C ++中的内存分配失败（使用new和delete）

Question

I am an ordinary student trying to code a Monte-Carlo Tree Search.

To do this, I made a struct named "Node", and tried to make new Node* pointers and allocate memory to them with "new", but my program keeps on crashing, and I would reeeeally appreciate some help.

Here's what my code looks like. Locations of failure are marked.

Thank you in advance.

Node* knell;
Node* treePolicy(Node* currentnode){
    puts("treepolicy");
    Node* temp=(Node*)malloc(sizeof(Node));
    puts("Mem Allocated");
    Move save=MCTBoard.generateRandomLegalMove(currentnode->player);      ///works very well up to here.
    save.printMove();
    temp=findNode(currentnode,save);
    puts("Node Found");
    knell=new Node; 
    if(temp==NULL){
        free(temp);
        temp = new Node; ///crashes the second time treePolicy(Node*) is called.
        temp->setMove(save);
        temp->child.clear();
        currentnode->child.push_back(temp);
        temp->parent=currentnode;
        MCTBoard.playMove(save,currentnode->player);
        for(int i=0;i<4;i++){
            for(int j=0;j<=i;j++){
                for(int k=0;k<=i;k++){
                    temp->board[i][j][k]=MCTBoard.a[i][j][k];
                }
            }
        }
        temp->value=temp->visited=temp->win=temp->draw=0;
        temp->player=3-currentnode->player;
        knell=temp;
        //delete temp; -> even with this enabled, still crashes.
        return knell;///an infinite loop happens here, but that is another problem so...
    }
else{
    ///not important,and besides,I've not even reached here yet once.
}

}

actually there does exist a same problem in another function, but I feel that that is some other problem.

So, can anyone please tell me why it crashes??

Answer 1

Let's walk through the life cycle of temp .

You allocated memory using malloc .

Node* temp=(Node*)malloc(sizeof(Node));

and then you are overriding the temp by return value of below function.

temp = findNode(currentnode,save);

then you are deleting it using free .

free(temp);

Memory above which you are freeing using free is returned from findNode . Most probably reason for the crash is that function returned memory allocated using new rather than malloc .

Answer 2

A couple of comments:

Node* knell;

why make knell a global variable? Why not handle the assignment of the global outside of the function (perhaps something that looks like:

knell = treePolicy(currentNode);

) or something similar

Node* treePolicy(Node* currentnode){
    puts("treepolicy");
    Node* temp=(Node*)malloc(sizeof(Node));

No reason to malloc here. You're going to overwrite later, so lets remove this line.

    puts("Mem Allocated");
    Move save=MCTBoard.generateRandomLegalMove(currentnode->player);      ///works very well up to here.
    save.printMove();
    temp=findNode(currentnode,save);

Here's a great place for auto.

    puts("Node Found");
    knell=new Node; 

Like we said above, remove this.

    if(temp==NULL){

good place for nullptr

        free(temp);

we just determined temp was null. No need to free

        temp = new Node; ///crashes the second time treePolicy(Node*) is called.
        temp->setMove(save);
        temp->child.clear();
        currentnode->child.push_back(temp);
        temp->parent=currentnode;
        MCTBoard.playMove(save,currentnode->player);
        for(int i=0;i<4;i++){
            for(int j=0;j<=i;j++){
                for(int k=0;k<=i;k++){
                    temp->board[i][j][k]=MCTBoard.a[i][j][k];
                }
            }
        }
        temp->value=temp->visited=temp->win=temp->draw=0;
        temp->player=3-currentnode->player;
        knell=temp;

Let's get rid of this

        //delete temp; -> even with this enabled, still crashes.
        return knell;///an infinite loop happens here, but that is another problem so...

Ok, so now there's no reason to return knell , but just return temp .

    }
else{
    ///not important,and besides,I've not even reached here yet once.
}

So the fixed up code should look like:

   Node* treePolicy(Node* currentnode){
        puts("treepolicy");
        Move save=MCTBoard.generateRandomLegalMove(currentnode->player);      ///works very well up to here.
        save.printMove();
        auto temp=findNode(currentnode,save);
        puts("Node Found");
        if(temp==nullptr){
            temp = new Node; ///crashes the second time treePolicy(Node*) is called.
            temp->setMove(save);
            temp->child.clear();
            currentnode->child.push_back(temp);
            temp->parent=currentnode;
            MCTBoard.playMove(save,currentnode->player);
            for(int i=0;i<4;i++){
                for(int j=0;j<=i;j++){
                    for(int k=0;k<=i;k++){
                        temp->board[i][j][k]=MCTBoard.a[i][j][k];
                    }
                }
            }
            temp->value=temp->visited=temp->win=temp->draw=0;
            temp->player=3-currentnode->player;
            return temp;
        }
    else{
        ///not important,and besides,I've not even reached here yet once.
    }

Or something to that effect.