返回的指针属性中类实例上的矢量属性消失

Question

我正在实现一种树搜索，它需要能够从树中获取"最有前途的节点"，然后对该节点执行某些操作，以便为下一次迭代更新树的其余部分。

问题：对象指针Board*具有矢量属性，这些属性似乎在生成它们的函数的return与调用环境中保存它们的Board*值之间发生变化。

我的输出：

>>g++ -std=c++17 -o tmp.out tests/test.cpp // <- require c++17 for other parts of the project
>>./tmp.out 
Best leaf by tree traversal has score: 8
Best leaf associated state has -1977735524 values in its attribute vector though! (Should be 4)

我所期望的：

>>g++ -std=c++17 -o tmp.out tests/test.cpp // <- require c++17 for other parts of the project
>>./tmp.out 
Best leaf by tree traversal has score: 8
Best leaf associated state has 4 values in its attribute vector though! (Should be 4)

#include <iostream>
#include <vector>
#include <queue>
using namespace std;
class Board{
vector<int> attribute;
string name;
public:
Board(){
attribute = {1,2,3,4};
name = "nonempty name";
}
Board getcopy(){
return *this;
}
int AttrLen(){
return attribute.size();
}
};
class Node{
Board* board;
Node* parent;
std::vector<Node*> children;
int depth=0;
int score=0;
bool terminal=false;
public:
Node(Node* _parent, Board* _board,int _depth){
parent = _parent;
board = _board;
depth = _depth;
// randomize score
score = rand() % 10;
if(depth<2){
for(int _=0;_<2;_++){
Board new_state = board -> getcopy();
children.push_back(new Node(this,&new_state,depth+1));
}
} else {
children = {};
terminal=true;
}
}
int getScore(){
return score;
}
bool isTerminal(){
return terminal;
}
Node* getBestChild(){
if(!terminal){
if(children[0] ->getScore() > children[1] -> getScore()){
return children[0];
} else {
return children[1];
}
} else {
return nullptr;
}
}
Board* getStateptr(){
return board;
}
};
int main(){
// make a board
Board my_board = Board();
// make a root node
Node root = Node(nullptr, &my_board, 0);
Node* best_child = root.getBestChild();
while(!best_child -> isTerminal()){
best_child = best_child -> getBestChild();
}
cout << "Best leaf by tree traversal has score: " << best_child -> getScore() << endl;
cout << "Best leaf associated state has " << best_child -> getStateptr() ->AttrLen() << " values in its attribute vector though! (Should be 4)" << endl;
}

Answer 1

不确定这是否是唯一的问题，但在这里

for(int _=0;_<2;_++){
Board new_state = board -> getcopy();
children.push_back(new Node(this,&new_state,depth+1));
}

您创建一个仅存在于for循环内的Board副本。它在}时自动销毁。因此，您存储在节点中的指针是悬空的。指向早已消失的对象。

请注意，您的getcopy有点奇怪。应改用复制构造函数。通常，当您的类型通过原始指针管理资源时(不确定是否确实如此(，则需要遵守 3/5 规则。在任何情况下，您都可以简单地通过编写Board new_state = *board;来复制电路板(假设编译器生成的复制构造函数做了正确的事情(。