有没有办法在C ++中通过引用传递rvalue？

Question

我正在尝试为小型数据库分配创建“回滚”功能。我有一堆二进制搜索树，我用它来存储数据库的备份：

GenStack<GenBST<Student>> masterStudentStack;

堆栈和BST都是我自己的实现（根据我的指令）。

我把BST的副本推到堆栈上没有问题，

masterStudentStack.push(*masterStudent);

但是，当我尝试检索此BST并使用以下命令将其返回到我的主BST指针时：

void rollBack() {
    masterStudent = new GenBST<Student>(masterStudentStack.pop());
}

我收到一个错误。

Menu.cpp:419:63: error: invalid initialization of non-const reference of 
type ‘GenBST<Student>&’ from an rvalue of type ‘GenBST<Student>’
 masterStudent = new GenBST<Student>(masterStudentStack.pop());
                                     ~~~~~~~~~~~~~~~~~~~~~~^~
In file included from Menu.h:7:0,
             from Menu.cpp:1:
GenBST.h:49:1: note:   initializing argument 1 of 
‘GenBST<T>::GenBST(GenBST<T>&) [with T = Student]’
 GenBST<T>::GenBST(GenBST<T>& other) {
 ^~~~~~~~~

当通过引用传入左值时，BST的复制构造函数是有效的（这是构造函数的声明）

GenBST(GenBST<T>& other);

但我不知道如何以一种复制构造函数接受它的方式从堆栈中弹出一些东西。所以，我的问题是：我可以用rvalue“stack.pop（）”创建一个新的BST吗？

谢谢，马修

编辑：

将“const”添加到我的BST复制构造函数后，我收到此错误

In file included from Menu.h:7:0,
             from Menu.cpp:1:
GenBST.h: In instantiation of ‘GenBST<T>::GenBST(const GenBST<T>&) [with T = 
Student]’:
Menu.cpp:419:65:   required from here
GenBST.h:50:22: error: passing ‘const GenBST<Student>’ as ‘this’ argument 
discards qualifiers [-fpermissive]
   if(other.getRoot() == NULL) {

GenBST.h:77:17: note:   in call to ‘GenTreeNode<T>* GenBST<T>::getRoot() 
[with T = Student]’
 GenTreeNode<T>* GenBST<T>::getRoot()
                 ^~~~~~~~~
GenBST.h:54:32: error: binding ‘GenTreeNode<Student>* const’ to reference of 
type ‘GenTreeNode<Student>*&’ discards qualifiers
     copyTree(this->root, other.root);
                          ~~~~~~^~~~
GenBST.h:108:6: note:   initializing argument 2 of ‘void 
GenBST<T>::copyTree(GenTreeNode<T>*&, GenTreeNode<T>*&) [with T = Student]’
 void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *& 
otherNode) {

这是我的构造函数及其调用的方法：

template <class T>
GenBST<T>::GenBST(const GenBST<T>& other) {
  if(other.getRoot() == NULL) {
    root = NULL;
  }
  else {
    copyTree(this->root, other.root);
  }
}

template <class T>
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *& 
otherNode) {
  if(otherNode == NULL) {
    thisNode = NULL;
  }
  else {
    thisNode = new GenTreeNode<T>(otherNode->key);
    copyTree(thisNode->left, otherNode->left);
    copyTree(thisNode->right, otherNode->right);
  }
}

有任何想法吗？

编辑2：

非常感谢大家的帮助。我将const添加到我的getRoot（）和copyTree（）方法中，现在归结为一个错误。

GenBST.h: In instantiation of ‘GenBST<T>::GenBST(const GenBST<T>&) [with T = 
Student]’:
Menu.cpp:419:65:   required from here
GenBST.h:54:32: error: binding ‘GenTreeNode<Student>* const’ to reference of 
type ‘GenTreeNode<Student>*&’ discards qualifiers
     copyTree(this->root, other.root);
                          ~~~~~~^~~~
GenBST.h:108:6: note:   initializing argument 2 of ‘void 
GenBST<T>::copyTree(GenTreeNode<T>*&, GenTreeNode<T>*&) const [with T = 
Student]’
 void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *& 
otherNode) const {