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c++ - C++ 标准是否对向量赋值函数或构造函数有明确要求？

问题描述

在可能有歧义的地方，我将参考 C++14 标准。

以https://en.cppreference.com/w/cpp/container/vector作为参考，T对于vector的template参数要求比较稀疏

对元素施加的要求取决于对容器执行的实际操作。一般要求元素类型是完整类型，满足Erasable的要求，但很多成员函数的要求更严格。

事实上，许多页面（例如用于插入的页面）都非常明确地说明了所放置的要求T。可悲的是，构造函数或分配函数并非如此。这是网站遗漏还是标准遗漏？

动机
这个问题的动机有点复杂。我有一个Node代表树中节点的类。每个节点都有一个父节点（由 a 表示Node*）和一个子节点列表（由 a 表示std::vector<Node, MyAllocator>）。最后一点是有趣的地方。为了确保Nodes 继续指向正确的父母，我实现了一个自定义分配器，它使用指向正确的指针“预加载”每个构造函数调用Node。代码如下

#ifndef PRElOAD_ALLOCATOR_H
#define PRElOAD_ALLOCATOR_H

#include <tuple>
#include <utility>
#include <memory>
#include <vector>

/**
 * @class PreloadAllocator
 * Provides a generic allocator that is able to 'preload' some arguments for the
 * construct function. Note that these arguments are *always* loaded at the
 * front of any constructor call so modified move, copy constructors need to
 * created to match.
 * This allocator will only work with simple containers like vector that do not
 * modify the type of the allocator being used. *It will not work* with types
 * like list and map that internally represent the data in a different format.
 * This is because the constructors for these objects behave differently and it
 * is not simple to know how to insert the extra arguments into such a call.
 */
template <typename T, typename... Args>
class PreloadAllocator {
  public:

    // Standard allocator typedefs
    using value_type      = T;
    using pointer         = T*;
    using const_pointer   = const T*;
    using reference       = T&;
    using const_reference = const T&;
    using size_type       = std::size_t;
    using difference_type = std::ptrdiff_t;

    PreloadAllocator(Args&&... args)
      : tup(std::forward_as_tuple(args...) ) {}

    template <typename U>
      PreloadAllocator(const PreloadAllocator<U, Args...>& other)
      : tup(other.tup) {}

    pointer allocate(size_type count, const_pointer hint = 0) {
      return m_defaultAlloc.allocate(count, hint);
    }

    void deallocate(pointer ptr, size_type count) {
      return m_defaultAlloc.deallocate(ptr, count);
    }

    template <typename... Ts>
      void construct(T* ptr, Ts&&... ts) {
        // First preload the constructor arguments with the allocator's
        // versions, then add on the rest, and let the default allocator do the
        // rest.
        return construct_impl<Ts...>(
            ptr, std::forward<Ts>(ts)..., std::index_sequence_for<Args...>{});
      }

    // Hold the arguments that we will preload into every constructor call
    const std::tuple<Args...> tup;
  private:
    // Compose the default STL allocator to use its version of allocate and
    // deallocate.
    std::allocator<T> m_defaultAlloc;
    // Actual function that does the constructing
    template <typename... Ts, std::size_t... Is>
      void construct_impl(T* ptr, Ts&&... ts, std::index_sequence<Is...>) {
        return m_defaultAlloc.construct(
          ptr, std::get<Is>(tup)..., std::forward<Ts>(ts)...);
      }
};

template <typename T, typename... Args>
bool operator==(
    const PreloadAllocator<T, Args...>& lhs,
    const PreloadAllocator<T, Args...>& rhs)
{
  return lhs.tup == rhs.tup;
}

template <typename T, typename... Args>
bool operator!=(
    const PreloadAllocator<T, Args...>& lhs,
    const PreloadAllocator<T, Args...>& rhs)
{
  return !(lhs==rhs);
}

#endif //> !PRElOAD_ALLOCATOR_H

class Node {
  public:
    using alloc_t = PreloadAllocator<Node, Node*>;
    using vec_t = std::vector<Node, alloc_t>;

    Node(Node* parent, int data)
      : parent(parent), data(data), children(alloc_t(this) ) {}
    Node(Node* parent, const Node& other)
      : parent(parent), data(other.data), children(other.children, alloc_t(this) ) {}
    Node(Node* parent, Node&& other)
      : parent(parent), data(std::move(other.data) ), children(std::move(other.children), alloc_t(this) ) {}

    // Copy/Move constructing is not going to give us the correct parent!
    Node(const Node&) = delete;
    Node(Node&&) = delete;

    Node* parent;
    int data;
    vec_t children;
};

int main() {
  Node root(nullptr, 0);
}

在 g++ 中编译此代码时，它会编译并运行良好。但是，当我在 clang 中编译时，出现以下错误

In file included from alloc_test.cxx:3:
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:1275:9: error: no matching member function for call to 'assign'
        assign(_Ip(__x.begin()), _Ip(__x.end()));
        ^~~~~~
alloc_test.cxx:15:55: note: in instantiation of member function 'std::__1::vector<Node, PreloadAllocator<Node, Node *> >::vector' requested here
      : parent(parent), data(std::move(other.data) ), children(std::move(other.children), alloc_t(this) ) {}
                                                      ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:588:10: note: candidate function not viable: no known conversion from '_Ip' (aka 'move_iterator<__wrap_iter<Node *> >') to
      'std::__1::vector<Node, PreloadAllocator<Node, Node *> >::size_type' (aka 'unsigned long') for 1st argument
    void assign(size_type __n, const_reference __u);
         ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:576:9: note: candidate template ignored: requirement '!__is_forward_iterator<move_iterator<__wrap_iter<Node *> > >::value' was not satisfied
      [with _InputIterator = std::__1::move_iterator<std::__1::__wrap_iter<Node *> >]
        assign(_InputIterator __first, _InputIterator __last);
        ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:586:9: note: candidate template ignored: requirement 'is_constructible<value_type, typename iterator_traits<move_iterator<__wrap_iter<Node *>
      > >::reference>::value' was not satisfied [with _ForwardIterator = std::__1::move_iterator<std::__1::__wrap_iter<Node *> >]
        assign(_ForwardIterator __first, _ForwardIterator __last);
        ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:592:10: note: candidate function not viable: requires single argument '__il', but 2 arguments were provided
    void assign(initializer_list<value_type> __il)
         ^
1 error generated.

那么，这是 g++ 和 clang 决定以不同方式解释的标准中的歧义，还是其中一个真正的错误？

尝试编写这种预加载分配器是对标准的不可原谅的滥用吗？

*编辑：*修复了代码片段中缺失的数据成员。

标签： c++g++clang

解决方案

您的代码似乎对 C++17 标准有效。最强的要求来自这个拷贝构造函数std::vector：

children(other.children, alloc_t(this))

该标准说（表 65）这Node需要Cpp17CopyInsertable进入std::vector<Node, PreloadAllocator<Node, Node*>>：

T is Cpp17CopyInsertable intoX意味着，除了T是Cpp17MoveInsertable into之外X，以下表达式是格式正确的：

allocator_traits<A>::construct(m, p, v)

并且它的求值导致以下后置条件成立： 的值v不变并且等价于*p。

Cpp17MoveInsertable的要求类似，只是v是一个右值，后置条件不同。

查看错误时clang，编译器似乎正在检查MoveConstructible的要求：

is_constructible<
    value_type, 
    typename iterator_traits<move_iterator<__wrap_iter<Node *> >::reference>::value

...这与Cpp17MoveInsertable的不同。

从设计的角度来看，我会亲自删除该自定义分配器，而是parent在需要时手动更新成员，例如：

Node(Node* parent, const Node& other) 
    : parent(parent), data(other.data), children(other.children) {
    fix_parent_in_childrens();
}

void fix_parent_in_childrens() {
    for (Node &node: children) {
        node.parent = this;
    }
}

由于您似乎已经有了正确的封装，因此（对我而言）这比自定义分配器更有意义。

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