从C ++中的结构中包含的指针数组动态分配和访问内存

Question

Sorry for the super-lengthy title, but... it DID say to be specific... anyway! I'm making an algorithm designed to solve sliding-block puzzle games. To do this, I need to store all possible variations of a game board within nodes. Each game board state is stored within a node that also contains pointers to its parent (ie the state right before its present state) and all of its children (all possible states available from its present state). Here's the node struct I've built:

struct node
{
    //Attributes
    char gameBoardState[5][4];
    node* parent;
    int numChildren;
    node* childArray[10];

    //Constructor
    node(char pGameState[][4], node* pParent = NULL, int pNumChildren = 0, node* pChildArray[] = NULL)
    {
        //For each row i
        for (int i = 0; i < 5; i++)
        {
            //For each column j
            for (int j = 0; j < 4; j++)
            {
                //Set each block equal to this
                gameBoardState[i][j] = pGameState[i][j];
            }
        }
    }
};

My issue is in this section of code here, designed purely to test whether or not I could access data properly and such (I've designed the functions that actually move the pieces around, but not yet implemented them):

//Sample board state
char sampleBoard[5][4];
char sampleBoard2[5][4];
//Sample character
char sample = 'a';
char sample2 = 'z';
//Initialize the sample board
for (int i = 0; i < 5; i++)
{
    for (int j = 0; j < 4; j++)
    {
        sampleBoard[i][j] = sample;
        sampleBoard2[i][j] = sample2;
        sample++;
        sample2--;
    }
}

//Test
cout << "\n\nERROR BEGINS\n\n";

//Create first node
node top = node(sampleBoard);
//Create a child node
top.childArray[top.numChildren] = new node(sampleBoard2, &top);

//Test
cout << "\n\nERROR ENDS\n\n";

For whatever reason, the lines right between the ERROR comments generate a memory access error. I've looked at all kinds of C++ tutorials regarding pointers, arrays, structs, and combinations thereof. I've also tried omitting the "&top" in the "new node" call; didn't help. I'm just not seeing how my code is generating this error in memory access. I also looked through over a dozen answers here, though none of them with solutions that seemed applicable to my situation. I get the feeling I'm missing something fairly obvious. Any help, even if its just redirection to a valid answer that I've somehow overlooked? Thanks in advance!

Answer 1

The issue is actually quite simple, you never initialized numChildren .

Therefore, attempting to say top.childArray[top.numChildren] ... will likely result in numChildren evaluating to some ridiculous (possibly negative) number.

The bigger issue here is that your code is poorly designed. It's very C-like in that you're using a lot of raw pointers and arrays where the sizing information is separate. Also, the onus of tracking the lifetime of these pointers is on you instead of happening automatically.

I suggest you look into refactoring your code with std::vector and std::string , which provide indexing operations as well as .size() , and your code will become much safer.

You could additionally encapsulate char gameBoardState[5][4]; into a class GameBoard that can be arbitrarily sized at construction (using vector as before). This has the additional benefit of you not needing to change code all over the place should you decide to test with different game boards.

Answer 2

In

top.childArray[top.numChildren] = new node(sampleBoard2, &top);

numChildren was never initialized. This means it has some garbage value. More than likely it is a value that is outside [0, 9] so you are accessing memory you do not own which is UB and will/can cause memory access errors.