使用队列实现树时出现编译器错误（C++ 的 STL）

Question

请告诉我为什么我的编译器会出现以下错误：

C:/Program Files/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/ext/new_allocator.h: In instantiation of 'void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = Node*; _Args = {Node**}; _Tp = Node*]':
C:/Program Files/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/alloc_traits.h:475:4:   required from 'static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = Node*; _Args = {Node**}; _Tp = Node*; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<Node*>]'
C:/Program Files/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/deque.tcc:168:30:   required from 'void std::deque<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {Node**}; _Tp = Node*; _Alloc = std::allocator<Node*>]'
C:/Program Files/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_queue.h:268:4:   required from 'void std::queue<_Tp, _Sequence>::emplace(_Args&& ...) [with _Args = {Node**}; _Tp = Node*; _Sequence = std::deque<Node*, std::allocator<Node*> >]'
binaryTree.cpp:23:20:   required from here
C:/Program Files/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev0/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/ext/new_allocator.h:136:4: error: cannot convert 'Node**' to 'Node*' in initialization
  { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

这是我的代码：

#include <iostream>
#include <queue>
using namespace std;

Node *root = NULL;

class Node
{
public:
    int data;
    Node *lchild;
    Node *rchild;
};

void create()
{
    Node *p, *t;
    int x;
    queue<Node *> q;
    cout << "Enter root value: ";
    cin >> x;
    Node *root = new Node();
    root->data = x;
    root->lchild = root->rchild = 0;
    q.emplace(&root);
    while (!q.empty())
    {
        p = q.front();
        q.pop();
        cout << "Enter value for left child (-1 for NULL): ";
        cin >> x;
        if (x != -1)
        {
            t = new Node();
            t->data = x;
            t->lchild = t->rchild = 0;
            p->lchild = t;
            q.emplace(t);
        }

        cout << "Enter value for right child (-1 for NULL): ";
        cin >> x;
        if (x != -1)
        {
            t = new Node();
            t->data = x;
            t->lchild = t->rchild = 0;
            p->rchild = t;
            q.emplace(t);
        }
    }
}

void preorder(Node *p)
{
    if (p)
    {
        cout << p->data << " ";
        preorder(p->lchild);
        preorder(p->rchild);
    }
}

int main()
{
    create();
    preorder(root);
    return 0;
}

Answer 1

问题是队列需要 Node * 值，而您正在传递 Node ** 值。

将q.emplace(&root); 替换为q.emplace(root);