首先,我要提前感谢任何回答这个问题的人。非常感谢您的帮助。这是我第一次在这里发帖,所以如果我发布不良礼仪,请原谅我。
我的问题是关于方法原型:
void copySubtree(Node<T> * & target, Node<T> * const & original);
当我稍后在copySubtree()
致电combineTrees()
时。正如代码目前一样,它构建。但我最初的原因是:
void copySubtree(Node<T> * & target, const Node<T> * & original);
给了我错误:
error C2664: 'RootedBinaryTree<T>::copySubtree' : cannot convert parameter 2 from 'RootedBinaryTree<T>::Node<T> *const ' to 'const RootedBinaryTree<T>::Node<T> *&'
我知道当你将const
放在参数中的数据类型之前时,它会阻止你在方法中修改所述参数,但是当你把它放在数据类型之后我也不知道它做了什么,也没有我确定我的代码在数据类型之后以const
的位置构建并不仅仅是一个侥幸。在数据类型之后放置const
有什么作用?我的代码是否会以当前编写的方式出现可怕的运行时问题?
[另外:我正在尝试编写带根的二叉树模板类方法定义(这就是为什么有些方法是空的,并且在评论中有一些随机的注释)。因此,我为此造成的任何不便表示歉意。]
以下是相关代码:
RootedBinaryTree.h
#ifndef ROOTEDBINARYTREE_H
#define ROOTEDBINARYTREE_H
template <class T>
class RootedBinaryTree
{
private:
template <class T>
struct Node
{
T nodeData;
Node<T> * leftChild;
Node<T> * rightChild;
};
Node<T> * root;
Node<T> * currentPosition;
void copySubtree(Node<T> * & target, Node<T> * const & original);
void deleteSubtree(Node<T> * n);
public:
RootedBinaryTree(const T & rootData);
RootedBinaryTree(const RootedBinaryTree<T> & original);
~RootedBinaryTree();
void toRoot();
bool moveLeft();
bool moveRight();
T getData() const {return currentPosition->nodeData;};
RootedBinaryTree<T> & operator=(const RootedBinaryTree<T> & RHS);
void combineTrees(const RootedBinaryTree<T> & leftTree, const RootedBinaryTree<T> & rightTree);
void setNodeData(const T & nodeData);
};
#endif
RootedBinaryTree.cpp
#ifndef ROOTEDBINARYTREE_CPP
#define ROOTEDBINARYTREE_CPP
#include "RootedBinaryTree.h"
template<class T>
void RootedBinaryTree<T>::copySubtree(Node<T> * & target, Node<T> * const & original)
{
// later add something here to delete a subtree if the node we are trying to assign to has children
// perhaps a deleteSubtree() method
target = new Node<T>;
if(original->leftChild != 0L)
{
copySubtree(target->leftChild, original->leftChild);
}
else
{
target->leftChild = 0L;
}
// ^^^ copy targets left (and right) children to originals
if(original->rightChild != 0L)
{
copySubtree(target->rightChild, original->rightChild);
}
else
{
target->rightChild = 0L;
}
target->nodeData = original->nodeData;
}
template <class T>
void RootedBinaryTree<T>::deleteSubtree(Node<T> * n) // Done
{// Assumes that n is a valid node.
if(n->leftChild != 0L) deleteSubtree(n->leftChild); // Delete all nodes in left subtree
if(n->rightChild != 0L) deleteSubtree(n->rightChild); // Delete all nodes in right subtree
delete n;
}
template <class T>
RootedBinaryTree<T>::RootedBinaryTree(const T & rootData) // Done
{
root = new Node <T>;
root->leftChild = 0L;
root->rightChild = 0L;
root->nodeData = rootData;
currentPosition = root;
}
template <class T>
RootedBinaryTree<T>::RootedBinaryTree(const RootedBinaryTree<T> & original)
{
}
template <class T>
RootedBinaryTree<T>::~RootedBinaryTree()
{
deleteSubtree(root); // root will be valid because of our constructor and other methods
root = currentPosition = 0L;
}
template <class T>
void RootedBinaryTree<T>::toRoot() // Done
{
currentPosition = root;
}
template <class T>
bool RootedBinaryTree<T>::moveLeft() // Done
{
if(currentPosition->leftChild == 0L) return false;
currentPosition = currentPosition->leftChild;
return true;
}
template <class T>
bool RootedBinaryTree<T>::moveRight() // Done
{
if(currentPosition->rightChild == 0L) return false;
currentPosition = currentPosition->rightChild;
return true;
}
template <class T>
RootedBinaryTree<T> & RootedBinaryTree<T>::operator=(const RootedBinaryTree<T> & RHS)
{
}
template <class T>
void RootedBinaryTree<T>::combineTrees(const RootedBinaryTree<T> & leftTree, const RootedBinaryTree<T> & rightTree)
{ // Copies leftTree into root's left tree and rightTree into root's right tree.
if(root->leftChild != 0L) deleteSubtree(root->leftChild);
if(root->rightChild != 0L) deleteSubtree(root->rightChild);
copySubtree(root->leftChild, leftTree.root);
copySubtree(root->rightChild, rightTree.root);
}
template <class T>
void RootedBinaryTree<T>::setNodeData(const T & nodeData)
{
currentPosition->nodeData = nodeData;
}
#endif
再次感谢!
答案 0 :(得分:3)
规则是const
将自己附加到左边的东西上,如果左边没有任何东西,它会将自己附加到右边的东西上。所以在这种情况下,我们有:
const Node *p; // p is a pointer to a const Node
Node const *p; // same again, p is a pointer to a const Node
Node * const p; // p is a const pointer to a (mutable) Node
const Node * const p; // p is a const pointer to a const Node
Node const * const p; // same again, p is a const pointer to a const Node
大多数人都会写const Type
,因为这是我们倾向于考虑它们的方式,但有些人更喜欢写Type const
因为这个规则。