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This patch adds memoization to these four views so that their begin() has the required amortized constant time complexity. The cache is enabled only for forward_ranges and above because we need the underlying iterator to be copyable and multi-pass in order for the cache to be usable. In the general case we represent the cached result of begin() as a bare iterator. This takes advantage of the fact that value-initialized forward iterators can be compared to as per N3644, so we can use a value-initialized iterator to denote the "empty" state of the cache. As a special case, when the underlying range models random_access_range and when it's profitable size-wise, then we cache the offset of the iterator from the beginning of the range instead of caching the iterator itself. Additionally, in drop_view and reverse_view we disable the cache when the underlying range models random_access_range, because in these cases recomputing begin() takes O(1) time anyway. libstdc++-v3/ChangeLog: * include/std/ranges (__detail::_CachedPosition): New struct. (views::filter_view::_S_needs_cached_begin): New member variable. (views::filter_view::_M_cached_begin): New member variable. (views::filter_view::begin): Use _M_cached_begin to cache its result. (views::drop_view::_S_needs_cached_begin): New static member variable. (views::drop_view::_M_cached_begin): New member variable. (views::drop_view::begin): Use _M_cached_begin to cache its result when _S_needs_cached_begin. (views::drop_while_view::_M_cached_begin): New member variable. (views::drop_while_view::begin): Use _M_cached_begin to cache its result. (views::reverse_view::_S_needs_cached_begin): New static member variable. (views::reverse_view::_M_cached_begin): New member variable. (views::reverse_view::begin): Use _M_cached_begin to cache its result when _S_needs_cached_begin. * testsuite/std/ranges/adaptors/drop.cc: Augment test to check that drop_view::begin caches its result. * testsuite/std/ranges/adaptors/drop_while.cc: Augment test to check that drop_while_view::begin caches its result. * testsuite/std/ranges/adaptors/filter.cc: Augment test to check that filter_view::begin caches its result. * testsuite/std/ranges/adaptors/reverse.cc: Augment test to check that reverse_view::begin caches its result. |
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| doc | ||
| include | ||
| libsupc++ | ||
| po | ||
| python | ||
| scripts | ||
| src | ||
| testsuite | ||
| acinclude.m4 | ||
| aclocal.m4 | ||
| ChangeLog | ||
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| ChangeLog-2019 | ||
| config.h.in | ||
| configure | ||
| configure.ac | ||
| configure.host | ||
| crossconfig.m4 | ||
| fragment.am | ||
| linkage.m4 | ||
| Makefile.am | ||
| Makefile.in | ||
| README | ||
file: libstdc++-v3/README New users may wish to point their web browsers to the file index.html in the 'doc/html' subdirectory. It contains brief building instructions and notes on how to configure the library in interesting ways.