C++ 中的记忆代码：为什么它不起作用？

Question

我收到了this example 关于在 C++ 中实现通用记忆的信息。但是，正如有人在this 评论中指出的那样，原始代码进行了 2 次查找，而下面的代码仅进行了一次。

唯一的问题是第二个return有一个我看不懂的错误。

template <typename ReturnType, typename... Args>
std::function<ReturnType (Args...)> memoize(std::function<ReturnType (Args...)> func)
{
    std::map<std::tuple<Args...>, ReturnType> cache;
    return ([=](Args... args) mutable  {
            std::tuple<Args...> t(args...);
auto range = cache.equal_range(t);
if (range.first != range.second) return (*range.first).second;
return (*cache.insert(range.first, func(args...))).second;

    });
}

编译器错误：

 In instantiation of 'memoize(std::function<_Res(_ArgTypes ...)>)::<lambda(Args ...)> mutable [with ReturnType = int; Args = {int, int}]':
14:36:   required from 'struct memoize(std::function<_Res(_ArgTypes ...)>) [with ReturnType = int; Args = {int, int}]::<lambda(int, int)>'
16:6:   required from 'std::function<_Res(_ArgTypes ...)> memoize(std::function<_Res(_ArgTypes ...)>) [with ReturnType = int; Args = {int, int}]'
34:56:   required from here
14:9: error: no matching function for call to 'std::map<std::tuple<int, int>, int, std::less<std::tuple<int, int> >, std::allocator<std::pair<const std::tuple<int, int>, int> > >::insert(std::_Rb_tree_iterator<std::pair<const std::tuple<int, int>, int> >&, int)'
14:9: note: candidates are:
In file included from /usr/include/c++/4.9/map:61:0,
                 from 1:
/usr/include/c++/4.9/bits/stl_map.h:629:7: note: std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<std::pair<const _Key, _Tp> >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const value_type&) [with _Key = std::tuple<int, int>; _Tp = int; _Compare = std::less<std::tuple<int, int> >; _Alloc = std::allocator<std::pair<const std::tuple<int, int>, int> >; typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<std::pair<const _Key, _Tp> >::other>::iterator = std::_Rb_tree_iterator<std::pair<const std::tuple<int, int>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<const std::tuple<int, int>, int>]
       insert(const value_type& __x)
       ^
/usr/include/c++/4.9/bits/stl_map.h:629:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/4.9/bits/stl_map.h:637:9: note: template<class _Pair, class> std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<std::pair<const _Key, _Tp> >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(_Pair&&) [with _Pair = _Pair; <template-parameter-2-2> = <template-parameter-1-2>; _Key = std::tuple<int, int>; _Tp = int; _Compare = std::less<std::tuple<int, int> >; _Alloc = std::allocator<std::pair<const std::tuple<int, int>, int> >]
         insert(_Pair&& __x)
         ^
/usr/include/c++/4.9/bits/stl_map.h:637:9: note:   template argument deduction/substitution failed:
14:9: note:   candidate expects 1 argument, 2 provided
In file included from /usr/include/c++/4.9/map:61:0,
                 from 1:
/usr/include/c++/4.9/bits/stl_map.h:650:7: note: void std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::initializer_list<std::pair<const _Key, _Tp> >) [with _Key = std::tuple<int, int>; _Tp = int; _Compare = std::less<std::tuple<int, int> >; _Alloc = std::allocator<std::pair<const std::tuple<int, int>, int> >]
       insert(std::initializer_list<value_type> __list)
       ^
/usr/include/c++/4.9/bits/stl_map.h:650:7: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/4.9/bits/stl_map.h:679:7: note: std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator, const value_type&) [with _Key = std::tuple<int, int>; _Tp = int; _Compare = std::less<std::tuple<int, int> >; _Alloc = std::allocator<std::pair<const std::tuple<int, int>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const std::tuple<int, int>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<const std::tuple<int, int>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<const std::tuple<int, int>, int>]
       insert(const_iterator __position, const value_type& __x)
       ^
/usr/include/c++/4.9/bits/stl_map.h:679:7: note:   no known conversion for argument 2 from 'int' to 'const value_type& {aka const std::pair<const std::tuple<int, int>, int>&}'
/usr/include/c++/4.9/bits/stl_map.h:690:9: note: template<class _Pair, class> std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::map<_Key, _Tp, _Compare, _Alloc>::const_iterator, _Pair&&) [with _Pair = _Pair; <template-parameter-2-2> = <template-parameter-1-2>; _Key = std::tuple<int, int>; _Tp = int; _Compare = std::less<std::tuple<int, int> >; _Alloc = std::allocator<std::pair<const std::tuple<int, int>, int> >]
         insert(const_iterator __position, _Pair&& __x)
         ^
/usr/include/c++/4.9/bits/stl_map.h:690:9: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/stl_map.h:686:32: error: no type named 'type' in 'struct std::enable_if<false, void>'
       template<typename _Pair, typename = typename
                                ^
/usr/include/c++/4.9/bits/stl_map.h:705:9: note: template<class _InputIterator> void std::map<_Key, _Tp, _Compare, _Alloc>::insert(_InputIterator, _InputIterator) [with _InputIterator = _InputIterator; _Key = std::tuple<int, int>; _Tp = int; _Compare = std::less<std::tuple<int, int> >; _Alloc = std::allocator<std::pair<const std::tuple<int, int>, int> >]
         insert(_InputIterator __first, _InputIterator __last)
         ^

Answer 1

错误在您的 lambda 中：

auto range = cache.equal_range(t);
...
... cache.insert(range.first, func(args...)) ...

并且是

14:9: error: no matching function for call to
'std::map<std::tuple<int, int>, int,...>
    ::insert(std::_Rb_tree_iterator<...>&, int)'

它告诉你你调用 insert 错误。

equal_range 返回的范围是一对迭代器——您需要取消对第一个迭代器的引用并获取第一个（键）元素，以获得您想要的键/值对。也就是说，像

cache.insert(range.first, make_pair(t, func(args...)))

---

一般来说，您可以通过以下方式帮助自己找出这些错误：

• 简化您的代码（这些大的复合语句意味着很多事情正在发生，并且有很多潜在的错误，在一行中），
• 通过仔细阅读错误消息（它确实列出了该列，并说问题出在insert，
• 如果所有其他方法都失败，则通过在一个最小示例中重现错误（删除与错误无关的代码可减少要考虑的问题数量）。