c++ pass a custom struct through a function

Question

I have been working with this C++ program for a while, and I have figured what exactly is happening, but I haven't figured out how to fix it exactly. Here is what I have set up:

struct entry {
    string foo;
    string bar;
    string num;
};
struct node {
    entry input;
    node* left;
    node* right;
};
node* home = new node;

This code takes place in a separate header file that is included in a main cpp file, which has the following:

home->input.foo="John";
home->input.bar="Doe";
home->input.name="1234";
printAll(home);

This is were the error pops up, trying to pass home through the function printAll in the header file:

void printAll(node* start){
    if(start==NULL) return;
    printAll(start->left);
    cout << start->input.foo;
    printall(start->right);
}

The error that Visual Studio gives me is 0xCDCDCDCD on start. I understand it's not home that is causing the issue, it's start, but I don't understand how to correct this error. I read around and I can assume that start has been thrown into heap memory but it is unintalized. I didn't think this was possible. And I also can guess that C++ doesn't know what start is and how to use it, how would I correct this?

You shouldn't just put "new" just anywhere. Where is the corresponding delete? And the code that ensures that this is always called, even in the case of exceptions? Use std::unique_ptr to handle that for you. — TamaMcGlinn, Oct 13 '18 at 22:27
"I read around and I can assume that start has been thrown into heap memory but it is unintalized. I didn't think this was possible." Formally whatever `start` points at is dynamically allocated, though msvc++ does have the concept of a heap. Anyway, the typical way to get around the unitialised memory problem is to provide a c'tor, either something like `node(): left(), right() {}` or `node(): left(nullptr), right(nullptr) {}`. Initialisation using a constructor will incur a performance penalty, but that's something that probably shouldn't be considered for a list node. — George, Oct 13 '18 at 22:46

score 1 · Accepted Answer · answered Oct 13 '18 at 22:30

1

You haven't initialized left or right. In debug builds, Visual Studio will set uninitialized memory to 0xCDCDCDCD. This is obviously not equal to NULL, so your comparison returns false.

answered Oct 13 '18 at 22:30

gsemac

852
7
17

^ which means that the printAll function calls itself infinitely. – TamaMcGlinn Oct 13 '18 at 22:35
@TamaMcGlinn Nope, it means undefined behaviour is invoked. – George Oct 13 '18 at 22:36

Bilal Saleem · Answer 2 · 2018-10-13T23:03:43.423

As noted in the other answer, the error you are getting may be because you haven't initialized the left and right node as NULL. However you have another error and that is you have created an infinite loop in your printAll.

Your printAll function will first move to the left-most node and print it. After that it will move one node to the right, and, before prints it anything, it'll move to the left again.

The proper way of printing all the nodes is to put list of nodes within a class that keeps track of the first and last node.

Class LList {
    node * startNode;

    void printAll(){
          if (startNode == NULL) 
                return;

          node * currNode = startNode;

         // print all the nodes moving left to right
         _printAll(currNode);
    }

    void _printAll(currNode){
          // print currNode and recursively go to next by calling 
          // _printAll(currNode->right)
     }
}

Additional things to note

Of course, you'll want to make printAll public and the rest of the above private.
You'll also need a function to add a node to the list. Look up linked lists attributes and methods to see what else you'll need.
It's better to avoid structs and use objects in place of them

As a note, the less intrusive fix for the op is to just remove `printAll(start->left);`. And "It's better to avoid structs and use objects in place of them" really has no meaning, instances of `struct` s are objects, and `class` s are more or less identical to `struct`s. It's just that the default access modifier and inheritance mode for a `struct` is `public`. And the fact that their declaration/definitions can't be merged i.e. `class A; struct A{};` is invalid while `struct A; struct A{};` is legal. — George, Oct 13 '18 at 23:09

c++ pass a custom struct through a function

2 Answers2