68,19 +70,39 @@ namespace cruft::iterator {
zipped_iterator operator++ (int); zipped_iterator operator++ (int);
decltype(auto) // Do NOT use std::forward_as_tuple here. It favours rvalue
// references which aren't useful after we've returned. decltype
// gives us lvalues and lvalue-references.
//
// If you insist on using another approach then you should test
// under a sanitizer (debug and release) before you commit the
// changes.
auto
operator* (void) operator* (void)
{ {
return std::forward_as_tuple ( return std::tuple<
decltype (*std::get<IndexV> (m_iterators))...
> (
*std::get<IndexV> (m_iterators)... *std::get<IndexV> (m_iterators)...
); );
} }
decltype (auto) // Do NOT use std::forward_as_tuple here. It favours rvalue
// references which aren't useful after we've returned. decltype
// gives us lvalues and lvalue-references.
//
// If you insist on using another approach then you should test
// under a sanitizer (debug and release) before you commit the
// changes.
auto
operator* (void) const operator* (void) const
{ {
return std::forward_as_tuple ( return std::tuple<
std::add_const_t<
decltype(*std::get<IndexV> (m_iterators))
>...
> (
*std::get<IndexV> (m_iterators)... *std::get<IndexV> (m_iterators)...
); );
} }