Unable to build: use of deleted function

[andviane]

Looks like a great thing but unfortunately I cannot build it:

[ 28%] Building CXX object examples/CMakeFiles/lowercase.dir/lowercase.cpp.o
In file included from /home/audrius/projects/lru_cache/lru-cache/include/lru/internal/base-cache.hpp:37:0,
                 from /home/audrius/projects/lru_cache/lru-cache/include/lru/cache.hpp:36,
                 from /home/audrius/projects/lru_cache/lru-cache/include/lru/lru.hpp:26,
                 from /home/audrius/projects/lru_cache/lru-cache/include/lru/lowercase.hpp:25,
                 from /home/audrius/projects/lru_cache/lru-cache/examples/lowercase.cpp:25:
/home/audrius/projects/lru_cache/lru-cache/include/lru/internal/base-ordered-iterator.hpp: In instantiation of ‘LRU::Internal::BaseOrderedIterator<Key, Value, Cache>::BaseOrderedIterator(LRU::Internal::BaseUnorderedIterator<AnyCache, AnyUnderlyingIterator>&&) [with AnyCache = LRU::Internal::BaseCache<int, int, LRU::Internal::Information, std::hash<int>, std::equal_to<int>, LRU::Tag::BasicCache>; UnderlyingIterator = std::__detail::_Node_iterator<std::pair<const int, LRU::Internal::Information<int, int> >, false, false>; <template-parameter-2-3> = void; Key = const int; Value = const int; Cache = const LRU::Internal::BaseCache<int, int, LRU::Internal::Information, std::hash<int>, std::equal_to<int>, LRU::Tag::BasicCache>]’:
/home/audrius/projects/lru_cache/lru-cache/include/lru/internal/base-cache.hpp:311:42:   required from ‘LRU::Internal::BaseCache<Key, Value, InformationType, HashFunction, KeyEqual, TagType>::OrderedConstIterator::OrderedConstIterator(LRU::Internal::BaseCache<Key, Value, InformationType, HashFunction, KeyEqual, TagType>::UnorderedIterator) [with Key = int; Value = int; InformationType = LRU::Internal::Information; HashFunction = std::hash<int>; KeyEqual = std::equal_to<int>; TagType = LRU::Tag::BasicCache]’
/home/audrius/projects/lru_cache/lru-cache/examples/lowercase.cpp:44:70:   required from here
/home/audrius/projects/lru_cache/lru-cache/include/lru/internal/base-ordered-iterator.hpp:128:12: error: use of deleted function ‘std::optional<LRU::Entry<const int, const int> >& std::optional<LRU::Entry<const int, const int> >::operator=(std::optional<LRU::Entry<const int, const int> >&&)’
     _entry = std::move(unordered_iterator._entry);
     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/7/bits/node_handle.h:39:0,
                 from /usr/include/c++/7/bits/hashtable.h:37,
                 from /usr/include/c++/7/unordered_map:47,
                 from /usr/include/c++/7/functional:60,
                 from /home/audrius/projects/lru_cache/lru-cache/include/lru/cache.hpp:28,
                 from /home/audrius/projects/lru_cache/lru-cache/include/lru/lru.hpp:26,
                 from /home/audrius/projects/lru_cache/lru-cache/include/lru/lowercase.hpp:25,
                 from /home/audrius/projects/lru_cache/lru-cache/examples/lowercase.cpp:25:
/usr/include/c++/7/optional:453:11: note: ‘std::optional<LRU::Entry<const int, const int> >& std::optional<LRU::Entry<const int, const int> >::operator=(std::optional<LRU::Entry<const int, const int> >&&)’ is implicitly deleted because the default definition would be ill-formed:
     class optional
           ^~~~~~~~
/usr/include/c++/7/optional:453:11: error: use of deleted function ‘std::_Enable_copy_move<true, false, true, false, _Tag>& std::_Enable_copy_move<true, false, true, false, _Tag>::operator=(std::_Enable_copy_move<true, false, true, false, _Tag>&&) [with _Tag = std::optional<LRU::Entry<const int, const int> >]’
In file included from /usr/include/c++/7/optional:43:0,
                 from /usr/include/c++/7/bits/node_handle.h:39,
                 from /usr/include/c++/7/bits/hashtable.h:37,
                 from /usr/include/c++/7/unordered_map:47,
                 from /usr/include/c++/7/functional:60,
                 from /home/audrius/projects/lru_cache/lru-cache/include/lru/cache.hpp:28,
                 from /home/audrius/projects/lru_cache/lru-cache/include/lru/lru.hpp:26,
                 from /home/audrius/projects/lru_cache/lru-cache/include/lru/lowercase.hpp:25,
                 from /home/audrius/projects/lru_cache/lru-cache/examples/lowercase.cpp:25:
/usr/include/c++/7/bits/enable_special_members.h:246:5: note: declared here
     operator=(_Enable_copy_move&&) noexcept                         = delete;
     ^~~~~~~~
examples/CMakeFiles/lowercase.dir/build.make:62: recipe for target 'examples/CMakeFiles/lowercase.dir/lowercase.cpp.o' failed
make[2]: *** [examples/CMakeFiles/lowercase.dir/lowercase.cpp.o] Error 1
CMakeFiles/Makefile2:294: recipe for target 'examples/CMakeFiles/lowercase.dir/all' failed
make[1]: *** [examples/CMakeFiles/lowercase.dir/all] Error 2
Makefile:140: recipe for target 'all' failed
make: *** [all] Error 2

I tried to force C++17 and C++14 in CMakeLists.txt but was not helpful

My compiler:

gcc --version
gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0
Copyright (C) 2017 Free Software Foundation, Inc.

cmake --version
cmake version 3.16.0

make --version
GNU Make 4.1
Built for x86_64-pc-linux-gnu

Let me known what else. I would be happy to assist in testing.

[OdyX]

I can confirm this is still the case some years later:

git clone https://github.com/goldsborough/lru-cache
cd lru-cache
git submodule update --init
cmake . -B build
cd build
make lowercase

Results in:

[ 50%] Building CXX object examples/CMakeFiles/lowercase.dir/lowercase.cpp.o
In file included from /tmp/lru-cache/include/lru/internal/base-ordered-iterator.hpp:32,
                 from /tmp/lru-cache/include/lru/internal/base-cache.hpp:37,
                 from /tmp/lru-cache/include/lru/cache.hpp:36,
                 from /tmp/lru-cache/include/lru/lru.hpp:26,
                 from /tmp/lru-cache/include/lru/lowercase.hpp:25,
                 from /tmp/lru-cache/examples/lowercase.cpp:25:
/tmp/lru-cache/include/lru/internal/base-iterator.hpp:63:34: warning: 'template<class _Category, class _Tp, class _Distance, class _Pointer, class _Reference> struct std::iterator' is deprecated [-Wdeprecated-declarations]
   63 | class BaseIterator : public std::iterator<IteratorTag, LRU::Entry<Key, Value>> {
      |                                  ^~~~~~~~
In file included from /usr/include/c++/12/string:45,
                 from /usr/include/c++/12/bits/locale_classes.h:40,
                 from /usr/include/c++/12/bits/ios_base.h:41,
                 from /usr/include/c++/12/ios:42,
                 from /usr/include/c++/12/ostream:38,
                 from /usr/include/c++/12/iostream:39,
                 from /tmp/lru-cache/examples/lowercase.cpp:23:
/usr/include/c++/12/bits/stl_iterator_base_types.h:127:34: note: declared here
  127 |     struct _GLIBCXX17_DEPRECATED iterator
      |                                  ^~~~~~~~
/tmp/lru-cache/include/lru/internal/base-ordered-iterator.hpp: In instantiation of 'LRU::Internal::BaseOrderedIterator<Key, Value, Cache>::BaseOrderedIterator(LRU::Internal::BaseUnorderedIterator<AnyCache, AnyUnderlyingIterator>&&) [with AnyCache = LRU::Internal::BaseCache<int, int, LRU::Internal::Information, std::hash<int>, std::equal_to<int>, LRU::Tag::BasicCache>; UnderlyingIterator = std::__detail::_Node_iterator<std::pair<const int, LRU::Internal::Information<int, int> >, false, false>; <template-parameter-2-3> = void; Key = const int; Value = const int; Cache = const LRU::Internal::BaseCache<int, int, LRU::Internal::Information, std::hash<int>, std::equal_to<int>, LRU::Tag::BasicCache>]':
/tmp/lru-cache/include/lru/internal/base-cache.hpp:311:42:   required from 'LRU::Internal::BaseCache<Key, Value, InformationType, HashFunction, KeyEqual, TagType>::OrderedConstIterator::OrderedConstIterator(LRU::Internal::BaseCache<Key, Value, InformationType, HashFunction, KeyEqual, TagType>::UnorderedIterator) [with Key = int; Value = int; InformationType = LRU::Internal::Information; HashFunction = std::hash<int>; KeyEqual = std::equal_to<int>; TagType = LRU::Tag::BasicCache]'
/tmp/lru-cache/examples/lowercase.cpp:44:70:   required from here
/tmp/lru-cache/include/lru/internal/base-ordered-iterator.hpp:128:12: error: use of deleted function 'std::optional<LRU::Entry<const int, const int> >& std::optional<LRU::Entry<const int, const int> >::operator=(std::optional<LRU::Entry<const int, const int> >&&)'
  128 |     _entry = std::move(unordered_iterator._entry);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /tmp/lru-cache/include/lru/internal/optional.hpp:29,
                 from /tmp/lru-cache/include/lru/internal/base-iterator.hpp:29:
/usr/include/c++/12/optional:705:11: note: 'std::optional<LRU::Entry<const int, const int> >& std::optional<LRU::Entry<const int, const int> >::operator=(std::optional<LRU::Entry<const int, const int> >&&)' is implicitly deleted because the default definition would be ill-formed:
  705 |     class optional
      |           ^~~~~~~~
/usr/include/c++/12/optional:705:11: error: use of deleted function 'std::_Enable_copy_move<true, false, true, false, _Tag>& std::_Enable_copy_move<true, false, true, false, _Tag>::operator=(std::_Enable_copy_move<true, false, true, false, _Tag>&&) [with _Tag = std::optional<LRU::Entry<const int, const int> >]'
In file included from /usr/include/c++/12/bits/hashtable.h:36,
                 from /usr/include/c++/12/unordered_map:46,
                 from /usr/include/c++/12/functional:61,
                 from /tmp/lru-cache/include/lru/cache.hpp:28:
/usr/include/c++/12/bits/enable_special_members.h:249:5: note: declared here
  249 |     operator=(_Enable_copy_move&&) noexcept                         = delete;
      |     ^~~~~~~~
make[3]: *** [examples/CMakeFiles/lowercase.dir/build.make:76: examples/CMakeFiles/lowercase.dir/lowercase.cpp.o] Error 1
make[2]: *** [CMakeFiles/Makefile2:283: examples/CMakeFiles/lowercase.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:290: examples/CMakeFiles/lowercase.dir/rule] Error 2
make: *** [Makefile:231: lowercase] Error 2

This, on a (very) recent Debian bookworm, but I can reproduce this easily in a Debian Buster (from 2019).

[davesrocketshop]

I encountered this issue in my own use of the LRU cache. The problem isn't in the LRU code per se, but in the underlying classes.

In my case I was attempting to use the code in a Qt based application. My cache was defined as:

LRU::Cache<QString, std::shared_ptr<MyObject>> _cache;

This build fine on Ubuntu 24.04 and 22.04, but not 20.04. Researching a little it showed the problem is likely caused by a missing move constructor, which makes sense given the older libraries in 20.04. I changed the key from QString to std::sting and everything built fine.

I haven't looked at the provided example code, but I wouldn't be surprised if it's something similar.