如何调整 std::vector<std::queue<std::unique_ptr<int>>> 的大小？

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【中文标题】如何调整 std::vector<std::queue<std::unique_ptr<int>>> 的大小？【英文标题】：How to resize a std::vector<std::queue<std::unique_ptr<int>>>? 【发布时间】：2020-12-04 09:00:02 【问题描述】：

我正在尝试执行以下操作：

#include <memory>
#include <vector>
#include <queue>

int main() 
    std::vector<std::queue<std::unique_ptr<int>>> v;
    v.resize(10);

但我在 GCC 10.2 中得到了这个：

$ g++ test.cpp -o test
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/memory:66,
                from test.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h: In instantiation of '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::_Deque_iterator<std::unique_ptr<int>, const std::unique_ptr<int>&, const std::unique_ptr<int>*>; _ForwardIterator = std::_Deque_iterator<std::unique_ptr<int>, std::unique_ptr<int>&, std::unique_ptr<int>*>]':
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:325:37:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = std::_Deque_iterator<std::unique_ptr<int>, const std::unique_ptr<int>&, const std::unique_ptr<int>*>; _ForwardIterator = std::_Deque_iterator<std::unique_ptr<int>, std::unique_ptr<int>&, std::unique_ptr<int>*>; _Tp = std::unique_ptr<int>]'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_deque.h:896:36:   required from 'std::deque<_Tp, _Alloc>::deque(const std::deque<_Tp, _Alloc>&) [with _Tp = std::unique_ptr<int>; _Alloc = std::allocator<std::unique_ptr<int> >]'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_queue.h:96:11:   required from 'void std::_Construct(_Tp*, _Args&& ...) [with _Tp = std::queue<std::unique_ptr<int> >; _Args = const std::queue<std::unique_ptr<int, std::default_delete<int> >, std::deque<std::unique_ptr<int, std::default_delete<int> >, std::allocator<std::unique_ptr<int, std::default_delete<int> > > > >&]'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:91:18:   required from 'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const std::queue<std::unique_ptr<int> >*; _ForwardIterator = std::queue<std::unique_ptr<int> >*; bool _TrivialValueTypes = false]'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:150:15:   required from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const std::queue<std::unique_ptr<int> >*; _ForwardIterator = std::queue<std::unique_ptr<int> >*]'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:325:37:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = const std::queue<std::unique_ptr<int> >*; _ForwardIterator = std::queue<std::unique_ptr<int> >*; _Tp = std::queue<std::unique_ptr<int> >]'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:347:2:   required from '_ForwardIterator std::__uninitialized_move_if_noexcept_a(_InputIterator, _InputIterator, _ForwardIterator, _Allocator&) [with _InputIterator = std::queue<std::unique_ptr<int> >*; _ForwardIterator = std::queue<std::unique_ptr<int> >*; _Allocator = std::allocator<std::queue<std::unique_ptr<int> > >]'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/vector.tcc:659:48:   required from 'void std::vector<_Tp, _Alloc>::_M_default_append(std::vector<_Tp, _Alloc>::size_type) [with _Tp = std::queue<std::unique_ptr<int> >; _Alloc = std::allocator<std::queue<std::unique_ptr<int> > >; std::vector<_Tp, _Alloc>::size_type = long unsigned int]'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_vector.h:940:4:   required from 'void std::vector<_Tp, _Alloc>::resize(std::vector<_Tp, _Alloc>::size_type) [with _Tp = std::queue<std::unique_ptr<int> >; _Alloc = std::allocator<std::queue<std::unique_ptr<int> > >; std::vector<_Tp, _Alloc>::size_type = long unsigned int]'
test.cpp:7:16:   required from here
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:137:72: error: static assertion failed: result type must be constructible from value type of input range
137 |       static_assert(is_constructible<_ValueType2, decltype(*__first)>::value,
    |                                                                        ^~~~~

与clang++11相同：

$ clang++-11 test.cpp -o test
In file included from test.cpp:1:
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/memory:66:
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:137:7: error: static_assert failed due to requirement 'is_constructible<std::unique_ptr<int, std::default_delete<int>>, const std::unique_ptr<int, std::default_delete<int>> &>::value' "result type must be constructible from value type of input range"
    static_assert(is_constructible<_ValueType2, decltype(*__first)>::value,
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:325:19: note: in instantiation of function template specialization 'std::uninitialized_copy<std::_Deque_iterator<std::unique_ptr<int, std::default_delete<int>>, const std::unique_ptr<int, std::default_delete<int>> &, const std::unique_ptr<int, std::default_delete<int>> *>, std::_Deque_iterator<std::unique_ptr<int, std::default_delete<int>>, std::unique_ptr<int, std::default_delete<int>> &, std::unique_ptr<int, std::default_delete<int>> *>>' requested here
     return std::uninitialized_copy(__first, __last, __result); 
                ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_deque.h:896:14: note: in instantiation of function template specialization 'std::__uninitialized_copy_a<std::_Deque_iterator<std::unique_ptr<int, std::default_delete<int>>, const std::unique_ptr<int, std::default_delete<int>> &, const std::unique_ptr<int, std::default_delete<int>> *>, std::_Deque_iterator<std::unique_ptr<int, std::default_delete<int>>, std::unique_ptr<int, std::default_delete<int>> &, std::unique_ptr<int, std::default_delete<int>> *>, std::unique_ptr<int, std::default_delete<int>>>' requested here
     std::__uninitialized_copy_a(__x.begin(), __x.end(),
            ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_queue.h:96:11: note: in instantiation of member function 'std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>::deque' requested here
    class queue
        ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:91:8: note: in instantiation of function template specialization 'std::_Construct<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>, const std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> &>' requested here
                std::_Construct(std::__addressof(*__cur), *__first);
                    ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:150:2: note: in instantiation of function template specialization 'std::__uninitialized_copy<false>::__uninit_copy<const std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *>' requested here
        __uninit_copy(__first, __last, __result);
        ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:325:19: note: in instantiation of function template specialization 'std::uninitialized_copy<const std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *>' requested here
     return std::uninitialized_copy(__first, __last, __result); 
                ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:346:19: note: in instantiation of function template specialization 'std::__uninitialized_copy_a<const std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>>' requested here
    return std::__uninitialized_copy_a
                ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/vector.tcc:659:14: note: in instantiation of function template specialization 'std::__uninitialized_move_if_noexcept_a<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::allocator<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>>>' requested here
                    std::__uninitialized_move_if_noexcept_a(
                        ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_vector.h:940:4: note: in instantiation of member function 'std::vector<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>, std::allocator<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>>>::_M_default_append' requested here
        _M_default_append(__new_size - size());
        ^
test.cpp:7:7: note: in instantiation of member function 'std::vector<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>, std::allocator<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>>>::resize' requested here
    v.resize(10);
    ^
In file included from test.cpp:1:
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/memory:65:
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_construct.h:109:38: error: call to deleted constructor of 'std::unique_ptr<int, std::default_delete<int>>'
     ::new(static_cast<void*>(__p)) _Tp(std::forward<_Args>(__args)...); 
                                    ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:91:8: note: in instantiation of function template specialization 'std::_Construct<std::unique_ptr<int, std::default_delete<int>>, const std::unique_ptr<int, std::default_delete<int>> &>' requested here
                std::_Construct(std::__addressof(*__cur), *__first);
                    ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:150:2: note: in instantiation of function template specialization 'std::__uninitialized_copy<false>::__uninit_copy<std::_Deque_iterator<std::unique_ptr<int, std::default_delete<int>>, const std::unique_ptr<int, std::default_delete<int>> &, const std::unique_ptr<int, std::default_delete<int>> *>, std::_Deque_iterator<std::unique_ptr<int, std::default_delete<int>>, std::unique_ptr<int, std::default_delete<int>> &, std::unique_ptr<int, std::default_delete<int>> *>>' requested here
        __uninit_copy(__first, __last, __result);
        ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:325:19: note: in instantiation of function template specialization 'std::uninitialized_copy<std::_Deque_iterator<std::unique_ptr<int, std::default_delete<int>>, const std::unique_ptr<int, std::default_delete<int>> &, const std::unique_ptr<int, std::default_delete<int>> *>, std::_Deque_iterator<std::unique_ptr<int, std::default_delete<int>>, std::unique_ptr<int, std::default_delete<int>> &, std::unique_ptr<int, std::default_delete<int>> *>>' requested here
     return std::uninitialized_copy(__first, __last, __result); 
                ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_deque.h:896:14: note: in instantiation of function template specialization 'std::__uninitialized_copy_a<std::_Deque_iterator<std::unique_ptr<int, std::default_delete<int>>, const std::unique_ptr<int, std::default_delete<int>> &, const std::unique_ptr<int, std::default_delete<int>> *>, std::_Deque_iterator<std::unique_ptr<int, std::default_delete<int>>, std::unique_ptr<int, std::default_delete<int>> &, std::unique_ptr<int, std::default_delete<int>> *>, std::unique_ptr<int, std::default_delete<int>>>' requested here
     std::__uninitialized_copy_a(__x.begin(), __x.end(),
            ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_queue.h:96:11: note: in instantiation of member function 'std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>::deque' requested here
    class queue
        ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:91:8: note: (skipping 2 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
                std::_Construct(std::__addressof(*__cur), *__first);
                    ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:325:19: note: in instantiation of function template specialization 'std::uninitialized_copy<const std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *>' requested here
     return std::uninitialized_copy(__first, __last, __result); 
                ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_uninitialized.h:346:19: note: in instantiation of function template specialization 'std::__uninitialized_copy_a<const std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>>' requested here
    return std::__uninitialized_copy_a
                ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/vector.tcc:659:14: note: in instantiation of function template specialization 'std::__uninitialized_move_if_noexcept_a<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>> *, std::allocator<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>>>' requested here
                    std::__uninitialized_move_if_noexcept_a(
                        ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/stl_vector.h:940:4: note: in instantiation of member function 'std::vector<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>, std::allocator<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>>>::_M_default_append' requested here
        _M_default_append(__new_size - size());
        ^
test.cpp:7:7: note: in instantiation of member function 'std::vector<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>, std::allocator<std::queue<std::unique_ptr<int, std::default_delete<int>>, std::deque<std::unique_ptr<int, std::default_delete<int>>, std::allocator<std::unique_ptr<int, std::default_delete<int>>>>>>>::resize' requested here
    v.resize(10);
    ^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/g++-v10/bits/unique_ptr.h:468:7: note: 'unique_ptr' has been explicitly marked deleted here
    unique_ptr(const unique_ptr&) = delete;
    ^
2 errors generated.

这似乎适用于 libc++，例如clang++ test.cpp -o test -stdlib=libc++.

这是一个已知的 libstdc++ 错误吗？关于如何解决这个问题的任何想法？

【问题讨论】：

即使v.reserve(10); 也失败了。充其量，std::vector&lt;std::queue&lt;std::unique_ptr&lt;int&gt;&gt;&gt; v(10); 有效（这使得std::vector 的意图在某种程度上没有用处）。 clang++ 错误（'unique_ptr' 已在此处明确标记为已删除：unique_ptr(const unique_ptr&) = delete;) 表明 resize() 似乎使用了复制并且显然失败了。我认为这是一个错误，因为应该只使用默认构造函数和移动，但我不是标准/编译器专家。 【参考方案1】：

这不是错误。 std::queue&lt;std::unique_ptr&lt;int&gt;&gt; 的移动构造函数在 libc++ 中是 noexcept(true)，但在 libstdc++ 中不是。因此，对于 libstdc++，std::vector 必须使用复制构造函数进行重新分配。但是它的调用会产生编译器错误，因为很自然地，唯一指针队列可能不会被复制。

证明：https://godbolt.org/z/9j9P59

请注意，这不是错误，因为标准没有规定 std::queue 的移动构造函数为 noexcept。但是，实现是allowed to strengthen the exception specification：

实现可以通过添加非抛出异常规范来加强非虚函数的异常规范。

这就是 libc++ 在这里所做的：https://github.com/llvm/llvm-project/blob/master/libcxx/include/queue#L241。

【讨论】：

【参考方案2】：

如果你通读libstdc++的源代码，在vector的resize中，它最终会调用，

std::__uninitialized_move_if_noexcept_a

从旧内存空间移动或复制到新内存空间的功能。

如果移动构造函数没有抛出，这个函数的作用是移动底层对象，否则复制它。

如果你通读了 std::queue 的 libstdc++ 实现，

template<typename _Alloc, typename _Requires = _Uses<_Alloc>>
queue(queue&& __q, const _Alloc& __a)
   : c(std::move(__q.c), __a)

后面显然没有noexcept，意思是它可能会抛出。因此libstdc++在resize时会调用queue的拷贝构造函数，std::unique_ptr不能被拷贝，所以会出现编译错误。

坦率地说，这在 c++11 之后应该可以工作，即使它不是错误，而是 c++11 之前的旧行为。

【讨论】：

这也不是移动构造函数。移动构造函数不能需要 2 个参数。

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